Assessment of Effectiveness and Suitability of Soil and Water Conservation Measures on Hillslopes of the Black Soil Region in Northeast China

Abstract

There are four sizable black soil regions throughout the world, all of which are valuable natural resources. The black soil region in Northeast China is a major foundation for grain production. Serious risks of soil erosion do exist, and they have an immediate impact on both the country’s food security and future ecological security. Many soil and water conservation measures have been put in place to control soil erosion. However, how effective and suitable are these measures? Currently, systematic analyses and assessments are lacking. The objective of this study was to assess the effectiveness and suitability of soil and water conservation measures on hillslopes using the comprehensive index method and the Pressure–State–Response model. The categorization of effectiveness and suitability of these measures were similar for both methods: that is, very effective and suitable measures included no-tillage + straw mulch and ridge belt or contour ridge. The two methods validated one another. Thus, this categorization standard is useful for choosing the best soil and water conservation measures for different soil erosion regions.

1. Introduction

Globally, soil erosion is a problem that leads to land degradation, reduction of cultivated land, deterioration of water quality, environmental pollution, etc. [1,2,3]. The existing research on soil erosion mainly focuses on types [4,5], processes [6,7], influence factors [8,9], hazards [10,11], and models [12,13]. These studies are useful in revealing the mechanism of soil erosion and in controlling it through the application of appropriate soil and water conservation measures. Thus, soil erosion is always a worldwide research hotspot.

To mitigate severe soil erosion, many soil and water conservation measures have been put into place. Three categories of these measures are tillage, engineering, and biological measures [14,15,16]. Representative tillage measures include contour ridge [17], crop rotation [18], minimal tillage and no-tillage [19], and straw return [20]. Representative engineering measures include terrace [21], fish-scale pit [22], sand barrier [23], and small storage and drainage project on hillslope [24]. Representative biological measures include ecological restoration [25], agricultural protection forest [26], plant hedge [27], and grass waterway [28]. According to [15], annual runoff and soil erosion are typically reduced by 53% and by 84% after applying soil and water conservation measures. However, different measures have unique regional efficacy and applicability. Therefore, more research on the assessment of effectiveness and suitability of soil and water conservation measures is still urgently needed to choose the best measures and configure them properly.

The assessment of effectiveness and suitability is the foundation of scientific use of soil and water conservation measures, which is connected to soil erosion control effects and agricultural sustainable development [29]. While accounting for economic and social benefits, the effectiveness and suitability are primarily determined by ecological benefits [30]. The assessment is a complex process that involves many factors of the social, economic, and natural ecological environment, and each factor includes multiple subfactors that interact with one other [31]. Many indicators have been used to assess different soil and water conservation measures [32,33,34], such as soil erosion degree, soil erosion intensity, soil erosion control area, soil organic matter content, soil conservation rate, biological diversity, frequency of drought and flood disasters, reservoir capacity loss, grain yield, gross farm production, per capita net income, gross value of social products, etc. Nevertheless, not much research has been performed to assess the effectiveness and suitability of these measures. Therefore, it is still challenging to determine which indicators to utilize and what techniques to apply to systematically assess the effectiveness and suitability of soil and water conservation measures.

There are four sizable black soil regions in the world, which are the Northeast Plain in China, the Ukrainian Plain in Ukraine, the Mississippi Plain in the United States, and the Pampas steppe extending from Argentina to Uruguay in South America [35]. Due to their great fertility, these black soil regions produce goods of superior quality [36]. However, serious erosion and degradation are plaguing black soil worldwide [20,37,38,39]. With regard to the black soil region in Northeast China, the primary causes of soil erosion are water force, wind force, and freeze–thaw cycles [5]. According to the China Soil and Water Conservation Bulletin from 2023, the area affected by soil erosion is 208,900 km², accounting for 19.2% of its total land area of 1,087,600 km². Soil erosion is most common in sloping farmland, which accounts for 46.39% of the soil erosion area [40,41]. This can lower the grain yield by up to 14.7% annually [42]. Therefore, both national food security and additional ecological security are directly affected by soil erosion. Soil and water conservation measures can protect the black soil resources and also preserve the capacity of land production [16,36,43]. However, the objective assessment of these measures is particularly deficient because the work was started later than expected in the black soil region of Northeast China.

