Conservation Tillage in Medicinal Plant Cultivation in China: What, Why, and How

Abstract

Ecological cultivation is a promising regime for medicinal plant production. For a long time, unreasonable farming methods have threatened soil health and medicinal agriculture and restricted the sustainable development of ecological agriculture for medicinal plants. However, there is a lack of comprehensive discourse and discussion about the pros and cons of different tillage regimes. Here, the research trend and application prospects of no-tillage (NT) are comprehensively reviewed, and the ecological benefits, challenges, and opportunities of the NT system in ecological agriculture of medicinal plants are scrutinized, aiming to call for an about-face in the sustainable conservation and utilization of both phytomedicine resources and agricultural/ecological resources. An exhaustive literature search in PubMed, Bing, Scopus, and CNKI was performed to outline the research trend in conservation tillage and medicinal plants during the recent four decades. The application of NT has a long history and can reduce tillage frequency and intensity and protect soil from erosion and deterioration. NT is often combined with organic mulch to significantly reduce soil disturbance. NT and stover mulching have the advantages of saving manpower and resources and improving soil quality, crop yield, and quality. The ecological and economic benefits of NT in long-term medicinal plant cultivation could be prominent. In developing medicinal plant cultivation, competing with food crops should be avoided as much as possible, and the impact on the production of major grain crops should be minimized. Therefore, the full utilization of soil resources in forests, mountains, and wasteland is advocated, and sustainable soil utilization is the core issue in the process of land reclamation. NT and stover mulching not only inherit the traditional concept of “natural farming”, conform to the basic laws of ecology, as well as the growth characteristics of medicinal plants, but also protect the ecological environment of the production area. It would become the core strategy of ecological agriculture for medicinal plants. Our summary and discussions would help propose countermeasures to popularize NT and organic mulch, promote relevant research and scientific allocation of resources, and adapt to local conditions to achieve precise management and harmonize conservation and production of medicinal plants.

1. Introduction

Presently, the robust development of global agriculture is challenged by a growing population, degraded farmland, and serious environmental issues, especially those related to climate change [1]. Since 2020, the COVID-19 pandemic and the fertilizer crisis have exacerbated soil degradation and crop yield decline [2], and the frequent outbreaks of major infectious diseases also require the sustainable production of plant-based drugs to meet the needs of disease prevention and treatment. Tillage is the manipulation of soil and plant residues by iron/steel tools to prepare an appropriate seedbed for crop cultivation, which has several advantages such as loosening soil, improving surface drainage, changing soil structure, increasing the release of nutrients from soil for crop growth, reducing competition from other plants, etc. [3,4]. However, intensive tillage practices may negatively affect the soil structure, soil porosity, storage of water in soil, aggregate stability, soil erosion, soil organic matter (SOM), soil micro- and macrofauna, and environmental quality by increasing greenhouse gas (GHG) emissions [5], and labor/production costs [6,7]. Such concerns have led to the search for tillage systems that minimize negative impacts on the environment while cost-effectively sustaining crop productivity [8]. Conservation tillage techniques, such as minimum tillage (MT ≈ reduced tillage (RT)) [9], no-tillage (NT; zero tillage) [10], and organic mulch [11,12], minimize the impacts of tillage on soil structure and soil biota [13], reduce soil erosion threats, increase soil organic carbon (SOC) [1], and may help reduce GHG emissions in comparison to conventional tillage (CT) [14]. To achieve these goals, conservation tillage should be adapted in terms of local conditions and flexibly implemented in crop production systems. Conservation tillage is any tillage and planting system that leaves at least 30% of the soil surface covered with crop residue after planting [15]. NT maintains more crop residue on the soil surface but might not allow profitable production if its other advantages cannot be fully utilized.

Medicinal plants have a long history of application, high medicinal value, and high market demand. Many herbs are commonly used in the prevention and treatment of COVID-19 [16,17], and the knowledge base of herbal medicines and their large-scale production would promote the translation of bench findings into the clinical practice of anti-COVID-19 and speed up anti-COVID-19 drug discovery and development. To meet the growing food or drug demand of consumers for phytoproducts such as phytomedicines, innovative strategies for ethnobiology/ethnomedicine and industrial development are needed to improve production capacity [18], while alleviating the pressure on natural pharmaceutical resources and avoiding ecosystem degradation. In China, only 1% of medicinal plants that are included in the Chinese Pharmacopoeia are mainly cultivated on farmland [19]. The most common habitats for medicinal plants fall into two categories. The natural habitats of around 42.5% of medicinal plants are at the forest edge/under the forest, while 43.8% of medicinal plants are on roadsides, hillsides, and wasteland/sandy land. The former has to adapt to environmental stresses such as weak light, pests, and diseases, while the latter is subject to drought, strong light, ultraviolet radiation, high/low temperatures [20], nutrient deficiency, and insect pests. For medicinal varieties in all kinds of habitats, conservation tillage can be attempted to ensure their quality and yield (Figure 1). To some extent, conservation tillage simulates the natural habitat of plants, in which environmental stresses elicit the synthesis and accumulation of specialized metabolites [21,22]. During long-term adaptation to specific stresses, medicinal plants continuously accumulate related genetic and epigenetic variations, which represent an important basis for their quality formation [19]. The NT/MT-based “pseudo environment cultivation” of medicinal plants could help balance the growth/development of medicinal plants and the accumulation of defense compounds, as well as the quality and yield. It could be developed as one of the basic modes of quality production for medicinal plants.

When cultivating medicinal plants, they are often collocated with food, vegetable, or cash crops [71,72]. According to the landform, climate, soil, and vegetation conditions, a variety of planting systems are flexibly adopted, including multiple cropping [14], monocropping, intercropping, under-crop sowing [73], mixed cropping, rotation [71,74] continuous cropping, and the like. This makes the production of medicinal herbs and various departments of agricultural production integrated [71,75], and new farming systems such as NT and RT make the cultivation of medicinal herbs more closely integrated with agriculture, forestry, and aquaculture, which is conducive to coordinated development and achieving better economic, social, and ecological benefits. Therefore, the objectives of this review are (i) to introduce novel insights into conservation tillage with a focus on NT, RT, and organic mulch; (ii) to provide details about the advantages and limitations of NT in crop plant production; and (iii) to discuss possibilities of promoting the application of NT in the ecological agriculture of medicinal plants. We summarized and commented on the recent progress of conservation tillage and its effects on food crops and medicinal crops. The burgeoning NT studies of grain crops provide important references for the NT exploration of medicinal plants at the budding stage, which has positive enlightening value. Meanwhile, it must be noted that the cultivation/tillage experiences of grain crops and cash crops cannot be simply copied, and the physiological and ecological characteristics of medicinal plants and specific requirements for their production must be considered.

2. Materials and Methods

An exhaustive literature search in PubMed, Bing, Scopus, and CNKI (http://cnki.net/, accessed on 10 June 2023) was performed to outline the research trend in conservation tillage and medicinal plants during the recent four decades, the golden age of the flourishing development of conservation tillage. Search terms ‘conservation tillage’, ‘no tillage’, ‘reduced/minimum tillage’, ‘straw/stover mulch’, ‘cover crop’, ‘understorey’, ‘soil’, etc., were used, combined with ‘medicinal plant’ and names of major families and genera.

3. NT: Concept and Development

3.1. CT vs. NT

Tillage and crop residue management profoundly influence soil physical, chemical, and biological properties that ultimately affect crop productivity [76,77]. A compatible combination of tillage and crop residue management should be explored to achieve sustainable food/medicine production (

Table 1. Examples of worldwide applications of conservation tillage in medicinal plant production.

