Suitability Analysis of Crops for Sloping Farmland in Arid Sandy Regions with Traditional Farming Methods

Abstract

Global agricultural systems are predominantly concentrated in regions characterized by fertile soils, abundant precipitation, and gentle slopes. However, a significant proportion of farmland is situated in areas with poor soil quality, arid conditions, and steep slopes. In such challenging environments, particularly sandy-arid sloping farmlands, selecting native crops that are well-adapted to local conditions is critical for sustainable agricultural practices. This study categorizes local crops in arid regions into four distinct types: tall-stem monocotyledonous plants (represented by maize, Zea mays L.), short-stem monocotyledonous plants (represented by millet, Setaria italica), tap-rooted dicotyledonous plants (represented by soybean, Glycine max (L.) Merr.), and tuberous dicotyledonous plants (represented by potato, Solanum tuberosum L.). A quantitative evaluation framework was developed using five key indices: nitrogen fixation, anti-wind erosion, roots reinforcement, anti-water erosion, and water conservation. These indices were used to calculate the suitability index values for each crop type. The findings revealed that in sandy-arid sloping farmland regions, maize and millet emerged as the most suitable crops for cultivation, followed by soybean, while potato was identified as the least suitable. Maize exhibited high values across all five indices, particularly demonstrating exceptional performance in nitrogen fixation. Additionally, the study demonstrated that traditional farming practices are highly effective in sloping farmlands, since they not only promote crop growth but also mitigate soil erosion. This research offers insights into agricultural management in regions affected by drought, soil erosion, and steep terrain. The results highlight the feasibility of employing traditional farming methods to cultivate maize in such challenging environments, providing practical guidance for sustainable agricultural development.

1. Introduction

The global agri-food system heavily depends on synthetic nitrogen (N) fertilization to boost crop yields. However, the use of synthetic N fertilizer is not sustainable. It is estimated that the synthetic N fertilizer supply chain leads to annual emissions of 1.3 × 10⁹ Mt CO₂, which accounts for 10.6% of agricultural emissions and 2.1% of global greenhouse gas (GHG) emissions. In terms of the sources of these emissions, the production of synthetic N fertilizer makes up 38.8% of the total emissions. Field emissions are responsible for 58.6%, and transportation contributes the remaining 2.6% [1]. Reducing the overall production and use of synthetic N fertilizers offers large mitigation potential, and in many cases, realizable potential to reduce emissions.

Biological nitrogen fixation (BNF) offers a sustainable mechanism for providing crops with nitrogen while preserving crop quality and yields without the overuse of fertilizer. Diazotrophs are capable of forming symbiotic relationships with specific plant species. They reduce atmospheric nitrogen (N₂) to ammonium (NH₄⁺) through a nitrogenase metalloenzyme complex, in exchange for a carbon source provided by the plant [2,3]. This symbiotic interaction has been most extensively studied in leguminous plants, where rhizobia bacteria colonize root nodules. These nodules create an environment with low oxygen pressure, which is necessary for optimal nitrogenase activity. Rhizobia bacteria supply 50–80% of the nitrogen required for host plant growth [4].

Maize has been reported to obtain 29–82% of its nitrogen requirements through BNF [5]. While diazotrophs have been documented to fix nitrogen in cereal crops through endophytic or rhizospheric associations, the amount of fixed nitrogen is typically variable and significantly lower than that observed in legume symbioses [6]. Maize is characterized by the development of aerial roots on up to 10 stem nodes, which secrete mucilage following rainfall [7]. These aerial roots are adventitious nodal structures that, unlike brace roots, do not contact the soil. The mucilage surrounding aerial roots creates a sugar-rich, hypoxic environment conducive to harboring a nitrogen-fixing microbiome [5].

Millet, a short-stemmed monocotyledon, has been reported to exhibit associative nitrogen fixation through rhizospheric diazotrophs, though contributions remain modest compared to legumes [8]. Its dense, shallow root system rapidly forms a soil-covering mat, significantly reducing surface erosion in arid regions by stabilizing loose topsoil and intercepting rainfall impact [9]. Potato, a tuberous-rooted dicotyledon, develops extensive lateral root networks that bind soil particles, improving aggregation in degraded soils. Though lacking significant nitrogen fixation, its tubers and fibrous roots create microhabitats that enhance soil organic matter and water retention [10].

Roots are critical for plant health and productivity, playing a pivotal role in anchoring plants in the ground [11,12,13,14]. Root failure occurs when the mechanical load exceeds the structural tolerance of the roots, particularly through breaking and buckling [15]. Despite the apparent role of root mechanics in limiting root lodging [16], the mature maize root system is composed of root whorls that develop from stem nodes both below (crown roots) and above (brace roots) the ground, collectively referred to as nodal roots. Brace roots exhibit higher structural mechanical properties and a lower bending modulus compared to crown roots [17]. There is variation in the contribution of brace roots to the anchorage between whorls, with the whorl closest to the ground contributing the most [18].