The process of calculating the comprehensive index involves first choosing representative indicators based on the multilevel continuous assessment [30] and comparative analysis of soil and water conservation [44]. Next, the weight of each indicator value is determined using a combination of subjective weighting (expert experience method, analytic hierarchy process) [45,46] and objective weighting (coefficient of variation method, principal component analysis method) [47,48]. Finally, the comprehensive index is obtained using weighted summation and the geometric average method. A framework model made up of pressure, state, and response indexes is the Pressure–State–Response (PSR) model. The pressure refers to the load caused by human activities to the environment and natural resources; the state refers to the quality of the environment and natural resources; the response refers to the natural, economic, and social measures taken by human beings. The PSR model is typically used to assess flood resilience [49], ecosystem health [50], ecosystem resilience [51], and agricultural sustainability [52]. When assessing the benefits of soil and water conservation measures, the comprehensive index is typically employed, while the PSR model is rarely used.

This study closes the knowledge gap regarding the assessment of effectiveness and suitability of soil and water conservation measures on hillslopes of black soil regions. By analyzing the data based on field monitors, radioactive tracers, simulated rainfall, and simulated inflow from our research team and the other pertinent literatures on the black soil region of Northeast China, the comprehensive index method and PSR model were applied and compared. The aims were (1) to obtain the values of soil erosion reduction ratio, gradation changes of soil erosion intensity, energy value, and proportion of cultivated land occupied; (2) to establish an assessment index system based on the PSR model, and analyze its coordination degrees; and (3) to calculate both the comprehensive indexes and assessment values of the PSR model and categorize the effectiveness and suitability of soil and water conservation measures.

2. Materials and Methods

2.1. Study Area

The study area is situated in the black soil region of Northeast China (Figure 1), which extends across Heilongjiang, Jilin, and Liaoning provinces and into part of the Inner Mongolia Autonomous Region with an area of 1,087,600 km². There is a large distribution of black soil (Chinese Soil Taxonomy) with a sticky soil texture and a high soil organic matter concentration with values of 17.25, 24.12, 26.15, and 40.43 g kg⁻¹ for Liaoning, Inner Mongolia, Jilin, and Heilongjiang, respectively [53,54]. When compared to other regions, the topography’s gentle slope gradients and long slope lengths produce distinct soil erosion processes [7]. The climate of this region is a northern continental monsoon that is moderate and semi-humid. The external agency of soil erosion includes water, wind, freeze–thaw cycles, gravity, etc. [5]. Soil erosion has gotten worse since the 1950s as a result of natural factors and human activities [55].

The mean annual temperature is 4.8 °C. The frost-free period lasts approximately 140 days each year. The mean yearly rainfall fluctuates between 400 and 850 mm, with about 60% of precipitation falling between July and August, and 90% falling between April and September. The maximum wind speeds, which range from 20 to 25 m s⁻¹, happen between March and May. There is a noticeable freeze–thaw cycle as a result of larger daily and annual temperature variations. Gravitational erosion happens all year round; it is most prevalent in the summer. Freeze–thaw action and gravitational erosion usually occur in tandem with water erosion [46]. Soil and water conservation measures mostly include contour ridge, wide ridge, straw return, minimum tillage, no-tillage, terrace, ridge belt, agricultural protection forest, etc., for controlling soil erosion on hillslopes of the black soil region in Northeast China [14].

2.2. Data Sources

The data were obtained from our research team [56,57,58] and other pertinent literature [59,60,61,62,63,64,65] on the Chinese black soil region. These data were from four experimental methods: that is, field monitors, radioactive tracers, simulated rainfall, and simulated inflow. The control treatments included no-ridge (bare fallow), narrow ridge along hillslope, traditional up- and downslope tillage, and the location before the forest belt based on various concrete measures. With regard to each treatment, there were 3–6 repetitions.