Region	Forms of Protective Cultivation	Types of Advantages (Effects)	Types of Cultivated Plants	Reference
Southern Italy (Basilicata Region)	NT	XS of tree crops significantly responds to a shift in soil management; NT plants showed an upregulated secondary metabolism.	Olea europaea	[80]
South Korea	NT/RT (coniferous, mixed, and deciduous broad-leaved stand sites)	Optimal soil conditions were achieved via NT/RT and understory planting.	Mountain-cultivated ginseng	[81]
India, Chir pine forests of the Northwest Himalaya	Understory planting (degraded Chir pine forests)	More economic returns from medicinal plants; great potential to improve the productivity of degraded forests.	Andrographis paniculata, Mucuna pruriens, Solanum khasianum, Spilanthes acmella, Withania somnifera, Cymbopogon nardus, and Ocimum basilicum	[9]
Coimbra County, State of Minas Gerais, Brazil	NT/corn stover mulch	Phytophagous pest species and beneficial arthropods abundance were higher under NT.	Phaseolus vulgaris	[82]
India, Lucknow (26.5°N, 80.5°E, 120 m altitude)	Paddy straw mulch	Paddy straw mulch permitted geranium crops to produce 18.4 kg/ha more oil, giving an additional return of 53,600/ha over that of unmulched control.	Pelargonium graveolens	[83]
Mediterranean Region	Legume cover	Gradually enhances soil quality for organic rainfed almond orchards in marginal areas.	Prunus armeniaca	[84]
Mediterranean Region	Combining zeolites with early-maturing annual legume cover	Ensure adequate crop yield and olive oil quality; preserve soil health.	Olea europaea	[85]
Northwest China	NT	Beneficial for water storage and moisture preservation, as well as for plant root development and soil water infiltration, so as to reduce runoff erosion and prevent secondary soil salinization.	Lycium chinense, Ephedra sinica, and Glycyrrhiza uralensis	[20]
Northeast China (Panax), Southwest China (Paeonia lactiflora), Central China (Polygonatum sibiricum), and North China (Forsythia suspensa)	Understory planting (Panax: Natural growth in forests of mixed coniferous and broad-leaved trees, shrubs, and weeds; P. lactiflora: Under Phellodendron amurense; Polygonatum: Under Chinese fir, Chinese chestnut, Camellia oleifera, and Phyllostachys pubescens)	Improve the yield and quality of ginseng. Utilizing the forest environment to develop polygonatum cultivation can regulate forest microclimate, improve soil, increase fertility, conserve water sources and soil, and improve site conditions.	Panax ginseng, P. lactiflora, Polygonatum sibiricum, and F. suspensa,	[73]
Gansu, China	NT/straw mulch (wheat-Isatis indigotica dual sequence rotation)	Improve soil environmental conditions and water storage to create a good foundation for the cultivation of strong seedlings of I. indigotica.	Isatis indigotica	[71]
Hunan, China	NT/organic mulch (Straw, mountain green, fungus chaff, chaff, and other organic substances are collected, and 1% superphosphate and 2% quicklime are added, then they are composted and fermented for 2–3 months to make organic mulch)	Ensure drug quality	Lilium brownii var. viridulum	[86]
Yulong County, Lijiang, Yunnan Province, China	Understory planting (Under fir forest)	Improve quality of medicinal materials and expand the planting area.	Nardostachys jatamansi	[87]
Gansu, China	2BM-5 type NT seeder	Protect soil, increase yield, and fully maintain soil moisture.	Glycyrrhiza uralensis, Crocus sativus, and Lamiophlomis rotata	[88]
Chongqing, China	NT (Rice semi-dry NT, wheat semi-dry NT, and paddy field NT comprehensive utilization)	Can better ensure drug quality and medication safety.	Lysimachia christinae	[89]

). The optimal tillage mode is conducive to improving soil properties and providing a favorable environment for crop plants. Moreover, managing crop residues effectively helps reduce environmental pollution due to crop residue burning. For sustainable agriculture in China’s Mollisols (Chernozems in the WRB system) region, NT, straw returning, and optimized nitrogen fertilization are necessary in continuous or rotation cropping systems of corn and soybean [78]. NT sowing following rice harvest was found to be the best sowing method for forage wheat and Italian ryegrass in winter fallow paddy fields in South China [79].

Conservation tillage has been applied in the production of various medicinal plants in most of the eight major production regions of terrestrial TCM plants (Figure 1). In NT cultivation of Bulbus lilii, the field was not plowed [86], and the bulbs were covered with organic mulch, such as straw, stover, mountain green, mushroom bran, and husk. In order to create organic mulch that supported and fixed lily plants, kept a specific amount of water and breathability, and gave nutrients for lily growth and development, 1% superphosphate and 2% quicklime were added, and a composting fermentation of 2–3 months was carried out. The experiences and preliminary results of Hungary in NT (direct drilling) and strip-tillage (a kind of RT) suggest that the difficulties of new tillage modes can gradually be reduced through site-specific technology solutions [90]. The necessity of subsoiling (deep loosening; ST) is beyond doubt, although the timing of operation and duration of effects should be investigated in detail. The complex advantages of conservation tillage attract farmers worldwide. In Québec, Canada, 77.0%, 21.5%, and 1.3% of maize and soybean farmers, respectively, are partial adopters, full adopters, and non-adopters of conservation tillage [91].

CT refers to tillage operations considered standard for a specific location and crop, in which the crop residues are usually buried and soil inversion is generally involved [92]; it is usually considered a base for determining the cost-effectiveness of erosion control practices. In England, 42% of agricultural fields may be overworked by CT, as indicated by the absence/rarity of epigeic and anecic earthworms [93], and organic matter management did not mitigate CT impacts. In Brazil, after CT for 18 years in soybean fields, the soil C stock in the 0–20 cm layer was reduced by 0.64 t/ha/year [94]. In principal component analysis, CT was significantly separated from other land use systems, which was strongly influenced by the low C contents in the different SOM fractions and higher N₂O emissions promoted by CT. In conventional olive growing, frequent soil tillage strongly reduces the complexity and diversity of the agroecosystem [80]. By combining and configuring soil tillage measures such as rotary tillage [95], ST, and NT, the drawbacks of traditional long-term tillage can be circumvented [96].

3.2. Development of NT and Related Conservation Tillage

The methods of conservation tillage are becoming increasingly diverse as standardized approaches have to be tailored to fit local conditions. In the NT system of many places, soil vegetation cover brings plentiful benefits, e.g., erosion protection, intake of organic material into the soil, nutrient cycling, and maintenance of an ideal microclimate for microbial development [97]. In Brazil, Crotalaria juncea, wild beans, Cajanus cajan, millet ‘BR05’, and sorghum ‘BR 304’ were used as cover crops cultivated after the corn crop. In the upper Mississippi River Basin of the United States, NT with a rye (Secale cereale) cover crop modified the relationship between drainage volume and nitrate losses by reducing nitrate concentrations [98], lowering nitrate losses by 59% compared with CT-oat and 67% compared with NT alone.

Intermittent deep tillage (DT) with a spiral drill, a new type of conservation tillage, could effectively promote agricultural production [99] through optimizing soil structure, underground ecosystems, and soil fertility. Such new farming methods require the use of newly developed agricultural machinery and tools as a complete set. For instance, there was a lack of supporting agricultural machinery for the large-scale planting of herbal medicine on the Gannan grassland in Gansu, China; a 2BM-5 type ditch cutting grassland medicinal NT seeder has been developed [88]. A new sowing machine was proposed that uses a disc knife to cut ditches and a compactor to close ditches on damp and rocky grasslands. The field tests showed that the 2BM-5 type NT seeder can ensure the basic number of seedlings per unit area with a seed depth qualification rate of 82.5% and an increase in seedling emergence rate of 6.25%, which can meet the needs of producing local medicinal plants such as Glycyrrhiza uralensis, Crocus sativus, and Lamiophlomis rotata (Figure 1).

The development of new farming machinery brings more diverse solutions to conservation tillage [95]. In CT, plowing, raking, or cross-axis rotation are used with soil breaking by plowing or longitudinal soil breaking. Fenlong-ridging is a smash-ridging farming method performed by the “self-propelled intelligent Fenlong machine” with a vertical drill, horizontal soil breaking by deep rotary tillage, ultra-DT, and ST without disturbing the soil layers [100]. In 21 Chinese provinces, Fenlong-ridging has been applied to over 40 kinds of crops to improve yield, quality, and efficiency. The tool geometry and working depth significantly impacted the performance of conservation tillage [95]. The vertical disc showed a higher average straw cutting efficiency, lower tillage forces, and less soil disturbance width than the disc coulter. Rippled disc, a kind of vertical disc, had the highest straw cutting efficiency, moderate tillage force, and appropriate soil disturbance width among the five tools, especially at the 10 cm working depth. For NT seeding in the intensive rotation system, a rippled disc would be a suitable rotary tool.

In mature forests, overstory diversity is important for controlling soil-borne pathogens, and understory biodiversity is critical and beneficial for soil nutrient availability, plant production, soil-plant mutualistic relationships, and maintaining ecosystem multi-services [101]. Many medicinal plants, such as Panax, Sinopodophyllum hexandrum, Polygonatum, and Amomum villosum, are suitable for understory cultivation [73,102,103] (Figure 1), in which NT or RT is implemented. Long-term, unreasonable farming methods threaten soil health and restrict the sustainable development of ecological agriculture [104]. The application of NT has a long history since the beginning of human civilization and reduces tillage frequency and intensity. NT has many advantages in saving manpower and resources, improving soil quality, crop yield, and quality, and the ecological and economic benefits of NT in long-term cultivation are particularly prominent. The full utilization of soil resources in forests, mountains, and wastelands is advocated by the ecological agriculture of medicinal plants, which does not compete with farming [104,105]. Sustainable soil utilization is the core issue in the process of land reclamation. NT not only inherits the traditional concept of “natural farming”, but also conforms to the basic laws of ecology. It conforms to the growth characteristics of medicinal plants and can protect the native environment of the production area. It will undoubtedly become one of the core strategies for the sustainable development of medicinal agriculture.

4. Advantages and Benefits of Conservation Tillage in Medicinal Plant Production

Soil health is associated with various agronomic and environmental benefits [106,107], which are deemed essential for the optimal production of medicinal plants. Soil health indicators include physical, chemical, biological, and other ones, all of which can be influenced by NT and relevant manipulations (Figure 2). Importantly, such influences are desired to achieve the target outcomes, e.g., the robust growth of medicinal species and preferred plant-based medicinal compounds [102,108].

4.1. Effects on Soil Structure and Physical Properties

Various conservation tillage approaches, such as straw mulching and ST, can protect soil and improve soil structure [77]. In the Loess Plateau of Northwest China, the alternation pattern NT/CT/ST significantly increased the mechanically stable aggregates and water-stable aggregates, with an increased mean weight diameter and geometric mean diameter. In the 0–20 cm soil layer, NT/CT/ST reduced the soil bulk density (SBD) and increased the soil porosity. In a study of long-term maize production modes suitable for loess dryland, the SBD was lower under NT/ST rotational tillage [96]; in the 0–60 cm soil layer, it averaged 1.31 g/cm³. In a continuous tillage cultivation positioning trial, the mean weight diameter, geometric mean diameter, and >0.25 mm aggregate content of ST straw cover were 27.3%, 17.5%, and 7.6% more than CT straw cover, respectively [109].