However, current research on the functions of herbaceous plant root systems mainly focuses on soil stabilization and nitrogen fixation, with few similar studies on the selection of crops for special environments. Specifically, for sloping farmland in sandy and dry areas, which native crops are more suitable for planting? This question has not been scientifically answered. The sloping farmland in sandy and dry areas is characterized by low soil fertility, scarce rainfall, and susceptibility to soil and water loss, which puts forward more stringent requirements for the selection of native crops.

This study focuses on sloping farmland in sandy and dry areas. In order to mitigate soil and water loss and nutrient loss on sloping farmland, four native crops with high planting frequency were selected to evaluate which crop is more suitable for sloping farmland. Using theoretical analysis, laboratory tests, and field tests, the four representative native crops were compared in terms of nitrogen fixation, anti-wind erosion, root-reinforcement, anti-water erosion, and water conservation, and their suitability scores were quantified. The crop with the highest comprehensive score is deemed most suitable for planting on sloping farmland, and the research conclusion provides a scientific reference for the planting choices of agricultural managers.

2. Materials and Methods

2.1. Study Area

The study was conducted in Qingshuihe County (39°35′00″–40°12′30″ N, 111°18′45″–112°07′30″ E), situated within the Inner Mongolia Autonomous Region, China. This region lies at the critical transitional ecotone between the Inner Mongolia Plateau and the Loess Plateau of Shanxi-Shaanxi, characterized by gentle-slope terrain where the majority of cultivated lands are distributed across hillside slopes [19]. Climatically, the area experiences a continental inland regime, marked by weakened and delayed incursions of the Pacific monsoon. These monsoon dynamics result in short, concentrated rainy seasons and limited annual precipitation (averaging 391 mm), which, coupled with high evaporation rates (1850 mm/yr) and a mean annual wind speed of 3.5 m/s, exacerbate regional aridity and predispose the landscape to soil erosion. Surface soils (0–100 cm depth) are classified as sandy loam with a bulk density of 1.46 g/cm³. Groundwater tables remain below 10 m, rendering subsurface contributions to soil moisture negligible. Soil total nitrogen content ranges from 0.124 to 0.192 g/kg, reflecting nutrient-poor conditions typical of erosion-prone environments.

These pedological and hydrological constraints, coupled with climatic stressors, define the study area as a representative model for soil degradation and conservation challenges in arid ecosystems.

2.2. Selection of Crops

The selection of crops for cultivation in challenging agroecological zones, such as sandy-arid sloping farmlands, requires a systematic evaluation of plant functional traits and their interactions with environmental stressors. In this study, four categories of indigenous crops were selected based on their morphological adaptations, root system architectures, and historical cultivation practices in arid regions. These include tall-stemmed monocotyledons (represented by maize), short-stemmed monocotyledons (represented by millet), tap-rooted dicotyledons (represented by soybean), and tuberous dicotyledons (represented by potato). This classification aligns with ecophysiological strategies that influence crop performance under water scarcity, nutrient limitations, and slope-induced soil instability (Figure 1).

The categorization integrates two critical axes of plant adaptation: aboveground biomass allocation and belowground root architecture. Monocotyledons, characterized by fibrous root systems and vertical stem growth, are contrasted against dicotyledons, which exhibit diverse root morphologies and broader canopy structures. Tall-stemmed monocotyledons (maize): These crops allocate resources to vertical stem elongation, reducing interplant competition for light while forming dense root mats that stabilize surface soils [15]. Their height mitigates wind erosion by reducing near-surface wind velocity. Short-stemmed monocotyledons (millet): compact stature minimizes water loss through transpiration, while shallow but extensive fibrous roots enhance topsoil retention. Tap-rooted dicotyledons (soybean): deep-penetrating taproots access subsoil moisture and nutrients, while symbiotic nitrogen fixation improves soil fertility [20]. Tuber-rooted dicotyledons (potato): shallow tuber systems prioritize carbohydrate storage but disrupt soil cohesion, increasing erosion risks on slopes [21].

2.3. Evaluation Indicators

Screening indigenous crops suitable for cultivation on sloping farmland in the Loess Plateau is necessary, and quantitatively evaluating the suitability of crop cultivation is also essential. In this study, five basic indicators were selected to assess the crops based on the two environmental issues of soil erosion and nutrient loss faced by sloping farmland:

Nitrogen fixation: This indicator measures the ability of crops to convert atmospheric nitrogen (N₂) into a form that can be used by plants, such as ammonium (NH₄⁺) and nitrate (NO₃⁻). Nitrogen fixation is crucial for maintaining soil fertility and reducing the need for synthetic fertilizers. For example, leguminous crops like soybeans have a well-developed symbiotic relationship with rhizobia bacteria, which enhances their nitrogen-fixing ability [6,22].

Anti-wind erosion: This indicator evaluates the crop’s ability to resist wind erosion. Crops with strong anti-wind erosion capabilities can help protect the soil surface from being blown away by strong winds [23]. This criterion is critical for the Loess Plateau due to prevalent wind erosion [24,25]. The evaluation of this indicator can be based on the plant’s physical characteristics, such as stem strength and root system structure.

