Conservation Agriculture as a System to Enhance Ecosystem Services

: Conservation agriculture (CA) is considered a sustainable practice with the potential to maintain or increase crop productivity and improve environmental quality and ecosystem services. It typically improves soil quality and water conservation; however, its effect on crop productivity is highly variable and dependent on local conditions/management. Crop residue retention plays a crucial role in CA and can help to improve overall soil health and ultimately crop productivity and sustainability. However, weed control, herbicide resistance, and weed shift under residue retained ﬁelds is a major challenge. Moreover, CA can increase water inﬁltration and reduce soil loss and runoff. This reduces the surface transport of nitrate and phosphorus from agricultural ﬁelds and the eutrophication of water bodies, although leaching of nitrate to groundwater can potentially increase. In addition, CA has been proposed as one of the components in climate-smart agriculture, owing to its reduced period to seed/plant next crop, reduced soil disturbance and low consumption of fossil fuels. Therefore, compared to the conventional intensive tillage, CA has a greater potential for soil C sequestration, favors higher soil biodiversity, lowers greenhouse gas emission, and can assist in mitigating climate change. However, not all experiments report a positive impact. The understanding and decoding the site-speciﬁc complexities of CA system is important and requires a multidisciplinary approach.


Introduction
Globally, conservation agriculture (CA)/no-till (NT) farming has been widely adopted and practiced (about 180 M ha of cropland,~12.5% of total global cropland area in 2015/16 and an increase of 69% globally since 2008/09) [1] as it provides various benefits to agricultural production driven by soil and water conservation and improvement in soil health [1,2]. CA is often advocated as a sustainable farming practice that can not only maintain or increase crop productivity, but also improve carbon storage, environmental quality, and ecosystem services (ES) [2][3][4][5][6]. However, despite the proven benefits of CA, its adoption has been mainly limited to developed countries [1,7,8]. With the exception of South America, uptake in developing countries is often very low due to various socio-economic and logistical barriers to its implementation (e.g., insufficient access to finance and appropriate machinery, poor extension services, and poor crop yield due to problems with weed/residue/soil fertility management) [9,10]. Other issues such as weed shift, herbicide resistance, nutrient stratification [11], residue borne pest and diseases also hinder the adoption of CA in both developed and developing regions. However, in regions where CA Moderation of extreme events (droughts/floods) Supporting Services Soil Biodiversity ↑ Indicates higher; ↓ indicates lower; ↔ No effect Arrows on an angle indicate more gradual change over time (Source: [10,22,23]).

Water Storage
One of the most well-established benefits of CA systems is their ability to improve soil water storage. Reduced soil disturbance coupled with increased residue retention typically leads to increases in SOC at the soil surface in CA systems [24]. This increases aggregate stability, helps preserve macropores capable of rapidly transmitting water into  components (NT alone). However, CA implementation in dry, rainfed areas lead to a 7.3% increase in crop yield, likely due to improvements in soil water storage. These results highlight the importance of implementing all components of the CA system to achieve maximum yield benefits and the need to implement it in a location-specific manner and appropriately adapt management practices to avoid yield loss. It may also take a number of years for yield improvements to be realized as until the soil quality is improved through these CA components, the yield gain under CA can be minimal [10].

Water Storage
One of the most well-established benefits of CA systems is their ability to improve soil water storage. Reduced soil disturbance coupled with increased residue retention typically leads to increases in SOC at the soil surface in CA systems [24]. This increases aggregate stability, helps preserve macropores capable of rapidly transmitting water into the soil profile, and can improve rates of water infiltration and thus the capture of rainfall for crop use [25][26][27][28]. In addition, the retention of crop residues on the soil surface decreases rates of soil water evaporation [29], also contributing to increases in soil water storage. In drier rainfed regions, where water availability is one of the main factors limiting plant production, this increase in water storage can have a major positive impact on crop productivity and potentially help agricultural systems adapt to the increasing incidence of drought under climate change. In irrigated regions, it can reduce the amount of water required for crop production and help conserve water resources. However, in cold regions or where soils are prone to waterlogging, these improvements can lead to no, or reduced, yield benefit, as discussed below.