In the chickpea field of India, it is essential to find a compatible combination of tillage and crop residue management for achieving sustainable food production by improving soil properties and providing a favorable environment to crop plants [76]. NT with 60 cm residue height (NT60) led to lower SBD and higher soil moisture content than that of NT30, and the SBD under NT30, NT60, RT30, and RT60 was higher than that under CT. In the maize fields of Iraq, compared with RT and DT, CT decreased SBD but increased soil water content [110]. The soil surface under NT/RT with residue retention is usually colder and wetter, and SBD is higher than that under CT, which could affect the growth of plant roots and the absorption of nutrients. While paying attention to reducing the frequency and intensity of tillage, DT and ST have the potential to break the compacted zone in the soil, leading to a better soil structure and crop yield.

The effects of NT/RT on SBD have been inconsistent among studies, as the subjective and objective factors that affect the implementation of NT/RT are very complex. In the trial of Daodi medicinal materials Radix glycyrrhizae and Radix isatidis, NT straw mulching reduced the SBD and aggregate damage rate [111], increased soil total porosity, and saturated hydraulic conductivity. The intercropping of spring wheat and licorice, and potato and alfalfa can improve soil physical properties. In the trial of wheat-Isatis indigotica dual sequence rotation, conservative tillage reduced the SBD, increased total porosity [71], and significantly improved saturated hydraulic conductivity. These results can only be observed in long-term experiments, and some short-term studies of one to two years cannot observe such effects.

The ecological environment is closely related to the growth, development, and quality formation of medicinal herbs. Ginseng has stringent requirements for the natural environment, often growing in damp valleys with abundant forests. Various ecological factors, e.g., light, temperature, altitude, water, and edaphic factors, impact the shape and ginsenosides of ginseng. It was found that soil physical factors, especially SBD, were more important ecological factors affecting the shape of ginseng [103]. When SBD was between 0.85 and 0.95 g/cm³, the ginseng roots had the best growth and development, with excellent yield and quality. When SBD was above 1.0 g/cm³, the development of ginseng roots was limited, with a significant increase in short branches, resulting in a noteworthy decrease in ginseng quality and yield. The optimal SBD and other soil conditions could be achieved via NT/RT and understory planting [81]. Temperature is the dominant ecological factor affecting the accumulation of ginsenosides in ginseng [103] and could also be regulated by conservation tillage.

During the potato tuber formation stage in southern Ningxia, China, the straw mulch significantly reduced the average soil effective accumulated temperature by 18.7% [112], in comparison with no mulching plots. In the trial of wheat-I. indigotica rotation, NT and straw cover significantly increased the soil temperature during low temperature seasons [71] and effectively reduced the soil temperature during high temperature seasons, which helped crop emergence and root growth.

4.2. Effects on Soil Erosion

Presently, the total planting area of Chinese medicinal materials (CMMs) in China is about 5.56 million hm² [113], with trees and shrubs accounting for about 58% and herbs and vines accounting for about 42%. The planting area of 191 commonly used CMMs is about 3.85 million hm², accounting for 69.2% of the total area, with herbaceous plants accounting for a relatively large proportion. As a series of policies that prevent “non-grain” of cultivated land have been promulgated [114], farmland-grown medicinal plants are being replaced by food crops. Many medicinal plants are very much like weeds, which are self-growing, do not need to compete with food crops for farmland, and have strong adaptability to the NT/RT environment [115]. Therefore, in some areas of rolling topography, especially in mountainous and hilly rural areas in the central and western regions of China, abandoned land on slopes can be fully utilized to plant perennial/biennial herbaceous medicinal plants under NT/RT, e.g., I. indigotica, Pseudostellaria heterophylla, and Nepeta cataria (Figure 1). When tillage is at right angles to the direction of the slope, it is referred to as contour tillage [116], an acknowledged form of conservation tillage. According to local climate and soil, medicinal plants that are suitable for local growth can be selected for NT/RT cultivation, which not only reduces soil erosion and runoff and loosens the soil but also brings certain economic benefits to farmers.

Rainfall is scarce in the arid inland areas of Northwest China, and water is the main factor limiting agricultural production. When producing perennial medicinal plants, e.g., Lycium chinense, Ephedra sinica, and Glycyrrhiza uralensis [117] (Figure 1), in saline fields, NT measures should be adopted to help achieve plant coverage throughout the year, reduce surface evaporation, and prevent secondary soil salinization [20]. NT is beneficial for water storage and moisture preservation, as well as for plant root development and soil water infiltration, so as to reduce runoff erosion. The northwest inland agricultural region suffers from frequent sandstorms and severe wind erosion, where conservation tillage is highly recommended for reducing water/wind erosion and boosting soil fertility [118].

In the Loess Plateau, the runoff and sediment content under CT treatments were greater than those under NT straw mulching [111]. NT straw cover effectively reduced the runoff and erosion of slope farmland and reduced the loss of soil nutrients. Under artificial simulated rainfall conditions, NT straw cover reduced the runoff by 20.3% to 56.2% and sediment yield by 38.1% to 76.8% as compared to CT; it reduced the runoff by 5.0% to 28.5% and sediment yield by 12.9% to 52.3% as compared to NT alone. In winter wheat monoculture in dry areas, when compared to CT, conservation tillage during the fallow period increased precipitation storage efficiency, soil water storage at wheat planting, and wheat yield by 31.0, 6.4, and 7.9%, respectively [119], but did not affect evapotranspiration or water use efficiency (WUE). In the common beans field of Uganda, NT decreased the surface runoff volume and suspended sediment concentration while increasing the infiltration rate and soil moisture content [120]. The effects of tillage and mulching on soil water conservation, crop yield, and water use varied with soil and climate conditions.

4.3. Effects on Soil Moisture

The positive effects of NT on soil moisture are closely associated with its effects on soil structure and erosion. In Mollisols of Northeast China, NT with stover mulching significantly increased the soil water content and root-associated organic carbon [10] and decreased soil pH. In Henan, China, NT showed better effects on soil moisture conservation and yield increment than ST treatment in dry years [121]. In winter fallow paddy fields in South China, the enzyme activities and total soil porosity in the NT forage wheat and Italian ryegrass fields decreased, and the water content and soil capillary porosity increased when compared to those of the CT field [79]. NT also increased the number of species and aboveground weed biomass. In India, MT and the accumulation of leaf litter result in moisture conservation and low weed infestation [9]. In the trial of wheat-I. indigotica rotation, under different rotation modes, NT straw cover significantly increased the soil moisture content of the tillage layer [71]. In the semi-arid Loess Plateau of Gansu, China, the NT straw mulch treatment led to the lowest soil bulk and pH and the highest total nitrogen (TN), NO₃−-N, and available P [122], which were accompanied by higher soil water content as compared with that of CT. In the chickpea field of India, NT60 led to a higher soil moisture content of 22.7% [76].

In the common beans field of Uganda, NT and stubble-mulching improved soil water storage by 46 and 45%, respectively [123], compared with CT in the 0–100 cm soil depth over 14 months. In the study of long-term maize production mode, the NT ↔ ST treatment showed a good water storage effect [96]. Compared to CT, the NT ↔ ST treatment led to good soil moisture status during the growth period of spring maize. In the arid region of Southern Ningxia, ST plowed at 40 cm with straw mulch significantly increased soil water storage by 33.4% compared with plowing 15 cm without mulch [112]. In the tea garden of East Anhui, DT significantly increased soil water storage space and enhanced the water holding capacity of the soil [124]. Compared with NT, the soil moisture of the 15–30 cm soil layer increased by 7.7% under DT. These data suggest that the most appropriate conservation tillage practice varies for different crops and different locations.

4.4. Effects on Soil Fertility

The positive effects of NT on soil fertility are intricately linked with its impact on the physical/biological properties of soil. It is well accepted that conservation tillage often improves soil fertility in agriculture [125,126] (Figure 3). In Sweden, compared with CT for moldboard plowing, NT and non-inversion MT increased the concentrations of SOC, TN, and microbial biomass carbon (MBC) in the upper 20 cm [92]. In Germany, MT increased SOC, TN, and MBC in the top 10 cm, while CT increased MBC contents and SBD between 20 and 30 cm of soil depth. In Romania, MT did not significantly change soil parameters, whose working depth of 25 to 30 cm was similar to that of CT. In Spain, NT pointedly increased the concentrations and stocks of C, N, and MBC. In Southern Italy, in the surface layer (0–15 cm depth), the SOC content and TN were significantly increased by NT [127], but no such effects were observed in the deep layers (30–60 and 60–90 cm). The C/N ratio showed a more equilibrated rate in the NT system, which was accompanied by the best physical characteristics of soil, showing a higher stability index compared to CT and RT. The effectiveness of NT/MT could heavily rely on site conditions, e.g., pH, soil texture, and climate.