Roots reinforcement: This indicator assesses the crop’s ability to reinforce the soil through its root system. Crops with extensive and strong root systems can bind the soil particles together, enhancing soil stability and reducing the risk of soil erosion [21,26]. The evaluation of this indicator can be based on root density, root length, and root thickness [15].

Anti-water erosion: This indicator measures the crop’s ability to resist water erosion [27]. Crops with strong anti-water erosion capabilities can reduce soil loss caused by heavy rainfall [28,29]. This indicator is evaluated based on the plant’s canopy coverage, which can reduce the impact of raindrops on the soil surface, and the root system’s ability to anchor the soil.

Water conservation: This indicator evaluates the crop’s ability to retain water in the soil. Crops with strong water retention capabilities can help maintain soil moisture, which is crucial for plant growth and reducing the risk of drought [30,31]. The evaluation of this indicator can be based on the plant’s root system’s ability to absorb and store water, as well as the plant’s canopy’s ability to reduce evaporation.

These five indicators provide a comprehensive framework for evaluating the suitability of indigenous crops for cultivation on sloping farmland in the Loess Plateau. By quantitatively assessing these indicators, we can identify the crops that are most suitable for the specific environmental conditions of the region, thereby promoting sustainable agricultural practices and soil conservation efforts.

2.4. Experimental Design

The study was conducted through an integrated approach combining theoretical analysis, controlled laboratory experiments, and field trials from April 2023 to September 2024. A comprehensive evaluation was conducted for four native crops: maize, millet, soybean, and potato, and the field size per plot was 14 m × 12 m. Six critical agroecological parameters were systematically assessed: (1) root system morphological and functional traits (e.g., depth, biomass, and root length density), (2) nitrogen fixation potential, (3) anti-wind erosion via stubble dynamics and soil surface roughness, (4) roots reinforcement capacity measured through shear strength testing, (5) anti-water erosion mitigation efficiency based on sediment yield reduction, and (6) water conservation efficiency linked to root-mediated moisture storage and canopy evaporation suppression. The crops were cultivated under distinct planting configurations to reflect regional agricultural practices: Maize and millet were planted along the slope on ridges, with row spacings of 60 cm (maize; 6 plants/m²) and 30 cm (millet; 15 plants/m²). Soybean and potato were randomly distributed along contour lines to optimize soil stabilization.

(1)

Root system characterization: Root traits were assessed through systematic field sampling. Sampling strategy: Nine randomized sampling points were selected across varying slope positions and elevations, with six plants per crop species randomly selected from each. Crop height and root depth were recorded using calibrated tools. Root-to-shoot ratio (R/S) was calculated by separating, oven-drying (65 °C, 48 h), and weighing aboveground biomass and roots. Root length density and root density were quantified via high-resolution root imaging. Topsoil bulk density (0–20 cm depth) was measured using undisturbed soil cores extracted via the ring knife method.

(2)

Nitrogen fixation: Data compiled from peer-reviewed studies on nitrogen fixation rates in arid-region crops.

(3)

Anti-wind erosion assessment: Wind erosion resistance was evaluated through field measurements of four key parameters: stubble height (measured using graduated poles at 20 random points per plot), land cover percentage (quantified via drone-based multispectral imaging (SZ DJI, Shenzhen, China)), soil bulk density (determined by the core method at 0–15 cm depth), and surface roughness (assessed using a 1 m profile laser scanner with 2 mm resolution (Creality 3D, Shenzhen, China)). Wind speed attenuation (%) was derived from empirical relationships between stubble height and aerodynamic drag coefficients. Wind erosion intensity (t ha⁻¹ yr⁻¹) was calculated using the Revised Wind Erosion Equation (RWEQ) [32], calibrated with site-specific soil texture, stubble height, and climatic data.

(4)

Roots reinforcement: The shear strength of root–soil composites was measured using a 2012-HPF large-scale direct shear apparatus (Geocomp, Boston, USA). Undisturbed soil samples containing intact root systems were subjected to incremental normal stresses (50–200 kPa) to determine cohesion (c) and internal friction angle (φ).

(5)

Anti-water erosion capacity was assessed through the following: plant coverage rates (%) were quantified using ground-truth quadrat surveys; soil erosion rates were measured via sediment traps (Hohhot, China) installed in the Study Area. Data normalization: Erosion values were converted to relative erosion rates (%) by comparing them with bare soil control plots.

(6)

Water conservation efficiency was evaluated based on the following: the evaluation of this indicator can be based on the plant’s root system’s ability to absorb and store water, as well as the plant’s canopy’s ability to reduce evaporation, based on field observations and measurements. All field data were collected in situ.

Statistical Analysis: All data were analyzed using ANOVA (α = 0.05) and nonlinear regression models. Field data were collected in situ to ensure ecological relevance.