Yield and Productivity
Although CA has been delivering positive results on improving soil water conservation, the effects of CA on crop productivity are less clear cut [9,30] (Table 1). CA systems have been observed to increase [31][32][33], decrease [34] and lead to no change in yield [35,36]. The increase or decrease in crop yield following the adoption of CA largely depends on whether CA has been partially or fully implemented, regional climatic conditions, and the type of cropping systems and management practices followed [7,34,37,38] (Figure 3). For example:

•
Climatic conditions: In cooler regions, crop residue retention can lower soil temperatures, delay plant maturity and negatively affect yield [20,35,39]. Similarly, in higher rainfall regions with poorly drained soils, the increased infiltration, and lower evaporation in CA systems can lead to waterlogging and yield loss [35,[39][40][41][42], although in suitably drained soils, CA can also bring yield advantages in wet climates [43,44]. In contrast, when CA is implemented in warmer and drier regions, higher yield is often observed due to a lowering of soil temperatures and increases in soil water storage [22,45,46]. • Management practices: In conventional systems, cultivation is used to control many weeds, pests and diseases. If CA is implemented without suitable modifications to tillage/weed/pest/disease management systems, it can lead to increases in infestations and losses in yield [47][48][49]. Similarly, the high carbon (C):nitrogen (N) ratio of crop residue retained in CA systems can immobilize N and lead to N deficiency. If fertilizers or crop rotations incorporating legumes are not used to maintain available N, yield decreases can also occur [22]. However, when weeds/pests/diseases and nutrient availability are successfully managed, the improvements in soil physicochemical properties often observed with CA (e.g., improved soil aggregation, soil structural stability, SOC, water storage, and nutrient supply) can lead to yield increases, particularly in CA systems that have been operating for a number of years and the physico-chemical benefits have increased over time [20,30]. • Full versus partial implementation. Where the components of CA are only partially implemented, yield increases may be reduced. For example, no-till with crop residue retention can generate a higher yield than no-till (without residue) alone, owing to the improved soil qualities in crop residue mulching [45]. Similarly, when appropriate crop rotations are not implemented to help control weeds/pests/diseases and maintain soil fertility, yield loss can occur [50]. These variations in response highlight the need for an integrated approach and to apply caution when interpreting the results of research into CA. There is a need to standardize research methodologies (definitions/different components and techniques of CA) to avoid conflicting results due to incorrect classification of systems as CA [51].
A meta-analysis conducted by Pittelkow et al. [9] observed that globally there was an average yield reduction of −2.5% under CA practices (NT+ residue retention + crop diversification), which increased to~9.9% when there was only partial implementation of CA components (NT alone). However, CA implementation in dry, rainfed areas lead to a 7.3% increase in crop yield, likely due to improvements in soil water storage. These results highlight the importance of implementing all components of the CA system to achieve maximum yield benefits and the need to implement it in a location-specific manner and Agriculture 2021, 11, 718 6 of 14 appropriately adapt management practices to avoid yield loss. It may also take a number of years for yield improvements to be realized as until the soil quality is improved through these CA components, the yield gain under CA can be minimal [10].