In the Loess Plateau, NT/CT/ST significantly increased the soil TN and SOM contents [77], with reduced soil total phosphorus and total potassium contents. The multi-year average grain yield of spring maize in NT/CT/ST was 10.2% higher than ST and 4.8% higher than ST/CT. In Northeast China, the treatments of NT33 (33% stover mulching ≈ 2500 kg/hm²) and NT100 (full mulching) increased the polysaccharide content of the top layer and mid-layer soils [128]. NT33 reduced the diversity of carbon components in topsoil, while NT100, if corn stover resources were sufficient, maintained the carbon stability of three soil layers. NT67 increased carbon stability in the deep layer of soil. In the study of long-term maize production mode, the NT ↔ ST treatment effectively increased the organic carbon storage in the 0–60 cm soil layer [96], which was 54.3 t/hm². In Moso bamboo forests, the combined intensive management, i.e., DT, fertilization, and organic material mulching, significantly increased the concentrations of available potassium, available nitrogen, available phosphorus, NH₄⁺, NO₃⁻, SOM, TN, and total phosphorus [129], but decreased pH, which was accompanied by a lowered Shannon diversity of the soil and rhizome/root microbiota and a relatively stable community composition and function.

In the soybean fields of Brazil, the NT systems were more efficient in accumulating labile and stable C fractions with values close to those of native soil [94] and were directly related to lower soil N₂O emissions. NT increased the SOM fractions such as MBC, permanganate-oxidizable carbon, particulate organic carbon, inert carbon, and humic substances. Cover crops, e.g., pea, rapeseed, and wheat, can increase the contents of SOM, alkali-hydrolyzable nitrogen, and enzymatic activities [12], and suppress bacterial wilt. The performance of different cover crops differed in recovering soil properties. For instance, rapeseed inhibited bacterial wilt more effectively than wheat and pea, while wheat was the best at increasing SOM, urease, and invertase, and pea improved catalase better than the other two. The medicinal plant Perilla frutescens was used as a cover crop in ginseng farmlands [130]. When compared with the control, the SOM content was increased, the SBD was decreased, and the fertility in the 0–20 cm of soil layer was increased. The soil’s microbial diversity improved. Compared to the untreated lands, the survival rate of ginseng seedlings increased by 21.4%, and the physiological indices of ginseng were better than the controls. The application plan for cover crops needs to be carefully considered from various angles, such as technology, economy, and environment.

4.5. Effects on Biodiversity and Soil Biota

Conventional agriculture contributes worryingly to biodiversity losses, partially due to high stock densities, heavy use of pesticides and fertilizer, monocultures, and the CT approach [131]. Radical changes in tillage modes are required, not only to address the loss of biodiversity but also to ensure food and medicine security in the future. NT/RT and organic mulch could be the “back to basics” solution to improving soil quality and biodiversity while maintaining or improving productivity and profitability. The impact of NT/RT on biodiversity and soil organisms largely determines its success or failure, as soil organisms largely shape the physical and chemical properties of the soil.

4.5.1. Bacteria and Archaea

In Mollisol, Northeast China, NT without stover mulch (NT0) and NT stover mulch, as low-disturbance practices, manifestly promoted soil bacterial species richness and diversity [10] and enriched potential metabolic diversity. When compared to the bacterial communities in CT, the vertical dissimilarity of bacterial communities in NT0 decreased, but straw mulch enhanced the uniqueness of community composition at each layer. In redundancy analysis (RDA), it was shown that NT and stover mulch had differential effects on the dominant nitrogen metabolism community in black soil [132]. The abundance of the denitrifying gene nirS was positively correlated with Nocardioides (Actinobacteria), a β-Proteobacteria genus, and an Acidobacteria genus, and was negatively correlated with soil pH, ammonia nitrogen, and a Chloroflexi genus [132]. The soil pH was positively correlated with ammonia nitrogen, TOC (total organic carbon), TN, a Candidatus Rokubacteria genus and a Gemmatimonadetes genus, and was negatively correlated with nitrate nitrogen, Nocardioides, and Solirubrobacter. In pairwise comparison, a Nitrosopumilales genus and a Thaumarchaeota genus were more abundant in NT0 than in NT100 [132], while the ammonia-oxidizing archaea (AOA) Nitrososphaeraceae genus and Candidatus nitrosocosmicus were more abundant in NT100, suggesting the effect of stover mulch on the archaea community.

In the indoor incubation of Mollisol, the moisture/gas regime meaningfully affected the microbial community diversity rather than community richness [133]. The key responsive bacterial classes under different gas conditions were γ-proteobacteria, Bacteroidia, and α-proteobacteria, in contrast to Actinobacteria, α-proteobacteria, and Thermoleophilia under different moisture regimes. The abundance of Piscinibacter, Chujaibacter, Symbiobacteraceae, and Acidobacteriales was positively correlated with moisture and N₂O emission, and denitrification, nitrate reduction to ammonium, nitrification, and nitrogen mineralization/fixation were the dominant processes of the nitrogen cycle in black soil. The hub nodes and connection modes of the microbial nitrogen-cycling network differ under six moisture/gas regimes, and the same species could be active in multiple major nitrogen turnover processes simultaneously. NT and stover mulch may influence the soil’s microbial diversity/richness by regulating soil moisture and/or gas conditions.

In the trial of wheat-I. indigotica rotation, conservation tillage effectively improved the soil enzyme activity and microbial count [71]. The activities of urease, alkaline phosphatase, sucrase, and catalase in the soils of different depths were expressed as NT straw mulch > NT > CT. The average activities of hydrolases and oxidoreductases in the 0–30 cm soil were also NT straw mulch > NT > CT. The rotation of conservation tillage with I. indigotica → wheat improved soil hydrolase and catalase activities. From CT to NT straw mulch, the overall number of microorganisms in each layer of the rotation soil gradually increased. The average number of bacteria, actinomycetes, fungi, and total number of microbes in the 0–30 cm soil were NT straw mulch > NT > CT. The number of soil bacteria and total number of microbes in the wheat field were reduced by about 10 times compared to those in the I. indigotica field. In Argentina, soil enzymatic activities were higher in NT than in CT, and enzymatic profiles responded to the changes much earlier than the overall prokaryotic community structure [134]. Comparable β-diversity was observed in both 27 year old NT soils and new NT soils 30 months after switching from CT to NT. The NT responsive bacteria and archaea OTUs were associated with coarse soil fractions, SOC, and C cycle enzymes, while CT responders were related to fine soil fractions and S cycle enzymes. In Uruguay, only after five years of RT + cover crops has soil health improved under intensive vegetable farming systems [135]. RT + cover crop increased soil aggregation, SOC, nutrient availability, and microbial α-diversity, rendering soils more similar to an adjacent undisturbed site.

In Brazil, enzyme activities can be used to estimate the soil quality of the Cerrado biome [136], where cotton, soybeans, and corn are cultivated. Cover crops such as Brachiaria grass and NT improved the activities of β-glucosidase, acid phosphatase, and arylsulfatase, suggesting the importance of regenerative management practices for the sustainability of agroecosystems in sandy soils. The cover crop treatments increased the relative abundance of plant-beneficial bacteria, which was negatively correlated with the disease index [12]. Wheat was the best at improving the growth of plant-beneficial bacteria, followed by rapeseed. The pea treatments enriched nitrogen-cycling bacteria. On the other hand, cover crops reduced the relative abundance of pathogens and denitrifying bacteria. The count of bacteria genes involved in nutrient cycling, antibiotic synthesis, and biodegradation of toxic compounds was increased by cover crops, especially wheat. Microbial activity, prokaryotic community, and soil health could be regulated by conservation tillage; meanwhile, water and nitrogen management in the soil need to be coordinated to achieve sustainable production [137].

4.5.2. Fungi

In agricultural soil, bacteria, archaea, and fungi do not share a common response to land management change [138]. In Spain, the vineyard tillage inhibited the disease-resistant Trichoderma populations in the soil [139], which makes us wonder whether NT would boost beneficial fungi to attain the disease-suppressive soil [140]. In the south of West Siberia, half of the identified OTUs (operational taxonomic units) were Ascomycota [13], and the phyla Basidiomycota, Zygomycota, Mortierellomycota, Chytridiomycota, and Glomeromycota showed tillage-related differential abundance. Wheat residues could increase the abundance of dominant genera Mortierella, Chaetomium, Clonostachys, Gibberella, Fusarium, and Hypocrea as compared with undisturbed soil. NT shifted the soil mycobiome composition closer to the undisturbed soil.

In winter wheat fields, around one-third of the fungal genera differed greatly between CT and non-inversion RT [141]. The influences of tillage and pre-crop were much greater than those of fertilization; for example, the phytopathogen Fusarium was significantly enriched in the intensively fertilized RT fields with the pre-crop maize, but Phoma showed a significant association with CT and pre-crop rapeseed. In another winter wheat study, the non-inversion RT enhanced the effects of the preceding crop on plant growth and fungal communities on plant roots and in the soil [142]. RT increased the abundance of putative phytopathogens such as Parastagonospora sp. but reduced Fusarium culmorum/graminearum, thereby impacting crop health and yield. Many beneficial arbuscular mycorrhizal fungi (AMF) of Glomeromycota also reacted differentially to farming practices [141].

In a 10 year NT mulching experiment, the frequency of stover mulching, rather than its amount, significantly influenced the soil microorganism and nematode communities [143]. The high mulch frequency treatments promoted the correlation between bacterial PLFAs (phospholipid fatty acids) and bacterivores as well as more carbon flow into the soil micro-food web; low mulch frequency increased the correlation between fungal PLFAs and fungivores, and a relatively stable micro-food web was developed. The structure and activity of decomposition pathways are determined by the bottom-up effects of different stover mulching frequencies.