3. Results

3.1. Root Characteristics

The root systems of crops play a crucial role in their growth and adaptation to different environments. Understanding the characteristics of various crop root systems is essential for agricultural practices and crop selection (Figure 2). The following is a summary of the root characteristics and biomass allocation of four crops: maize, millet, soybean, and potato.

Maize has a fibrous root system, typical of monocotyledons. In arid zones, its primary roots can penetrate vertically to 0.2–0.5 m under optimal conditions. However, lateral roots mainly occupy the top 0–30 cm of soil, with 60% in the upper 30 cm. Root density peaks at 20–40 cm depth (2.8 ± 0.4 cm/cm³). It has a high root hair density, which enhances water and nutrient absorption. The lateral roots form a dense mat with an inter-root distance of 2–4 mm, improving soil aggregation. The root-to-shoot ratio (R/S) is 0.25–0.35 under water stress, prioritizing shoot growth for light competition (

Table 1. Measurements of root system indicators for four indigenous crops.

Parameter	Maize	Millet	Soybean	Potato
Height (m)	1.5–2.0	1.2–1.5	0.5–1.0	0.2–0.6
Max root depth (m)	0.2–0.5	0.1–0.3	0.15–0.3	0.3–0.6
Root density (cm/cm3)	2.8 ± 0.4	4.2 ± 0.6	1.5 ± 0.3	0.4 ± 0.5
Root-to-shoot ratio (R/S)	0.25–0.35	0.45–0.55	0.20–0.25	/

).

Millet features a fibrous root system with shallow proliferation. The maximum root depth is 0.1–0.3 m, with 85% of roots confined to 0–20 cm. The root density in the topsoil is 4.2 ± 0.6 cm/cm³, higher than that of maize due to its compact growth. It has thin roots (diameter: 0.2–0.4 mm) with a high specific root length (SRL: 25–30 m/g), optimizing water foraging in dry topsoil. There is limited root branching beyond the crown region. The R/S ratio is 0.45–0.55, reflecting greater resource allocation to roots under aridity.

Soybean has a taproot system with lateral branching. The taproot can penetrate 0.15–0.3 m, with laterals extending 5–8 cm horizontally. The root density is 1.5 ± 0.3 cm/cm³ in the top 10 cm, decreasing sharply below 20 cm. The taproot diameter is 5–8 mm at the base, providing strong mechanical anchorage. The R/S ratio is 0.20–0.25, prioritizing photosynthetic biomass.

Potato possesses a tuberous system with adventitious roots. The root depth ranges from 0.3 to 0.6 m, with 90% of roots in 0–25 cm. The tuber depth is 10–20 cm. It has fine roots with a low SRL (8–12 m/g). The tuber roots lack lignin, contributing minimally to soil binding.

3.2. Nitrogen Fixation Effect

BNF provides a sustainable means of supplying crops with nitrogen and maintaining crop quality and yields without the excessive use of fertilizers [3]. Diazotrophic microorganisms can form symbiotic relationships with specific plant species, reducing atmospheric nitrogen (N₂) to ammonium (NH₄⁺) through a nitrogenase metalloenzyme complex in exchange for a carbon source [33]. The ability of crops to fix atmospheric nitrogen varies significantly across different plant families, which has important implications for soil fertility management, particularly in arid regions (Figure 3). This symbiotic relationship has been most extensively studied in legumes, where rhizobia colonize root nodules, creating an environment with low oxygen pressure that is necessary for nitrogenase activity. Rhizobia can supply 50% to 80% of the nitrogen required for plant growth [4].

Maize has been reported to obtain 29–82% of its nitrogen needs through BNF [5]. Maize is characterized by the formation of aerial roots on as many as 10 stem nodes that produce mucilage after rain [7]. Aerial roots are adventitious nodal roots that, unlike brace roots, do not make contact with the soil. The mucilage associated with aerial roots provides a sugar-rich, hypoxic environment for hosting a nitrogen-fixing microbiome [5]. These findings in maize serve as the basis for exploring nitrogen fixation activity in other cereal crops that form aerial roots.

Four major crops, soybean [34], maize [5], millet [35], and potato [36], exhibit distinct nitrogen acquisition mechanisms. Understanding these mechanisms is crucial for optimizing fertilizer use and improving crop yields. The nitrogen acquisition mechanisms of crops can be broadly classified into two categories [34,37]: SNF and non-symbiotic nitrogen acquisition. SNF involves a mutualistic relationship between plants and nitrogen-fixing bacteria, while non-symbiotic nitrogen acquisition relies on the plant’s ability to uptake nitrogen from the soil through various transporters and microbial processes [38] (

Table 2. Comparison of nitrogen fixation methods and effects among four indigenous crops.

Parameter	Soybean	Maize	Millet	Potato
Fixation type	Symbiotic	Associative	Endophytic	None
Key organisms	Bradyrhizobium	Azospirillum	Diazotrophs	-
N2 fixed (kg ha−1 yr−1)	40–60	10–20	No data	≈0
Energy cost (photosynthate)	High (15% C)	Moderate (5% C)	-	-

Notes: Soybean’s symbiosis is evolutionarily advanced, while maize’s associative fixation is facultative and context dependent.