Conservation Agriculture and Water: From Erosion to Eutrophication
To feed the growing human population (~7.9 billion), natural ecosystems are being increasingly converted to cultivated areas. This agricultural expansion leads to more soil disturbance and soil erosion, which degrades soil quality, leads to the pollution of waterways, and damages infrastructure [52,53]. For example, in South Asian countries, water erosion affects 21% of the total land area and is one of the main forms of land degradation [52,54], while worldwide, approximately 75 billion tonnes of soil is eroded from arable land each year [53].
Soil erosion under conventional agriculture is mainly attributed to greater soil disturbance and non-adoption of site-specific soil and water conservation measures. The inclusion of crop residue retention under CA can increase surface roughness and reduce runoff and soil losses [10,55]. Improvement in soil aggregate stability and water storage under CA also directly or indirectly affects runoff and soil losses [25,56,57]. The effectiveness of CA in reducing soil water erosion varies according to the climate, cropping system and experimental duration [6], although in comparison to conventional systems, and particularly those incorporating extended periods of bare fallow, CA can reduce annual soil loss by over 90% (Table 2).
Nitrogen and phosphorus (P) loss from agricultural fields into waterways can lead to eutrophication and N and P are recognized as major water pollutants worldwide [58]. As CA offers several advantages over conventional tillage in various aspects of soil and water conservation, it is also expected to affect N and P export [59]. CA can reduce soil loss and runoff due to less soil disturbance, greater surface stability and increased rates of water infiltration [20,60] (Figure 4 and Table 2). Where this is the case, the surface transport of N and P from agricultural fields into surface water bodies is expected to be reduced. However, in some instances, CA has been observed to increase rates of runoff (Table 3), particularly in the early stages of adoption when surface sealing can be a problem and tillage can increase rates of infiltration. Greater snow capture by standing stubble can also increase runoff in CA systems [61]. Under these circumstances, CA can increase the potential for the transport of soluble forms of N to surface water. Table 3 depicts some examples of the changes in surface runoff (mm) that have been observed under conventional and CA practices.
Agriculture 2021, 11, x FOR PEER REVIEW transport. Overall, a recent review concluded that pesticide transport from CA syst runoff can be greater, reduced and no different from conventional systems depend the chemical in question, but that CA is more effective in reducing the transport o cides sorbed onto soil surfaces due to its ability to decrease erosion rates [72].   In addition, the increased infiltration of water into the soil profile under CA can also increase rates of profile leaching [62] and potentially increase the transport of nitrate into groundwater. Some studies have found increased rates of nitrate leaching from NT profiles [62,63], although others have also found that NT had no effect on nitrate loss [64] or could reduce nitrate concentration in groundwater [65]. The variability in these results is due to complex interaction and contribution of several factors, such as soil physical characteristics, rainfall patterns and soil management practices that directly or indirectly affect the nitrate mobility and export from the fields to another location [66]. In order to achieve the full potential of CA to reduce nitrate loss, cover crops and balanced fertilizer management needs to be incorporated into the CA system to improve soil nitrogen retention and water quality [67].
Conservation agriculture can also affect the transportation of pesticides to waterbodies. Crop residues can intercept pesticides, especially apolar pesticides or those with low polarity [68]. Therefore, retention of crop residues on the soil surface under CA can affect the efficiency of pesticide interception. When more than 30% of the soil surface is covered with crop residues, 40-70% of the applied pesticide can be intercepted [69]. Moreover, these crop residues have 10 to 60 times more sorption capacity than soil [70]. CA can also modify the concentration and transport of pesticides in the soil, although as pesticide behavior in soil is highly variable, the effect of CA on pesticide transport is also often inconsistent [58]. Conservation agriculture can enhance the soil organic matter content, especially in the top surface layers [2,25], which can increase the retention of pesticides and limit their susceptibility to microbial degradation. However, Alletto et al. [71] highlighted that the conservation tillage system has lesser effects than initial soil condition on pesticide transport. Overall, a recent review concluded that pesticide transport from CA systems in runoff can be greater, reduced and no different from conventional systems depending on the chemical in question, but that CA is more effective in reducing the transport of pesticides sorbed onto soil surfaces due to its ability to decrease erosion rates [72]. ↓ arrow indicates reduction compared to conventional tillage; * mean data t/ha/yr ** mean data of four years in t/ha/h.

Large Scale Crop Residue Burning in Conventional Farming: A Significant Threat to Air Quality
The large scale burning of crop residues has numerous adverse effects including air pollution, due to the increase in particulate matter (also known as 'black carbon') and carbon emissions, which contributes to regional and global climate change. In areas where burning is widespread, such as the Indo-Gangetic plains (IGP), it can be responsible for significant health (toxic smog) and environmental impacts [84,85]. For example, Kaskaoutis et al. [86] used AERONET imaging data from the Kanpur Research Station for identifying hot-spots of residue burning. They reported that large scale atmospheric emissions, pollution, and accumulation of aerosols have increased over the IGP during the past decade, especially from October to February [87,88]. Carbon monoxide (CO) and nitrogen dioxide (NO 2 ) are among the main components of smog. The smoke plumes from crop residue burning are mostly concentrated between the ground and about 800-900 m in altitude captured by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) instrument. Where the uptake of CA in a region is widespread, the elimination of residue burning in favor of residue retention can thus lead to significant reductions in air pollution.