Soil inversion tillage destroys fungal hyphae and negatively influences AMF production of glomalin protein [97]. Even after three years of the NT system, aggregate and glomalin t values were considerably lower as compared to values from continuous pasture for 15 years. With different cover crops, the diversity of AMF, values of spore density, root colonization, and glomalin content were not significantly different. Applying nitrogen fertilizer did not affect the AMF activity in the investigated cover crops.

4.5.3. Nematode

Many soil nematodes feed on bacteria, fungi, and herbs, all of which are essential parts of the soil micro-food web. Increasing physical disturbance is detrimental to nematode community abundance and diversity, subsequently affecting the stability and sustainability of the edaphic ecosystem. Tillage significantly reduced the overall abundance and richness of nematode communities over time [144]. The abundance and richness of bacterial feeders, predators, and omnivores were reduced. Unlike tillage, minimal disturbance, such as the removal of surface litter, only significantly decreased the abundance of three genera: Acrobeles, Aporcelaimellus, and Boleodorus. Tillage significantly reduced the functional metabolic footprint of nematodes, their metabolic activity, and C inflow and impaired the structures of the soil food web. In 10 European long-term field experiments, tillage, rather than organic matter addition, strongly influenced nematode communities [145]. Compared to CT, RT increased nematode diversity, nematode community stability, structure, fungal decomposition channel, and number of herbivorous nematodes. Total and labile organic carbon, available K, and microbial parameters had a large impact on nematode community structure. Nematode communities are sensitive indicators of soil quality, and molecular profiling helps reveal the effects of conservation tillage on soil quality. In a 12 year NT system, mulching significantly influenced the production and respiration of fungivorous nematodes [146], and the effects of 100% mulching were much stronger than those of no mulching, 33% mulching, and 67% mulching. More basal C flowed into the fungal decomposition pathways, and fungivores contributed more to soil carbon sequestration through the decomposition of recalcitrant nutrients from residue. Residue mulching increased the metabolic footprints of nematode communities and the carbon use efficiency of fungivores and omnivore-predators and enhanced the potential of soil carbon sequestration through the metabolic processes of different nematode trophic groups. The effects of different types of conservation tillage on population densities of various nematode species in monocropping and multicropping systems are being revealed [147], as are the effects of tillage on nematode distribution in the soil profile, as well as the effects of conservation tillage on nematode control. The role of nematology in conservation tillage research should be further highlighted.

4.5.4. Earthworm

CT decreases the abundance and biomass of earthworms and alters their community structure, and less intensive soil cultivation practices increase earthworm populations and their contributions to ecosystem functioning [148]. In spring corn planting areas of Northeast China, NT and full stover mulching (NT100) significantly increased the number and weight of earthworms per unit area of Mollisols [149], as compared to conventional ridge tillage. In an irrigated maize cropping system in Colorado, conservation tillage enhanced macrofauna, especially earthworm abundance and diversity [150]. Strip tillage led to a higher infiltration rate and higher abundance of macrofauna than CT, while MT had greater species richness than CT (4.1 vs. 2.0 taxa/sample). Residue cover was positively correlated with earthworm abundance, which was also positively correlated with aggregated stability and infiltration. Strip tillage and MT increased the net economic return relative to CT. In forty NT sites and six native Atlantic Forest fragments of Southern Brazil, eighteen earthworm species were found [151], belonging to the families Acanthodrilidae, Glossoscolecidae, Megascolecidae, Rhinodrilidae, and Ocnerodrilidae, of which ten were native and eight exotic. NT agroecosystems had larger earthworm populations and higher species richness than native forests [152], mainly due to the colonization of exotic species in the former.

4.5.5. Arthropod

Conservation tillage combined with cover crops or mulching may enhance natural enemy (e.g., arthropod) activity in agroecosystems by reducing soil disturbance and increasing habitat structural complexity [153]. Weed seed predation by arthropods can increase with vegetation cover and RT, as they may improve the quality of habitat for weed seed foraging. In Brazil, arthropod diversity and guild composition were similar between NT and CT [82], but their abundance was higher under NT, where residues from the preceding crop were maintained in the field. Thirty-four arthropod species were recorded, and Hypogastrura springtails, Empoasca kraemeri, Circulifer leafhoppers, and Solenopsis ants were significantly impacted by NT. The infestation levels of major insect pests, especially leafhoppers, were around seven-fold lower under NT + crop residues, whereas the abundance of predatory ants and springtails was much higher under NT than under CT. There was a significant trophic interaction among crop residues, detritivores, predators, and herbivores, which was associated with higher bean yield under NT. NT and crop residue retention can reduce infestation by foliar insect pests and increase the abundance of predators and detritivores, which are conducive to insect pest management.

The pollinator Peponapis pruinosa (squash bee) preferred excavating nests in the most disturbed soil zones, i.e., strip-tilled rows and CT edges [154]. In the RT system, the strip-tilled row had significantly more nests than the NT edge. These suggest that soil tillage practices influence P. pruinosa nesting choice, and production practices should be considered to protect pollinators. In a 17 year fertilization experiment, biennial organic amendments were insufficient for promoting soil organisms in the long run [155] and should be combined with NT or RT to attain a beneficial effect on soil quality.

4.6. Effects on Weeds

Effective control of weeds on farmland is one of the chief indicators for the successful implementation of NT technology. NT/RT and stover mulching/crop residue coverage help inhibit the growth of weeds and reduce the use of chemical herbicides [156,157]. NT causes less soil damage, and weed seeds are rarely exposed and difficult to germinate. The weed control through straw mulch benefits from various physical and chemical factors. Physical factors include shading and lower soil temperature [112], while chemical factors include microbial metabolites, pH changes, and plant allelopathy [22]. Plants produce allelochemicals, which are released into the soil after the decay and leaching of straw and inhibit the germination of weed seeds and the growth of seedlings. These allelochemicals include specialized metabolites such as glucosides, phenols, terpenoids, alkaloids, and hydroxamic acids, which are widely distributed in crop residues/cover plants. For example, benzoxazinone- β- D-glucoside is a typical allelochemical of Poaceae [158].

Ryegrass only releases a small amount of phytotoxic benzoxazinone compounds during its lifespan, but its straw mulch can release up to 1.2–2.0 g/m² [158], so it can be used to inhibit weeds before the next crop of corn. Under controlled conditions, rye straw mulch significantly inhibited the broad-leaved weeds; it inhibited the germination and seedling growth of Amaranthus retroflexus and Portulaca oleracea and strongly inhibited the growth of Chenopodium album. Retaining high residues of rye effectively inhibited weeds in NT soybean production [159]. Retaining high stubble significantly increased soybean yield when weed biomass was high [160]. In the Mid-Atlantic United States, cover crop-based, organic rotational NT production systems utilize cover crop surface mulch as the primary within-season weed control tactic [74]. High-residue cultivation reduced total weed biomass across locations; total weed biomass was negatively correlated to soybean yields.

In the soybean field of Iran, the tillage system and weed management regime significantly influenced the seed yield, pod number per plant, seed number per pod, weed density and biomass [4], while their interaction significantly influenced the weed density, weed biomass, and seed number per pod. In NT row crop seeding and NT seed drilling, non-weeding led to the highest weed density, while herbicide + hand weeding led to the lowest one. Thus, moderate weeding may be carried out according to the production purpose. In India, the positive returns from understory MT are attributed to low weed growth and less disturbance to the soil [9].

On the other hand, case studies in the United States suggest that weeds growing in fall-planted cover crops can provide ground cover [161], decrease potential soil losses, and effectively retain nitrogen. In certain circumstances, weeds in cover crops enhance ecosystem service provisioning. If weeds are herbicide-resistant, cover crops should be managed to limit weed biomass and prolificacy. Therefore, the extent to which weeds should be allowed to grow in a cover crop is largely context-dependent. In the northern grain region of Australia, the major weeds before tillage trials were Polygonum aviculare, Sonchus oleraceus, and Avena fatua [162]. Tillage promoted the germination of other weeds, such as Hibiscus trionum, Medicago sativa, Vicia sp., Phalaris paradoxa, and Convolvulus erubescens. As compared with CT/RT, SB (straw burning) + NT, and NT provided ideal media for weeds to germinate and resulted in heavy infestations of weeds, which might not be good in grain production but might be innocuous in medicinal plant production. Competition between weeds and medicinal plants can produce certain stress effects [163], which may elicit the biosynthesis of specialized metabolites [21,22], which is beneficial for improving the quality of herbal medicine products. The moderate retention of weeds could help maintain high biodiversity in the agroecosystem [164], which is advantageous for pathogen and pest control. A trade-off between weed suppression and the selection of more competitive weed communities by introducing agroecological service crops should be evaluated in the long run. In general, conservation tillage positively impacts crop productivity under adverse climate conditions and in various agroecological conditions [165], despite the increase in weeds.

4.7. Effects on Pests/Natural Enemies

NT and cover crops help protect annual crops from insect pests by supporting populations of resident arthropod predators [166]. The best pest management consequences may occur when biocontrol is boosted by planting cover crops and broad-spectrum insecticides are disused. Cover crops can promote natural-enemy populations against insect pests. The inclusion of winter and interseeded cover crops in organic agronomic crop rotations is recommended to gain environmental benefits without increasing the risks of damage by insect pests [167]. In reducing the intensity and frequency of tillage in an organic farming system, utilizing ecological processes to manage pests and fertility should be emphasized. At the Pennsylvania NT location, delaying corn planting dates increased the activity/density of predatory arthropods [74], which was conducive to increasing corn yields. The effects of NT + cover crop practices on entomopathogenic fungi, a short-term soil health indicator, and varied and adaptive pest management strategies should be used in NT systems.