).

Soybean is known for its efficient symbiotic nitrogen fixation (SNF) mechanism. The process begins with the secretion of flavonoids by soybean roots, which attract rhizobia bacteria. These bacteria infect the root hairs and form nodules. Inside the nodules, bacteroids convert atmospheric nitrogen (N₂) into ammonium (NH₄⁺) and nitrate (NO₃⁻) for plant use. Soybean exhibits fixation rates ranging from 40 to 60 kg ha⁻¹ yr⁻¹, depending on the nodulation density (15–35 nodules per plant) and soil moisture. Approximately 50–70% of soybean’s nitrogen demand is met through BNF, reducing synthetic fertilizer requirements by 50–70%. However, a key limitation of SNF in soybean is that it ceases under drought stress due to nodule senescence [39].

Maize, unlike soybean, lacks symbiotic nodules but hosts diazotrophic bacteria in its rhizosphere and aerial root mucilage [40]. These bacteria colonize the root surfaces and internal tissues (aerenchyma), converting atmospheric nitrogen (N₂) into ammonium (NH₄⁺) for plant use. A notable feature of maize is the secretion of aerial root mucilage by brace roots in humid conditions, creating a microaerophilic biofilm that enhances bacterial colonization. The fixation rate of maize ranges from 10 to 20 kg ha⁻¹ yr⁻¹, highly variable with soil moisture and microbial diversity. Under drought stress, diazotrophic bacteria can contribute 5–15% of the plant’s nitrogen uptake. However, nitrogen acquisition in maize via diazotrophic bacteria is highly dependent on environmental conditions, requiring more than 60% soil water content for mucilage production.

Millet relies entirely on soil nitrate and ammonium uptake via high-affinity transporters [8]. It exhibits root plasticity, with prolific lateral branching in nitrogen-poor soils to enhance foraging, but it does not fix nitrogen. Millet has a low nitrogen requirement, needing only 40–60 kg ha⁻¹ yr⁻¹ for grain production, minimizing its dependency on nitrogen fixation.

Potato, similarly, depends on inorganic nitrogen uptake (NO₃⁻, NH₄⁺) and organic nitrogen mineralization via soil microbes [10]. It has shallow roots, limited to 0–30 cm depth, accessing surface-applied fertilizers but not subsoil nitrogen reserves. Potato has a high nitrogen demand, requiring 120–180 kg ha⁻¹ yr⁻¹ for tuber production, necessitating heavy fertilizer inputs.

3.3. Anti-Wind Erosion Effect

The study area is characterized by a typical inland monsoon climate, with an average annual precipitation below 400 mm and evaporation approaching 2000 mm. Severe soil desertification has resulted in surface soils classified as loessal soil. Frequent strong winds exacerbate wind erosion, particularly during April and May when monthly average wind speeds reach 5.6 m/s, predominantly from northwest directions. Stubble retention, an agricultural practice of maintaining crop residues on fields post-harvest from October to April, demonstrates significantly mitigated soil wind erosion (Figure 4), especially in severely affected regions like the Loess Plateau.

Standing stubble increases surface roughness, effectively decelerating near-surface wind velocities (wind speed reduction). Enhanced stubble coverage correlates with reduced erosion rates. Under high wind conditions (12–18 m/s), maize stubble decreases wind erosion modulus by 49–62% compared to conventional tillage, while millet stubble achieves 51–76% reduction (

Table 3. The impact of crop stubble on soil physical properties, with wind erosion intensity calculated using the Revised Wind Erosion Equation (RWEQ).

Crop Type	Stubble Height (cm)	Land Cover (%)	Soil Density (g/cm3)	Surface Roughness (cm)	Wind Speed Attenuation (%)	Wind Erosion Intensity (t ha−1 yr−1)
Millet	6–12	38–50	1.3	4.2	51–76	1.3
Maize	10–18	21–36	1.1	3.3	49–62	1.4
Soybean	0	0	1.0	/	/	9.4
Potato	0	0	0.9	/	/	12.2

). The practice elevates aerodynamic roughness, diminishing wind shear forces on soil particles (surface roughness enhancement). Taller stubble significantly improves roughness parameters, creating a more erosion-resistant microtopography. Stubble barriers physically intercept saltating particles within the near-surface airflow (aeolian sediment interception). Wind–sand flow activity predominantly occurs within the stubble height range, with denser stubble coverage demonstrating superior particle-trapping efficiency. Stubble consolidates soil aggregates and enhances organic matter content, collectively strengthening erosion resistance (soil structural improvement). Concurrently, the practice improves soil moisture retention capacity, further reducing wind erosion susceptibility.

Comparative analysis reveals wind erosion resistance follows the hierarchy: millet > maize > soybean > potato. Notably, maize and millet stubble exhibit superior erosion control through their integrated effects on wind dynamics modification, sediment interception, and soil structural improvement. Implementing these stubble retention systems in loessal sloping farmland effectively mitigates soil loss, improves soil quality, and yields substantial ecological and economic benefits.