For the field monitors [56,59,60,61,62,63,64], natural rainfall processes, wind speeds and directions, temperatures, and other general indicators were recorded using different automatic meteorological stations. The runoff and sediment were observed by traditional runoff collectors such as buckets or automatic collecting devices (xyz-2 type, Harbin, China). The wind-eroded materials were collected by sand-taped instruments (HHH.SCC-X step type, Beijing, China). About 27–51 runoff and sediment samples or wind-eroded materials for each treatment of every year were weighed and oven-dried at 105 °C or 75 °C to calculate the amounts of soil erosion.

For the radioactive tracers [65], the ¹³⁷Cs and ²¹⁰Pb_ex were utilized. Soil samples were taken using the 5 cm diameter soil augers. Next, 704–716 soil samples for each time were put through 2 mm sieves to remove impurities such as grass roots and stones and were crushed after a drying process. The high purity germanium probe γ spectrometer (ORTEC GMX-50220, CANBERRA company, Toledo, USA), which had a measurement range of 3–3 × 10³ keV, was used to test for radioactive elements. Nuclide activity could be used to calculate soil erosion rates.

For simulated rainfall experiments [56], a lateral sprinkler rainfall simulator system was utilized to replicate rainfall. By adjusting the nozzle size and water pressure, rainfall intensities were varied between 30 and 165 mm h⁻¹. The rainfall height was 6 m, and the rainfall uniformity was >85%. A pre-rain with a 30 mm h⁻¹ rainfall intensity was applied to soil pans until surface flow occurred to maintain constant soil water contents. During the rainfall (50 or 100 mm h⁻¹), runoff and sediment samples were measured in 1–2 min intervals. Then, ~30 samples for each treatment were weighed and oven-dried to determine the amounts and rates of runoff and soil erosion.

For simulated inflow experiments [57,58], an overland tank equipped with a clapboard was attached to the upper end of experimental plot to supply inflow water. Additionally, a runoff collector was set up at the base of every runoff plot to gather runoff and sediment samples. Then, ~30 samples for each treatment were weighed and oven-dried to calculate the quantities and rates of runoff and soil erosion.

2.3. Research Methods

2.3.1. Comprehensive Index Method

Four representative indicators, that is, soil erosion reduction ratio, gradation changes of soil erosion intensity, energy value, and proportion of cultivated land occupied, were chosen based on methods of multilevel continuous assessment [30] and comparative analysis of soil and water conservation [44] to evaluate weights and calculate comprehensive indexes.

Each representative indicator’s weight was established by combining the subjective and objective weighting methods [46]. Then, the effectiveness and suitability comprehensive indexes of soil and water conservation measures were assessed using the weighted summation and geometric average method to establish how effective and suitable they were.

2.3.2. Pressure–State–Response Model

• Indicator standardization

There were positive and negative indicators affecting the effectiveness and suitability of soil and water conservation measures. The positive indicator was to promote the effectiveness and suitability, and the higher the value, the better the outcome. The negative indicator was to hinder the effectiveness and suitability, and the lower the value, the better the outcome.

To unify units and dimensions of each indicator, a standardization was required [43]. In this study, the maximum–minimum method was adopted to standardize indicators. The formula was as follows:

$$\mathrm{Positive}\mathrm{indicator}:Y_{ij}=(X_{ij}-X_{i\mathrm{m}\mathrm{i}\mathrm{n}})/\left(X_{i\mathrm{m}\mathrm{a}\mathrm{x}}{-X}_{i\mathrm{m}\mathrm{i}\mathrm{n}}\right)$$

(1)

$$\mathrm{Negative}\mathrm{indicator}:Y_{ij}=\left({X_{i\mathrm{m}\mathrm{a}\mathrm{x}}-X}_{ij}\right)/\left(X_{i\mathrm{m}\mathrm{a}\mathrm{x}}{-X}_{i\mathrm{m}\mathrm{i}\mathrm{n}}\right)$$

(2)

where Y_ij is the standardized value, X_ij is the original value, and X_imin and X_imax are the minimum and maximum values for the i-th indicator of the j-th soil and water conservation measure.