In paddy fields in Assam, India, the Oribatida populations of mites were significantly different between the NT and CT systems [168]. Aphididae and Formicidae pests were observed more on foliage, flowers, and pitfall traps of organic Cucurbita in full tillage, while RT methods, such as strip tillage, increased the abundance of natural enemies (e.g., Parasitica) and possibly pollinators [169], which may lead to enhanced biocontrol and pollination, but impacts may vary for different arthropod species and crops.

4.8. Effects on Yield and Quality of Plants

4.8.1. Yield and Output

A global meta-analysis showed that CT significantly decreased maize yield by 5% and nitrogen use efficiency by 15% [170], which could be alleviated under good hydrothermal conditions and straw mulching. When compared with traditional ridge cropping, long-term NT with stover mulching could increase maize yield [171]. NT100 had the highest increase rate of 11.4%, followed by NT67 and NT0. NT67 treatment significantly reduced the interannual fluctuation of maize yield and led to better sustainability of yield. NT stover mulching significantly increased soil total carbon and TN contents, which were significantly positively correlated with maize yield. The application of such low-carbon technologies significantly improved the income level of large-scale farmers [172].

In the chickpea fields of India, the maximal grain (2380 kg/ha) and biological output (5762 kg/ha) were attained under RT60 [76]. The net monetary benefit of conservation tillage was 24.3% to 37.7% higher than that of CT. The total N uptake was maximal under RT60, while the total P and K uptake was higher in NT30. In long-term maize production, the NT ↔ ST treatment effectively increased the plant height and dry matter accumulation of spring maize [96]. Compared with CT, NT ↔ ST significantly increased crop yield and WUE in continuous cropping fields of corn. In the common bean fields of Uganda, NT and stubble mulching improved WUE and grain yield [123].

In the trial of wheat-I. indigotica rotation, conservation tillage was beneficial for chlorophyll synthesis and delayed chlorophyll a degradation [71]. NT mulching significantly increased the activity of protective enzymes and proline content in the crops, reduced malondialdehyde damage and cell membrane permeability, and protected cell structure integrity, thus promoting crop growth and development. Conservation tillage significantly improved the crop yield as well as the WUE at the leaf level and yield level [71]. In Iowa, USA, the catnip (Nepeta cataria) plant height was significantly greater under the oat straw mulch than under other treatments at 4–6 weeks [173]; at 4 to 8 weeks of the second year, catnip plant height and width were significantly lower in the negative control compared with mulch treatments. Catnip yield was significantly higher in the flax straw mat than in other treatments. All organic mulch treatments significantly reduced weeds, with flax straw and wool mat having the best weed control. Meanwhile, the concentrations of medicinal compounds, nepetalactone in catnip and pseudohypericin-hypericin in St. John’s wort (Hypericum perforatum), were not affected, and flax straw mulch slightly increased their concentrations. However, there are few reports about the impact of NT/RT and organic mulch on other medicinal crops.

In winter wheat monoculture on dryland, NT performed better on soil water conservation during the fallow period but had a similar effect on wheat yield and WUE as compared to RT and ST [119]. Wheat WUE was improved by straw mulching but not affected by stubble mulching. In the spring wheat field of the Loess Plateau, NT straw mulch led to the highest dry matter accumulation throughout the whole growth stage of spring wheat as compared with CT, NT alone, and CT straw mulch [122]. The average grain yields and WUE of NT straw mulch across three years were 6.0–30.7% and 6.7–40.5% higher than other tillage practices, respectively. These advantages were achieved by improving the edaphic properties and increasing stress-related substances in wheat, e.g., catalase, peroxidase, and soluble protein. In the potato field of Ningxia, higher WUE was obtained in ST 30–40 cm with straw mulch [112], whereas the accumulated temperature use efficiency was increased significantly under different tillage depths with straw mulching. The impact of soil water on the total yield of potatoes was stronger than that of effective accumulated temperature during tuber formation. ST 30–40 cm with straw mulch improved the soil moisture and heat conditions and increased the potato yield and income by more efficiently using water and heat resources, which has application and popularization value in dryland cultivation. In the maize fields of Iraq, the wheat straw mulch increased the plant height, yield components, grain and biomass yield, as well as soil water content [110], and mulching of 8000 kg/ha had the largest positive effects on maize yield.

So far, limited emphasis has been placed on the production of medicinal plants for sustainable harvest and conservation in the understory of degraded forests [9]. In India, Chir pine forests are usually not managed to grow any medicinal plants, leaving a vast space to produce and conserve medicinal plants, which involve sustainable management techniques like MT under the canopy of trees. The medicinal plants were grown with three tillage depths, viz., minimum (0 cm), medium (up to 10 cm), and deep tillage (up to 15 cm), in open and below tree canopy conditions. The good yields of Andrographis paniculata [174], Mucuna pruriens, Solanum khasianum, and Spilanthes acmella were attained via MT. The positive average annual returns were achieved in understory conditions, and the system is conducive to utilizing vacant lands and increasing total productivity from them, as well as conserving medicinal varieties in situ. Conservation tillage can also be applied to other medicinal species in blank patches or the understory of degraded forests.

4.8.2. Medicinal Quality and Nutritional Value

The specialized metabolites are usually defensive weapons of plants (Figure 3), which play essential roles in host defense against various biotic and abiotic stresses. Many specialized metabolites display bioactivities and clinical efficacy, and NT-based ecological planting could increase the contents of specialized metabolites in plants [19,80]. The induction and utilization of specialized metabolism in ecological cultivation of medicinal plants could be attained via carrying out biocontrol of pathogens, pests, and weeds, promoting beneficial microbes in soil and plants, optimizing mixed planting, NT/RT, and organic mulch [22]. The biggest difference between the production of medicinal plants and that of food crops is that the specialized metabolism and contents of medicinal compounds must be highlighted in the former; it is also necessary to consider yield and sometimes nutritional indicators, usually involved in primary metabolism, in NT-related ecological agriculture. Compared with CT, the NT system might permit more biotic stresses, such as pathogens, pests, and weeds [21,175], as well as more abiotic stresses, e.g., drought and salinity [20], which could promote the accumulation of specialized metabolites of medicinal plants and improve the quality of medicinal crops [89] (Figure 1 and Figure 3), which is conducive to the prevention and control of diseases, pests, and weeds. However, systematic research on how conservation tillage improves the content of specialized metabolites is still very limited, and the long-term impacts and mechanisms of NT, RT, and organic mulch need to be widely verified and systematically studied in the ecological planting of medicinal plants.

Sustainable management with NT, fertigation, and internal C-inputs via spontaneous weeds and pruning residues was implemented in the olive plantation [80]. NT increased the concentrations of most phytometabolites in the xylem sap (XS) as compared to CT, most of which were involved in plant specialized metabolism, chemical defense, signal transduction, and growth regulation, including alkaloids, terpenoids, phytohormones, steroids, carotenoids, retinoids, and tocopherols, etc. The XS of the tree crop significantly responds to a shift in soil management, and the NT plants showed an upregulated, specialized metabolism. NT could enhance plant physiological status, increase yield, improve quality, safeguard the environment, and ultimately benefit human health. In Poland, NT did not decrease the levels of bioactive phytosterols, tocopherols, and squalene in four varieties of common buckwheat [176]. Functions associated with stress tolerance, including signal transduction and biosynthesis of some secondary metabolites, were significantly increased in Solanaceae plants under intermittent DT treatments (30, 40, and 50 cm) [99].

In Lucknow, India, the application of paddy straw mulch increased the herb and essential oil yields in geranium by 23% and 27%, respectively, as compared with the unmulched control [83]. The straw mulch significantly enhanced the plant response toward 160 kg N/ha and increased nitrogen uptake and apparent N recoveries by geranium crops. Citronellol and geraniol, the quality markers of essential oils, were not affected by organic mulch or nitrogen fertilization, and these components met standards for international trade. In Southeast Spain, NT and cover of Vicia faba, Vicia sativa, and Vicia ervilia improved soil properties [84], e.g., SBD, available water content, aggregate stability, electrical conductivity, SOM, N, P, K, micronutrients, and microbial properties, which increased the antioxidant activity and total polyphenol content of almonds, thus improving their nutritional/medicinal value. In vineyards in Southern Italy, cover crop treatments increased concentrations of ethyl esters, volatile fatty acids, and free terpenes in wines made from Vitis vinifera in the humid climate [177]. The leguminous cover crop and its combination with natural zeolites could be promising practices to guarantee yield and quality in olive orchards under rainfed conditions [85].

4.8.3. Hazardous Substances

To reduce occupying farmland, understory Panax notoginseng (UPn) was developed as an ecological planting model with no chemical input [178]. Root and rhizome are more prone to excessive heavy metal levels than other medicinal parts [179]. In Lancang County, Yunnan, the hazard index and target hazard quotient of eight heavy metals in the roots and leaves of UPn were less than one, and Upn showed no human health risk, suggesting that understory cultivation with MT creates a safe and healthy growth environment for medicinal plants, which is worth promoting. More research on how NT/RT affect the content of organic pollutants and heavy metals in medicinal plant products is warranted.