3.4. Roots Reinforcement Effect

Plant roots act as natural soil anchors, enhancing mechanical soil strength [41]. Crops typically exhibit shallow root growth, with the majority of roots located within the top 10–30 cm of the soil surface. As a result, crops primarily reinforce the surface layer of the soil. Crop roots also possess significant mechanical reinforcement capabilities, particularly for the soil surface [42]. Therefore, the mechanical reinforcement effect of crops deserves sufficient attention.

The mechanical reinforcement theory is primarily applied to the reinforcement of lateral roots in plants, that provided by crop roots [42]. The root system of crops is characterized by numerous roots, predominantly fine roots, which form a root–soil composite with the surrounding soil. The effect of the plant root system is similar to that of fiber reinforcement [14]. Consequently, research findings on fiber mechanical reinforcement can be utilized to study the mechanical reinforcement effect of herbaceous roots.

When applying the mechanical reinforcement theory to assess root mechanical reinforcement effect, the root mechanical reinforcement can be considered as the additional cohesion provided by the roots to the soil (Figure 5). When the soil contains roots, a root–soil composite is formed, and the shear strength of the root–soil composite is:

$${\tau }_{\mathrm{s}+\mathrm{r}}={\sigma }_{\mathrm{n}}\tan \phi +c_{\mathrm{s}}+c_{\mathrm{r}}$$

(1)

where ${\tau }_{\mathrm{s}+\mathrm{r}}$ is the shear strength of the root–soil composite, ${\sigma }_{\mathrm{n}}$ is the effective normal stress, $\phi$ is the friction angle of the soil, $c_{\mathrm{s}}$ is the cohesion of the soil, and $c_{\mathrm{r}}$ is the root additional cohesion.

Experimental research is a primary method for investigating the root mechanical reinforcement effect, as it directly obtains quantitative results through data collection and observation of experimental phenomena. The direct shear test is a crucial tool in geotechnical engineering research due to its simplicity, cost-effectiveness, effectiveness, and reliability [42,43]. This test quantifies the root mechanical reinforcement effect by shearing the root–soil composite and unrooted soil under identical experimental conditions, allowing for a comparison of the shear strength of the two materials.

In this study, the shear effect of the root–soil composite was determined using direct shear tests, and the influencing factors of the root mechanical reinforcement effect were further investigated (

Table 4. Shear strength characteristics of the root–soil composite: field-measured data.

Material	Soil	Soil + Maize	Soil + Soybean	Soil + Millet	Soil + Potato
Shearing strength (kPa)	9.69	79.78	22.97	85.26	19.43

). On one hand, the impact of root–soil interaction on the mechanical reinforcement effect was examined through direct shear tests. The results indicated that the enhancement of shear strength is closely related to root density, tensile strength, and root–soil interfacial friction. Under shear loading, roots on the shear surface experienced tension, with different roots exhibiting distinct failure modes. For coarse roots, which can withstand significant tensile forces, the root-to-soil mechanical reinforcement primarily relied on interfacial friction. In contrast, for fine roots, which have a lower tensile capacity, the mechanical reinforcement effect was mainly attributed to the maximum tensile force of the roots. As root density in the soil increased, the combined mechanical reinforcement effect of the roots on the soil intensified, thereby enhancing the overall mechanical reinforcement effect.

Current research suggests that the impact of root mechanical reinforcement on the internal friction angle of soil is minimal, leading to the conclusion that the root mechanical reinforcement effect on the internal friction angle can be considered negligible [44]. When the root–soil composite reached 80–85% saturation, its shear strength increased by 100% compared to that of unrooted soils [45]. This further confirms the reinforcement effect of plant roots under rainfall conditions [46].

3.5. Anti-Water Erosion Effect

Experiments with both above-ground and below-ground biomass indicated that vegetation cover was more effective in reducing splash detachment [47,48] while plant roots were more efficient in reducing rill erosion [49,50]. The roots of vegetation interweave, forming a dense network that holds large amounts of soil, thereby reducing water-induced erosion and gully formation. Figure 6a illustrates a structural model comparing the erosion-reducing potential of plant roots and plant cover. While plant roots are more effective in reducing rill and gully erosion rates, plant cover is more effective in reducing splash detachment and inter-rill erosion rates [51,52,53,54].

Most equations indicate a decreasing soil loss with increasing vegetation cover ($C$, %). A relationship was formed in Figure 6b after transforming absolute soil erosion values into relative erosion values ($E_r$). An exponential equation best describes the relationship between vegetation cover ($C$, %) and relative soil loss ($E_{\mathrm{r}}$):

$$E_r=e^{-0.04C}$$

(2)

Both soybeans and potatoes exhibit plant coverages of less than 30%, with relative erosion values exceeding 0.6. This indicates that in sloping cultivated areas, the above-ground parts of soybeans and potatoes contribute less to reducing soil erosion. In contrast, maize and millet have vegetation coverages of over 60%, and their relative erosion values are below 0.2, suggesting that in sloping cultivated areas, the above-ground parts of maize and millet make a greater contribution to mitigating soil erosion.