• Determination of assessment indicator weight

The assessment of effectiveness and suitability of soil and water conservation measures was a multi-factor comprehensive assessment process. The assessment region’s features were taken into account when analyzing the relative relevance of each factor in the assessment index system. The mean square error weighting was the primary method used in this study to calculate the weight of each indicator. The specific calculation process was as follows.

First, the degree of dispersion of the random variable y_i was calculated. The calculation formula was

$$E_i=\frac{1}{n}{\displaystyle \sum _{j=1}^n{Y}_{ij}}$$

(3)

where E_i is the mean value of the random variable y_i; j is soil and water conservation measure (j = 1, 2, ……, n); and Y_ij is the standardized value for the i-th indicator of the j-th soil and water conservation measure.

Secondly, the mean square error σ_i of the random variable y_i was calculated. The calculation formula was

$${\sigma }_i=\sqrt{\frac{1}{n}{\displaystyle \sum _{j=1}^n{(Y_{ij}-E_i)}^2}}$$

(4)

Finally, the weight coefficient of Y_ij relative to each subsystem was calculated. The calculation formula was

$$w_i={\sigma }_i/\sum _{i=1}^m{\sigma }_i$$

(5)

where w_i is the weight coefficient for indicator i (i = 1, 2, ……, m) and $\sum _{i=1}^m{\sigma }_i$ is the sum of the mean square error of total m indicators.

• Scores of effectiveness and suitability

The scores of effectiveness and suitability of soil and water conservation measures were calculated based on the formula as follows.

$$D_j={\displaystyle \sum _{i=1}^m{Y}_{ij}{w}_i}$$

(6)

where D_j is the score for the i-th indicator of the j-th soil and water conservation measure.

The indicator weight of the criterion layer was obtained in the same way, and finally, the weight coefficient of each indicator at each level was obtained. Then, assessment values of the pressure, state, response, and further scores of effectiveness and suitability of soil and water conservation measures were calculated.

• Coordination degree analysis of PSR model

The process of putting soil and water conservation measures into place was dynamic, and the only way to increase their efficiency was to repeatedly manage and regulate the situation. The coordination degree between the components of each subsystem of pressure, state, and response determined the effectiveness and suitability of soil and water conservation measures [51]. Any subsystem’s bias would have an effect on its effectiveness and suitability. Thus, this study constructed a coordination degree model of pressure index, state index, and response index and judged the coordination status of the three subsystems. The constructed model was as follows:

$$\mathrm{C}={\displaystyle \frac{Y_p+Y_s+Y_R}{\sqrt{Y_p{ }^2+Y_s{ }^2+Y_R{ }^2}}}$$

(7)

where C is the coordination degree index and Y_P, Y_S, Y_R are, respectively, the assessment scores of pressure, state, and response. The closer the scores of each index system are, the closer the C values are, and the higher the assessment value is, indicating the stronger the effectiveness and suitability of soil and water conservation measures.

2.3.3. Categorization of Effectiveness and Suitability

According to the quartile method (25%, 50%, 75%, and 100%) [66], four classes for effectiveness and suitability of soil and water conservation measures were separated: that is, ineffective and unsuitable (<25%), moderately effective and suitable (25–50%), relatively effective and suitable (50–75%), and very effective and suitable (>75%) classes for both the comprehensive index method and the PSR model.

2.4. Defining Indicators

2.4.1. Soil Erosion Reduction Ratio

Soil erosion reduction ratio refers to the proportion of soil erosion reduction amount of a soil and water conservation measure that is compared to that of the control treatment. The specific formula is

$$∆S={\displaystyle \frac{S_C-S_i}{S_C}}\times 100\%$$

(8)

where ∆S is the soil erosion reduction ratio (%); S_C is the soil erosion amount of control treatment (t km⁻² y⁻¹); and S_i is the soil erosion amount of the i-th soil and water conservation measure (t km⁻² y⁻¹).

2.4.2. Gradation Changes of Soil Erosion Intensity

Soil erosion intensity is classified into six classes by using the mean soil erosion modulus (t km⁻² y⁻¹), which are slight (≤200), light (200–1200), middle (1200–2400), strong (2400–3600), very strong (3600–4800), and severe (>4800) soil erosion based on ‘Techniques standard for comprehensive control of soil erosion in the black soil region (SL 446—2009)’ [67]. Then, the gradation changes of soil erosion intensity are calculated by subtracting the number of lower levels from the number of higher levels.