4.9. Long-Term Ecological Benefits

Besides the abovementioned advantages, NT/RT and organic mulch generally mitigate the spillover effects of pollutants and pesticides as well as the emission of GHG, thereby showing long-term ecological benefits in reducing air pollution, water pollution, and soil pollution, which ultimately benefit the planet and humanity.

4.9.1. Pollutants and Pesticides

In Mollisol of Northeast China, the mineral nitrogen declined with depth to 60 cm and then increased to its maximum at 250–300 cm under CT and at 120–150 cm under NT0 and NT straw mulch [10]. More mineral nitrogen at 0–150 cm under low-disturbance practices would provide more available nitrogen for crops in the coming growing season, while the accumulated nitrogen at 150–300 cm under CT may leach into the groundwater, which may aggravate the nitrogen contamination in deep groundwater, ultimately threatening the agricultural sustainability in Mollisol regions.

Under natural rainfall conditions, the combination of NT and grass hedgerow measures effectively reduced 69% of runoff loss [180], 62% of nitrogen loss, 77% of phosphorus loss, and 88% of atrazine loss, which were supported by simulation results. On runoff plots with slopes of 5%, 10%, 15%, and 20%, the reduction of agricultural non-point source pollution by combining NT and hedgerow was negatively correlated with the slope. In a meta-analysis of pesticide loss in runoff, the concentrations of atrazine, cyanazine, dicamba, metribuzin, and simazine, instead of alachlor, in runoff were higher under NT than under CT [181]. NT regulates soil properties that control pesticide retention and interactions with soils, and eventually their mobility in the environment. More understandings of pesticide-soil interactions in NT systems should be gained to inform the selection of pesticides by farmers and improve the predictive power of pesticide transport models.

In a cotton-tomato rotation, conservation tillage noticeably decreased dust emissions due to fewer field operations [182], and long-term sampling is necessary to determine the effects of cover crops on dust production.

4.9.2. GHG Emission

In the North China Plain, when compared to CT, NT significantly reduced the net CO₂ cumulative emissions and water consumption [183,184] but reduced the grain yield. NT decreased the N₂O emission by 22.6% in winter wheat seasons [185] but did not alter it in summer maize seasons. Crop residue retention increased N₂O emissions by 28.1% and 26.7%, respectively, as compared with residue removal in the winter wheat and summer maize seasons. The NT soils took up more CH₄ in the summer maize seasons, and total non-CO₂ GHG emissions at the area scale showed trends similar to those of N₂O emissions. In an organic clover-winter wheat cropping sequence in Switzerland, RT and manure compost application could mitigate GHG as long as SOC is sequestered [14].

In the indoor incubation of black soil, the N₂O/CO₂ emission under six moisture/gas regimes significantly varied [133], the processing time also theoretically influenced the GHG emission, and there were sizable interactions between moisture/gas regime and processing time. The impact of moisture/gas regimes, processing time, and interaction items on ammonia nitrogen and nitrate nitrogen was also conspicuous. NT and stover mulch may influence soil GHG emissions by regulating soil moisture and/or gas conditions. In pot experiments, the N₂O production and nitrifying-denitrifying microbial communities were influenced by the antecedent soil moisture and pattern of the dry-wet cycle [186]. The nitrifying-denitrifying microbial communities, especially members of β-/γ-Proteobacteria, Bacteroidetes, and Gemmatimonadetes, in black soil were important in explaining the variation of N₂O production. Acidobacteria, Sphingobacteriia, δ-Proteobacteria, Methylobacterium, Gemmatimonas, and Pseudarthrobacter, etc., were the key taxonomic groups in response to the moisture alteration. The nitrite/nitrate reduction to ammonium could be boosted by high moisture. Both nitrifier denitrification and heterotrophic denitrification could be substantially enhanced when the black soil moisture was increased to above 60% water holding capacity. NT and stover mulching may influence soil GHG emissions and relevant microbial communities via the combined effects of early and immediate moisture. Regional assessments of SOC trends and the carbon sequestration potential of NT and organic mulch are crucial in developing climate change mitigation strategies [187]. Developing simplified, scale-adapted assessments is necessary for cross-regional comparisons of conservation tillage and for communication with stakeholders and policymakers.

5. Disadvantages, Limitations, and Adverse Effects of Conservation Tillage

Due to the complex and varied application scenarios and diverse implementation methods of conservation tillage, despite decades of research, the application patterns of NT/RT and organic mulch have not been thoroughly understood. In addition, the current global climate change has brought more challenges to the implementation of conservation tillage. Although there are relatively few reports on the negative effects of NT/RT and organic mulch, recognizing the drawbacks, limitations, and chain reactions of conservation tillage can help maximize its benefits by leveraging its strengths and avoiding its weaknesses.

5.1. Medicinal Quality and Nutritional Value

In Pengzhou, Sichuan, China, the content of Cd and Pb in the Ligusticum chuanxiong soil was significantly higher under the vegetable/herb upland CT than under the rice/herb NT [72]. However, the Cd content was higher in the soil undergoing rice/herb NT than in that undergoing rice/herb plowing, as was the Pb content. The Cd content in the roots, stems, and leaves of L. chuanxiong basically met the regulatory standard, except in the vegetable/herb rotation, and NT was associated with a higher Cd content of L. chuanxiong as compared with the plowing method. Chuanxiong is not suitable for rotation with leafy vegetables. When rotating with rice, it should be plowed as much as possible, and the straw should be removed. In Greece, the crude protein of cold-pressed cake from Camelina sativa seed cultivated under RT was lower than that under CT [188], although RT did not reduce polyunsaturated fatty acids such as linoleic acid and α-linolenic acid.

5.2. Physical Properties of Soil

In Uganda, although NT and stubble mulch improved soil water storage and the yield of common beans, seasonal precipitation distribution had a greater influence on the final grain yield, soil water storage, and WUE [123]. In Iraq, as compared with RT, DT and CT could break the compacted zone in the soil, leading to a better soil environment and crop yield [110]. When compared with RT, wheat straw mulch could be a more efficient soil management practice for corn production in an arid subtropical climate region. At three sites in Northeast Germany with gentle, moderate, and steep slopes, non-inversion RT led to much more soil movement compared to inversion CT [189], with the soil translocation distance being increased by 1.3–2.1 times, which contradicts the general assumption of reduced erosion in RT. In tillage erosion-dominated areas, RT with high tillage speed and depth cannot fulfill the requirements of soil conservation.

On the silty clay loam of Eastern Nebraska, the water infiltration of corn fields under NT was not as good as that with chisel plow (MT/RT), disk, and moldboard plow CT systems [190]. In Italy, the new rotary ripper increased the porosity of the soil in the deepest layer and increased cost-effectiveness as compared with CT equipment [191]. However, because of the low soil segregation level with the new tool, additional tillage activities are required before planting.

5.3. Pests

Decreased pest pressure is sometimes associated with more diverse agroecosystems, including the addition of rye cover crops [192]. However, rye is the host of polyphagous pests, e.g., true armyworm (Mythimna unipuncta), black cutworm (Agrotis ipsilon), and common stalk borer (Papaipema nebris), which could cause injury to crops when rye is planted as the cover crop. Although NT generally improves soil properties and helps to control arthropod pests, it may intensify crop infestation by the common vole [193]. NT farming exacerbated the common vole infestation of winter rape fields [194], particularly in spring, which was related to damage to adjacent crops. NT creates favorable refuge habitats for fossorial rodents [195], benefiting from reduced soil disturbance between crop rotations and thus increasing burrow persistence. Tillage practices have more influence on common vole occurrence (NT > RT and CT) than other aspects such as crop type.

5.4. Others

In the sweet corn field, the cover crops, including crimson clover, forage radish, and rye, suppressed weeds in all three studied years [196]. Crop development and yield were similar among CT, NT + cover crop residue, living mulch + cover crop residue, and living mulch + winter-killed residue in year 2. However, reduced yields were encountered in all cover crop treatments during year 3. In a two year experiment, the presence of earthworms increased the combined cumulative emissions of CO₂ and N₂O from a simulated NT system to the same level as a simulated CT system [197]. The soil C storage was not increased in the presence of earthworms. Because NT agriculture stimulates earthworms presence, the potential of NT soils for GHG mitigation might be limited in some cases.

6. Discussion and Prospects: Countermeasures and Suggestions

6.1. Popularization of NT and Perception Change

Environmentally friendly medicinal crops are the basis of both human health and environmental soundness. However, to meet the increasing demand for phytomedicine products, it is necessary to adjust the parameters of the technological process by creating ecological/environmental agriculture. The NT/RT-related ecological cultivation of medicinal crops enables minimizing the pesticide/herbicide load on plants and soil [181], as well as the use of modern technical means and new agricultural techniques, which would increase the productivity of medicinal crops, preserve soil fertility, and increase the level of safety of phytomedicine products [17,86,178,198].

From a practical perspective, it is necessary to consider the production of medicinal crops in two parallel systems: organic farming, which excludes the use of mineral chemical fertilizers and protective equipment [199], and an environmentalized farming system, which is conducive to the reduction of chemical loads on plants and soil. NT and related methods fall within the framework of both systems, where a series of agricultural techniques are optionally used, achieving resource-saving multi-depth loosening of the soil without turnover of the soil layer. The stover mulch is used in the post-harvest period, the ecological and biological crop rotations are developed, the sowing schemes are unified, the working solutions of fertilizers and plant protection products are localized [200], and the weeds are destroyed by mechanical means in the pre-sowing and pre-planting periods [164,175].