For each of the four crops studied (maize, millet, soybean, and potato), rill erodibility decreased exponentially with increasing root density (RD) and root length density (RLD), similar to the trend observed with vegetation cover. However, specific regression equations were not provided. Among the crops, soybean and potato exhibited higher soil erosion rates at low root length densities compared to maize and millet. When the data were plotted on a relative scale, an exponentially decreasing trend was evident (Figure 6c,d). The root architecture of crops appears to provide the greatest erosion resistance at high root densities while offering the least resistance at low root densities. This suggests that the impact of plant roots on enhancing soil resistance to concentrated flow erosion primarily depends on the presence and distribution of effective roots within the top 30 cm of the soil profile. Plant roots reduce concentrated flow erosion by increasing soil resistance, enhancing soil permeability, and improving soil physical properties. This study successfully linked critical flow velocities for soil detachment with RD in loessal soils (Figure 6e). It was found that below a certain critical root density, the effect of roots on critical flow velocity and consequently on the erosion rate was significantly pronounced.

3.6. Water Conservation Effect

Topography significantly influences the distribution of organic carbon and its fractions across slope landscapes, thereby affecting soil erosion and sedimentation patterns [55]. The formation of erosion–deposition patterns is directly influenced by topographic factors such as slope gradient, surface roughness, and relief amplitude. The spatial variability in topography and soil composition modifies the convergence of slope runoff and erosion processes, leading to changes in erosion and sediment production, as well as particle size distributions [56]. The spatial distribution of topography and particle size ratios reflects long-term sediment movement and establishes the basis for subsequent sediment transport and deposition. Topography is a critical factor affecting sediment production in watersheds during erosive rainfall events and plays a key role in shaping erosion–deposition patterns on slopes. Improving soil physical and chemical properties can enhance soil resistance to erosion and lead to the development of new slope erosion patterns.

Topography exerts greater influence on erosion than the soil’s physicochemical properties (Figure 7). Slope gradient, surface roughness, and relief amplitude directly influence processes such as slope runoff generation, surface infiltration, runoff accumulation, flow production, and the production of erosion material. These factors can either mitigate or exacerbate soil loss from the slope [57,58]. The comprehensive process of flow generation and convergence on the surface slope constitutes the fundamental mechanism driving alterations in surface microtopography. Thus, processes such as slope surface flow generation, erosion material production, and microtopography modification are intricately interlinked and exert reciprocal influences on one another. Hoeing enhances erosion resistance factors and strengthens the interactions among them, leading to a more effective limitation on soil erosion occurrence.

4. Discussion

4.1. Crops Suitable for Sloping Farmland

The Loess Plateau is characterized by a vast expanse of sloping farmland, which faces significant challenges such as soil erosion and nutrient loss [19]. It is necessary to screen out indigenous crops suitable for cultivation on sloping farmland [23]. This paper selected the four most frequently planted indigenous crops (maize, millet, soybean, and potato) and used the indicator $A_i$ ($A_1$ for maize, $A_2$ for millet, $A_3$ for soybean, and $A_4$ for potato) to quantitatively evaluate their suitability for cultivation on sloping farmland. Each indicator ($A_{i1}$, $A_{i2}$, $A_{i3},$ $A_{i4}$, and $A_{i5}$) was scored from one to four, with higher scores indicating greater suitability for cultivation. The formula is as follows:

$$A_i=A_{i1}+A_{i2}+A_{i3}+A_{i4}+A_{i5}$$

(3)

where $A_{i1}$ represents the nitrogen-fixing ability of the crop; $A_{i2}$ represents its wind erosion resistance; $A_{i3}$ represents its soil-binding ability; $A_{i4}$ represents its water erosion resistance; and $A_{i5}$ represents its water retention ability. After scoring and calculating the four typical indigenous crops (with a full score of 18), the results showed that maize had the highest a score of 18, followed by millet with a score of 16, soybean with a score of 9, and potato a score of 5 (Figure 8). These results suggest that maize and millet are more suitable for cultivation in the sloping farmland areas of the Loess Plateau, and their effects on soil and water conservation are very significant, which is the primary contribution in this region. This aligns with [30], which corroborates the adaptability of these crops in erosion-prone environments. However, [30] further emphasizes that the erosion reduction effects of crops depend not only on species selection but also critically on planting density—a factor warranting deeper exploration in future studies to optimize conservation strategies.

The four crops selected in this paper are widely planted in the Loess Plateau region, and it is recommended that their cultivation be promoted in sloping farmland areas prone to soil and water loss. Other types of indigenous crops in this region can also be classified into these four types, and more planting suggestions can be provided. Crops similar to maize, such as sorghum and sunflower, are also highly recommended for planting. Crops similar to millet, such as wheat, panicle millet, foxtail millet, and oat, are also recommended for planting. Leguminous crops such as mung bean, cowpea, and pea are not recommended for planting in sloping farmland areas. Root crops, such as potatoes and peanuts, are completely not recommended for planting in sloping farmland areas.