2.4.3. Energy Value

Energy analysis is to transform all available energy from different products and resources into solar energy [68]. The energy analysis of soil and water conservation measures on hillslopes is to transform different forms of input into a unified energy value standard to create a unified assessment of various types of substances, energy, and information.

Based on the information gathered from the questionnaire, energy analysis was used to quantify the energy values of soil and water conservation measures on hillslopes. The specific formula is

$$E=TB$$

(9)

where E is the solar energy (sej); T is the solar energy conversion rate (sej/g or sej/J); and B is the mass (g) or energy (J) contained in a substance.

The energy calculation table [69] is used to compute the solar energy conversion rates (T). The mass or energy values of a substance (B) were computed based on the mechanical fuel consumption value, as well as other costs including labor, machinery, fertilizer, and seed costs. The input was then converted to the mechanical fuel consumption value.

2.4.4. Proportion of Cultivated Land Occupied

The proportion of cultivated land occupied refers to the ratio of land area taken by a soil and water conservation measure that cannot be cultivated to the total area of the original cultivated land. The specific formula is

$$∆A={\displaystyle \frac{A_i}{A_0}}\times 100\%$$

(10)

where ∆A is the proportion of cultivated land occupied (%); A_i is the land area taken by the i-th soil and water conservation measure that cannot be cultivated (km²); and A₀ is the total area of original cultivated land (km²).

3. Results and Discussion

3.1. Assessment of the Effectiveness and Suitability of Soil and Water Conservation Measures Based on the Comprehensive Index Method

In comparison to the control treatments, the soil erosion reduction ratios of soil and water conservation measures on hillslopes in the Chinese black soil region were 34.5–99.6%, with an average of 84.1% (

Table 1. Indicators of soil and water conservation measures on hillslopes and their categorization for comprehensive index method.

Soil and Water Conservation Measure	Experimental Condition	Control Treatment	Soil Erosion Modulus in Control Treatment (t km−2 y−1)	Soil Erosion Modulus in Measure Treatment (t km−2 y−1)	Soil Erosion Reduction Ratio (%)	Gradation Changes of Soil Erosion Intensity	Energy Value (108 sej)	Proportion of Cultivated Land Occupied (%)	Categorization of Effectiveness and Suitability
Contour ridge [62]	Field monitor	No-ridge (bare fallow)	1225.1	20.8	98.3	From middle to slight 1	3.24	0	Relatively effective and suitable 2
Wide ridge [64]	Field monitor	Narrow ridge along hillslope	501.6	172.5	65.6	From light to slight	3.74	0	Moderately effective and suitable
Straw mulch [58]	Simulated inflow	No-ridge (bare fallow)	1613.8	1057.0	34.5	From middle to slight	1.87	0	Moderately effective and suitable
Deep application of straw [56]	Simulated rainfall	Traditional up- and downslope tillage	13,672.0	2176.5	84.1	From severe to middle	2.81	0	Relatively effective and suitable
Smashed straw turnover [56]	Simulated rainfall	Traditional up- and downslope tillage	13,672.0	1376.0	89.9	From severe to middle	2.34	0	Relatively effective and suitable
No-tillage + straw mulch [63]	Field monitor	Traditional up- and downslope tillage	43,382.3	177.3	99.6	From severe to slight	1.87	0	Very effective and suitable
Plant hedge [57]	Simulated inflow	No-ridge (bare fallow)	4421.0	822.0	81.4	From very strong to light	9.85	6	Relatively effective and suitable
Ridge–furrow intervals [61]	Field monitor	Traditional up- and downslope tillage	456.0	112.0	75.4	From light to slight	3.27	0	Moderately effective and suitable
Minimal tillage with residue mulching [63]	Field monitor	Traditional up- and downslope tillage	43,382.3	4188.7	90.3	From severe to very strong	1.87	0	Relatively effective and suitable
Bench terrace [60]	Field monitor	No-ridge (bare fallow)	4962.0	130.0	97.4	From severe to slight	74.80	16.5	Relatively effective and suitable
Ridge belt [59]	Field monitor	Traditional up- and downslope tillage	5947.0	123.0	97.9	From severe to slight	19.70	6	Very effective and suitable
Agricultural protection forest [65]	Radioactive tracer	The location before the forest belt	5.3	0.3	94.4	From slight to slight	1.06	0	Relatively effective and suitable