Conservation Agriculture and Greenhouse Gas Emissions: Decoding the Complexities
Crop and soil management practices affect the release as well as capture of greenhouse gases (GHGs) from the soil to the atmosphere and vice-versa [89]. Consequently, agriculture has been identified as one of the four important sectors that could contribute to reducing global GHGs emissions [90]. The reduction in fuel usage associated with the smaller number of tillage operations under CA is well established to reduce GHGs emissions. For example, fossil fuel emissions from agricultural operations under conventional tillage (moldboard plough) were estimated to be 0.05 Mg ha −1 yr −1 compared to 0.03 Mg ha −1 yr −1 under no-till conditions [91]. In addition, the conversion from conventional to CA has been reported to lead to soil C sequestration, although the amount sequestered is often highly variable and dependent on climate, soil type and management practices. Estimates of C sequestration can be negative (i.e., net C loss) in cool moist regions where tillage buries residues in regions of the profile with lower decomposition rates and/or lowers yield [92], but positive in regions where soil and climate are favorable for Agriculture 2021, 11, 718 9 of 14 biomass production, CA has a positive impact on yield, and the reduced soil disturbance helps protect organic matter from microbial decomposition [93,94]. Many worldwide estimates of average sequestration are around~0.3-0.5 Mg ha −1 yr −1 [94][95][96][97], although in individual regions, higher rates may be observed (e.g., 0.85 Mg ha −1 yr −1 in Spain when NT has been in place for <10 years [98]). However, estimates are often highly uncertain due to the methodological challenges of measuring C change [24,99]. It should be noted that rates of sequestration will also decrease over time as sites approach their equilibrium C content.
In addition to C change, CA can also affect the flux of other GHGs, particularly methane (CH 4 ) and nitrous oxide (N 2 O). Similar to C storage, CA has been observed to both increase and decrease N 2 O emissions (Table 4), depending on the influence it has on soil moisture and microbial activities and, thus, nitrification and denitrification. For example, where CA increases soil moisture, microbial biomass, and labile carbon, there is potential for greater rates of nitrification and denitrification and thus N 2 O emission [6]. However, when CA lowers soil temperatures, and improves soil structure and drainage, denitrification and N 2 O emissions can decrease [6,100]. Less information is available regarding the impact of CA on CH 4 emissions; however, these are commonly observed to either remain unchanged or decrease due to improvements in aggregate stability/porosity and the subsequent uptake of CH 4 by methanotrophic bacteria [95,101].
Overall, it is the net impact that CA has on CO 2 , CH 4 and N 2 O flux that determines whether a CA system will act as a net sink or source of GHGs. However, relatively few studies consider the flux of all GHGs from the soil concurrently. One meta-analysis that summarized the results of nine studies conducted globally reported an average difference in global warming potential (GWP) of −2.39 Mg ha −1 yr −1 in NT compared to conventionally tilled systems when considering both soil GHGs flux and emissions from farm operations [102]. However, a second analysis noted greater GHGs emissions from the soil of NT compared to conventional systems during the first 5 years of practice (GWP of +0.39 and +1.51 Mg ha −1 yr −1 in humid and dry temperate regions, respectively), but lower or similar emissions after 20 years (GWP −2.07 and −0.36 Mg ha −1 yr −1 in humid and dry temperate regions, respectively) [103]. The decline in GHG emissions in NT systems over time was largely due to declines in N 2 O emissions, which have been found to reduce as soil aggregation and drainage improve in more established NT systems [103].

Can Conservation Agriculture Really Conserve Soil Biodiversity?
As CA promotes the accumulation of soil organic carbon at the surface of the profile, it is expected that the microbial activity and biomass must be higher in CA farms due to the increased availability of organic substrates [110]. The improvements in soil aggregation, aeration and moisture availability also create favorable conditions for increases in both the size and diversity of microbial populations, as can crop diversification through crop rotation or intercropping [111]. Full implementation of CA components has been reported to increase the diversity of both fungal and bacterial populations [112], with NT in particular favoring the increase in fungal diversity due to the absence of tillage [113]. The increase in microbial diversity has several significant implications for crop productivity and soil health. For example, several plant growth-promoting soil microbes proliferate in these favorable conditions and contribute to enhanced plant growth, disease suppression and abiotic stress tolerance [114]. Microbial-induced enzymes associated with nutrient cycling are also found in greater amounts under CA, leading to higher nutrient availability under CA [115].
CA has the potential to not only improve microbial diversity but also to influence such soil macro-fauna as earthworms, ants, termites and beetles [116]. These macrofauna improve soil health by breaking down plant residues, increasing macroporosity, and improving water infiltration, soil aggregation and nutrient cycling [117]. Intensive tillage practices often kill or disturb the functions of soil macro-fauna, exposing them to the soil surface and other predators. This loss of soil biodiversity severely affects the soil physicochemical properties and ultimately influences crop productivity. Therefore, biological parameters are often used as indices in characterization of CA soils. The significant effects of CA on soil macro-fauna could be greater in warm temperate zones and soils with higher clay content (>30%) and low soil pH (<5.5) [116].

Conclusions
Compared to conventional agriculture, CA improves several aspects of cropping systems that can enhance ES. Conservation agriculture improves soil structure and typically leads to reduced soil erosion and surface runoff. It is particularly advantageous in drier regions, where it helps to increase soil water storage and maintain greater crop yield. Compared to the intensive agriculture, CA also generally enhances soil organic carbon storage, particularly in the topsoil. This can help with climate mitigation through carbon sequestration, reduced emission of greenhouse gases (CO 2 , CH 4 , N 2 O) and water regulation. However, not all experiments report that CA has a positive impact on ES. This can be due to the duration of experiments, as well as cropping system, climate, soil type and land management practices. Therefore, understanding and decoding the complexities involved in soil-climate-management-dependent CA is important and requires a multidisciplinary approach. Whether CA can deliver significant ESs under a climate changing scenario is also an important question that needs to be addressed by studying the differential effects of temperature, warming and changes to rainfall patterns on soil processes and ES in CA-adopted farms/experiments.