For production conditions in the system of ecological farming, rebuilding the entire technological process as in organic farming is not necessary. Within the framework of this system, permitted fertilizers, biological methods against diseases and pests, and resource-saving, gentle methods of soil tillage can be applied. As a result, the pesticide/herbicide load is reduced, and high-quality, environmentally friendly medicinal products are assured. Eco-friendly medicinal plants are the basis of our health, for which it is compulsory to fine-tune the parameters of technological processes by creating environmental agriculture, e.g., the ecologized cultivation of medicinal crops [89,103,201], which enables maximizing the yields of medicinal compounds [108], reducing the pesticide/fertilizer load on plants and soil, and utilizing innovative techniques and new agricultural machinery. The misconceptions about NT/RT should be eliminated by extensively informing the public about their advantages and short-term and long-term ecological/economic outcomes, so as to truly understand and actively view conservation tillage. All stakeholders work together to promote NT/RT technology and systematically collect knowledge/experiences from all aspects. It is advised to strengthen technical training for CMM planting personnel, farmers, technical promotion personnel, researchers, etc., in order to overcome cognitive biases.

6.2. Research Reinforcement and Scientific Allocation of Resources

Here are just a few examples. NT/RT is thought to be beneficial to soil health (Figure 2 and Figure 3). However, it is still difficult to quantitatively assess soil health and link those assessments to outcomes [106]. Therefore, the quantitative link between tillage mode and soil health is still elusive. The soil health indicators suitable for medicinal plants might not be identical to those for food crops, and a quantitative approach should be developed to select soil health indicators that help connect management-induced changes in soil health to specific outcomes, e.g., yield, medicinal quality [17,198] or environmental quality. The randomized controlled studies of NT/RT application in medicinal plants are lacking, and only qualitative research and experience summaries are far from enough for gaining deeper insights into conservation tillage of medicinal crops. The nonlinear relationship between the amount of corn stover mulch and the mid-infrared spectral characteristics of the soil should be further investigated based on the microbial control over soil carbon/nitrogen cycling under different amounts of stover mulch [128]. The structural equation model can be constructed to hypothesize causal relationships between soil chemistry and microbial communities under different conservation tillage modes [125].

Nitrogen fertilizer is the most frequently applied chemical fertilizer in the cultivation of medicinal plants and has a prominent contribution to the yield of medicinal plants. Conservation tillage does not exclude the use of nitrogen fertilizer. The demand for nitrogen of different CMM varieties in different places of origin ranged between 0 and 1035.5 kg/hm² [202], which was related to both the varieties and soil fertility of different places of origin. NT and organic mulch improve soil fertility slowly, and soil fertility can be quickly improved through fertilization when necessary. However, biased or excessive nitrogen application must be avoided in the cultivation of CMMs, as only moderate nitrogen application could increase the yield, contents of nutrients, antioxidant enzymes, and resistance-related protein activities, as well as the level of nitrogen-containing specialized metabolites in CMMs [203]. The nutrient content, WUE, photosynthetic capacity, etc., of medicinal plants are profoundly affected by nitrogen application, which subsequently regulates the quality of CMM. The impact of nitrogen application on different specialized metabolites is mainly related to their biosynthesis pathways and plant nutritional status. It is necessary to plan nitrogen application concentration and formula fertilization in the context of NT, RT, and organic mulch.

6.3. Adapting to Local Conditions and Precise Management

The CMM division in China draws on the experience and achievements of the national agricultural conditions professional division, agricultural production department division, comprehensive natural division, and comprehensive agricultural division, which are of great significance for guiding the production of CMM. There are eight terrestrial CMM regions in China: Northeast China, North China, East China, Southwest China, South China, Inner Mongolia, Northwest China, and Qinghai Tibet. Northeast China is mainly composed of mountains, hills, and plains, with a large area of forest and grassland, providing advantages for ginseng understory planting [103,201] (Figure 3) and imitation wild planting such as Paeonia lactiflora and Saposhnikovia divaricata [73]. North China is mainly composed of plains, plateaus, and hills, among which the Huanghuaihai Plain is the largest plain in China and an important grain production base. Given the diverse natural environment and economic development of the region, it is possible to carry out ecological planting of intercropping/rotation of Radix astragali/Bupleurum chinense and crops [204], intercropping (under tree sowing) of Scutellaria baicalensis and fruit trees, under corn sowing of B. chinense, understory planting of Anemarrhena asphodeloides, and pseudo-wild planting of Forsythia suspensa [73]. East China is mainly composed of hills, basins, and plains; in the long-term production practice, representative ecological planting models have been formed in the region, mainly including intercropping/rotation of medicinal plants (e.g., P. lactiflora, Alisma orientalis, Atractylodes lancea, Salvia miltiorrhiza, and Fritillaria thunbergii) and grain [205], compound planting of Trichosanthes kirilowii, soybean, and wheat, bionic-facility cultivation and original ecological cultivation of Dendrobium officinale [206], and terraced embankment planting of honeysuckle. Southwest China is mainly composed of basins and plateaus, where representative ecological planting models include intercropping/rotation of crops and herbs such as L. chuanxiong and Aconitum carmichaeli [72,73], intercropping/undercrop sowing of Phellodendron chinense and P. lactiflora, intercropping/undercrop sowing of Ophiopogon japonicus and corn [207], underforest planting of P. notoginseng [178], Polygonatum kingianum, and Rhizoma paridis, simulated wild planting of rhubarb, and circular planting of Gastrodia elata and Phallus dongsun. In addition, representative ecological planting models in Central China include wild imitation planting of Polygonatum sibiricum under forest [208], intercropping of Pinellia ternata and corn, and rotation of Rehmannia glutinosa and grains (wheat, corn, millet, and sweet potato). There is increasing emphasis on the use of conservation tillage techniques in the above-mentioned planting patterns.

Southern China is dominated by mountains and hills with a tropical and subtropical monsoon climate, where representative ecological planting models include understory planting of Amomum villosum [75], intercropping of Ilex asprella and Lysimachia christinae, and interplanting of Siraitia grosvenorii and sugar orange. Northwest China is mainly composed of plateaus, basins, and mountains, most of which have a temperate monsoon climate and are relatively arid. The region mainly adopts simulated wild and wild tending planting modes, such as wild licorice tending planting [20], intercropping/under crop sowing of B. chinense and corn, and intercropping of Gentiana macrophylla and forest. Inner Mongolia is rich in Platycodon grandiflorum, Herba ephedrae, Radix glycyrrhizae, and Astragalus membranaceus, which are suitable for NT/RT cultivation. The Qinghai Tibet region is mainly composed of plateaus, and there is very little arable land. The Tibetan medicine produced in this area has unique therapeutic effects [209]. Currently, the relatively mature ecological planting models include the wild imitation planting of Rheum tanguticum and the wild imitation planting of Nardostachys jatamansi [87]. NT, RT, and organic mulch can be applied flexibly in all eight terrestrial CMM regions in China to promote the ecological agriculture of CMM. Timely summarizing and sorting out the commonly used methods of conservation tillage in various CMM regions can provide references for the application of NT, RT, and organic mulch in the cultivation of more CMMs.

In conclusion, conservation tillage represented by NT, RT, and organic mulch has shown versatile advantages in the agricultural practice of crop plants, economic plants, and medicinal plants. NT/RT coupled with organic mulch showed positive effects on soil structure and physical properties, mitigated soil erosion, and improved soil moisture and fertility. NT, RT, and organic mulch usually have positive effects on biodiversity and soil biota, such as bacteria, archaea, fungi, nematodes, earthworms, arthropods, etc. Under certain circumstances, NT, RT, and organic mulch help control weeds and regulate the balance between pests and natural enemies. Optimistically, NT, RT, and organic mulch, when applied properly, did increase the yield and output of medicinal plants without impairing their medicinal quality or nutritional value. The long-term ecological benefits of NT, RT, and organic mulch could be authentic, such as reducing organic pollutants and GHG emissions. Yet, the potential disadvantages, limitations, and cryptic effects of conservation tillage cannot be overlooked under any circumstances. In the ecological agriculture of medicinal plants, it is obligatory to comprehensively consider factors such as climate, terroir, medicinal plant types, soil conditions, and planting systems in different regions, and develop unique NT management measures according to local conditions. The harsh reality is that there is severe soil and water loss, a continuous decline in soil fertility, and a deterioration of the ecological environment in arable lands in China and other developing countries. On the other hand, in the 21st century, humanity is facing an increasing number of major infectious disease threats, and the demand for green and natural plant medicine products is skyrocketing [17,198], which requires a sustainable supply of quality drugs from land. Therefore, an innovative land use strategy must be adopted for the sustainably developed ecological agriculture of medicinal plants in the post-pandemic era, which involves planting under forests and in grasslands without competing for land with food crops [105,201], fully utilizing mountainous wasteland for wild nurturing or biomimetic cultivation, and developing reasonably according to local conditions and Chinese medical regionalization [210,211]. Only by doing so can people’s growing demand for herbal medicine consumption be met. NT, RT, and organic mulch should be promoted and utilized as a core strategy in the production practice of CMM ecological agriculture.