¹ Soil erosion intensity is divided into six classes using the mean soil erosion modulus (t km⁻² y⁻¹), which are slight (≤200), light (200–1200), middle (1200–2400), strong (2400–3600), very strong (3600–4800), and severe (>4800) soil erosion based on ‘Techniques standard for comprehensive control of soil erosion in the black soil region (SL 446—2009)’ [67]. ² The comprehensive index method was used to categorize the effectiveness and suitability into four classes according to the quartile method [66]: ineffective and unsuitable (<25%), moderately effective and suitable (25–50%), relatively effective and suitable (50–75%), and very effective and suitable (>75%).

). This finding was comparable to that reported by Xiong et al. [15], who found that yearly soil erosion reduction ratios were typically 84% based on a meta-analysis of 1589 sample plots across 22 countries to identify soil and water conservation measures.

The soil erosion intensity for various soil and water conservation measures could be gradually reduced from a high level to a low level [70]. In this study, the mean gradation changes of soil erosion intensity were 2–3 levels (Table 1). In the practical application of soil and water conservation measures, two or more measure types were often used for spatial allocation [16,71] because any individual measure could not meet the soil loss tolerance standard (≤200 t km⁻² y⁻¹) [43]. Quantifying soil erosion intensity enabled an assessment of effectiveness and suitability of soil and water conservation measures.

Energy values were from 1.06 × 10⁸ to 74.80 × 10⁸ sej for different soil and water conservation measures (Table 1). The capital investment in building terraces was the largest [72], and the input energy value was also the largest. Ridge tillage with common cost was ploughed and re-ridged each year. Straw mulch was also carried out in some areas every year [20], mainly during the autumn harvest, which costed mediumly. The agricultural protection forest measure did not need to be repaired every year after completion and could also create income [73], and thus, its cost was the lowest. Soil and water conservation measures with high energy values mainly included bench terrace, ridge belt, and ridge cultivation in low hilly areas. Those with medium energy values mainly included straw mulch and conventional ridge tillage. Those with low energy values mainly included agricultural protection forests. Therefore, energy values of different measures should be fully considered in the allocation of hillslope soil and water conservation measures.

The majority of soil and water conservation measures neither increase nor decrease the amount of land under cultivation [51], with the exception of ridge belt and plant hedge, which occupied approximately 6% of the cultivated land (Table 1). Moreover, the bench terrace occupied a larger percentage of cultivated land, between 13 and 20% on average [16]. Thus, the proportion of cultivated land occupied should also be considered to increase farmers’ acceptance of the measures.

Comparing comprehensive indexes and categorizing the effectiveness and suitability of various soil and water conservation measures yielded the following results: very effective and suitable measures included no-tillage + straw mulch and ridge belt; relatively effective and suitable measures included bench terrace, contour ridge, smashed straw turnover, deep application of straw, plant hedge, minimal tillage with residue mulching, and agricultural protection forest; and moderately effective and suitable measures included ridge–furrow intervals, wide ridge, and straw mulch (Figure 2, Table 1). It was noteworthy that no ineffective and unsuitable soil and water conservation measures were discovered. In the practice of soil and water conservation, the best soil and water conservation measures could be chosen based on the effectiveness and suitability categorizations mentioned above.

3.2. Assessment of Effectiveness and Suitability of Soil and Water Conservation Measures Based on the PSR Model

The PSR model takes into account the relationship between pressure (runoff, soil erosion, freeze–thaw conditions, etc.), state (soil organic matter content, soil nitrogen and phosphorus content, soil pH, etc.), and response (runoff reduction ratio, soil erosion reduction ratio, etc.) and has strong flexibility and practicability [51]. From an empirical perspective, this study constructed a scientific assessment index system based on the PSR model, chose a fair assessment method, and analyzed the effectiveness and suitability of various soil and water conservation measures on hillslopes on the basis of a substantial amount of experimental data. These findings could serve as a guide for the selection of scientific soil and water conservation measures in the black soil region.