4.2. Cropping Plan for Sloping Farmland

As a crucial agroecological zone in China, the Loess Plateau possesses agricultural development potential across most areas except localized mobile dune regions. The region confronts a critical environmental challenge stemming from severe soil erosion, whose sediment yield transported by the Yellow River induces continuous sedimentation in downstream reaches, forming distinctive elevated river channels. More notably, intensive soil erosion accelerates gully headward erosion processes, sculpting a unique “gully-slope system” geomorphological pattern. This erosional dynamic generates extensive sloping farmland, where accelerated topsoil loss directly diminishes land productivity and agricultural output.

Spatial analysis using DEM (30 m) data reveals the study area’s geomorphology comprises loess tablelands, loess ridges, loess hills, and loess gully systems, with slopes exceeding 5° occupying 78% of the total area in the Loess Plateau. In Qingshuihe County, slope distribution demonstrates distinct gradients: <5° (5%), 5–8° (7%), 8–15° (24%), 15–25° (38%), 25–35° (21%), and >35° (5%). Notably, in densely populated areas, the gradient of sloping land cultivation exhibits a significant positive correlation with erosion risks, rendering soil conservation on cultivated slopes a pivotal scientific issue for regional sustainable development. While current engineering measures (e.g., terrace construction) demonstrate effectiveness, they face practical constraints including high construction intensity and maintenance costs. Under such circumstances, selecting indigenous crops adapted to sloping farmland emerges as a pragmatic alternative (Figure 9). This conclusion resonates with [23], which underscores that mitigating land degradation can be achieved through strategic vegetation selection.

This study proposes a terrain gradient-based differentiated cultivation strategy: (1) in gentle slopes (<5°) with minimal erosion, economically prioritized cropping systems (e.g., maize, millet, soybean, and potato) are recommended without topographic modification; (2) for moderate slopes (5–15°), priority should be given to deep-rooted species (e.g., maize and millet) or low-fertilizer-demand crops (e.g., soybean) to enhance soil stabilization; (3) when slopes reach 15–25°, traditional ridge tillage (Figure 7) is advised to achieve dual benefits of soil conservation and rainwater harvesting through micro-topography modification; and (4) extremely steep slopes (>25°) warrant ecological restoration measures such as grassland conversion.

5. Conclusions

This study comprehensively evaluated the suitability of four indigenous crops (maize, millet, soybean, and potato) for cultivation on sloping farmland in sandy and arid regions, focusing on their nitrogen fixation, anti-wind erosion, root-reinforcement, anti-water erosion, and water-retention. Focusing on their root characteristics and the impact on soil erosion and nutrient loss, the findings provide valuable insights for agricultural practices in challenging.

(1)

The four indigenous crops, namely maize, millet, soybean, and potato, exhibit significant differences in their root systems, which determine their suitability for cultivation on sloping farmland. Maize and millet, with their extensive and deep root systems, are better equipped to anchor the soil and resist erosion. Specifically, maize exhibits peak root density (2.8 ± 0.4 cm/cm³) at 20–40 cm soil depth, while millet shows concentrated topsoil root density (4.2 ± 0.6 cm/cm³) within the upper 15 cm. In contrast, soybean and potato have shallower and less extensive root systems. Soybean displays a root density of 1.5 ± 0.3 cm/cm³ limited to the top 10 cm, whereas potato exhibits minimal root development (0.4 ± 0.5 cm/cm³) beyond 25 cm depth, making them less suitable for sloping farmland.

(2)

The evaluation of the five indicators (nitrogen fixation, anti-wind erosion, root reinforcement, anti-water erosion, and water conservation) revealed that maize demonstrated superior adaptability with the highest composite score of 18, exhibiting robust performance across all indices. Millet ranked second with a score of 16, showing particular strength in soil and water conservation. In contrast, soybean and potato displayed markedly lower suitability, scoring 9 and 5, respectively, with potatoes showing negligible soil and water conservation capacity. This indicates that maize and millet are the most suitable crops for sloping farmland in sandy and arid regions.

(3)

Sloping farmland in sandy and arid regions face severe challenges of soil erosion and nutrient loss. The cultivation of maize and millet can effectively mitigate these problems. Their deep and extensive root systems help to stabilize the soil, reduce water and wind erosion, and enhance water retention, thereby contributing to the conservation of soil and nutrients on sloping farmland.

In summary, this study provides valuable insights into agricultural practices in sandy and arid regions with sloping farmland. The findings highlight the importance of selecting crops with suitable root systems to combat soil erosion and nutrient loss. Maize and millet are recommended as the most suitable crops for cultivation on sloping farmland in these challenging environments, offering a sustainable solution for agricultural production and soil conservation.