3.2.1. Assessment Index System of Effectiveness and Suitability

Principles of scientificity, feasibility, independence, completeness, simplicity, comparability, and hierarchy should be adhered to while completely taking assessment objectives into account. Then, to build an assessment index system, indicators that could accurately convey the scientific meaning of the effectiveness and suitability of soil and water conservation measures were carefully chosen. Target, criterion, and indicator layers made up the index system, which also included pressure, status, and reaction index systems with various specialized indicators (Figure 3).

For the specific index selection, a frequency statistical method was adopted [74]. Corresponding indicators were listed after reviewing the literature on the assessment of benefits of soil and water conservation measures. Local farmers and specialists with extensive practical experience were then requested to take part in the questionnaire survey. The background, purpose, and basic principles of indication selection were informed to farmers and specialists. According to their own expertise and experience, they selected the indicators that significantly influenced soil and water conservation measures as a crucial point of reference for choosing assessment indicators. Ultimately, the assessment index system was constructed by choosing indicators with a high frequency of occurrence based on a thorough study of local conditions (

Table 2. Assessment indicator system of the effectiveness and suitability of soil and water conservation measures on hillslope for the Pressure–State–Response model.

Target Layer	Criterion Layer	Indicator Layer	Weight Function		Attribute of Indicator
			50 mm h−1	100 mm h−1
Effectiveness and suitability of soil and water conservation measures on hillslope	Pressure	A1 Soil erosion rate (g m−2 h−1)	0.12	0.13	Reverse indicator
		A2 Runoff (L)	0.14	0.13	Reverse indicator
	State	A3 Soil organic matter content (g kg−1)	0.12	0.12	Forward indicator
		A4 Soil total nitrogen (g kg−1)	0.14	0.13	Forward indicator
		A5 Soil total phosphorus (g kg−1)	0.12	0.13	Forward indicator
	Response	A6 Energy value (sej)	0.12	0.12	Reverse indicator
		A7 Runoff reduction ratio (%)	0.12	0.12	Forward indicator
		A8 Soil erosion reduction ratio (%)	0.12	0.12	Forward indicator

). Among them, the energy value of soil and water conservation measures in the response index was introduced for the first time in the assessment index system of effectiveness and suitability. The runoff reduction ratio and soil erosion reduction ratio were the mean indexes of reducing runoff and soil erosion obtained by methodically comparing multiple soil and water conservation measures.

3.2.2. Assessment of Effectiveness and Suitability

By using the aforementioned assessment method and model, the original data of eight indicators for contour ridge, wide ridge, deep application of straw, smashed straw turnover, no-tillage, and no-tillage + straw mulch were processed, respectively, to determine standardized values for each indicator and assessment values for each measure (

Table 3. Original data and standardized values of each indicator of soil and water conservation measures at the 50 mm h⁻¹ rainfall intensity.

Indicator Layer 1	Original Data (g m−2 h−1)						Standardized Value
	Contour Ridge	Wide Ridge along Hillslope	Deep Application of Straw	Smashed Straw Turnover	No-Tillage	No-Tillage + Straw Mulch	Contour Ridge	Wide Ridge along Hillslope	Deep Application of Straw	Smashed Straw Turnover	No-Tillage	No-Tillage + Straw Mulch
A1	7.53	900.30	113.55	77.84	68.58	28.10	1.00	0.00	0.88	0.92	0.93	0.98
A2	5.04	41.62	26.57	18.10	37.43	14.13	1.00	0.00	0.41	0.64	0.11	0.75
A3	24.18	37.20	25.90	25.90	25.52	30.85	0.00	1.00	0.13	0.13	0.10	0.51
A4	1.27	1.30	1.25	1.31	1.27	1.28	0.27	0.91	0.00	1.00	0.27	0.55
A5	0.52	0.55	0.51	0.46	0.49	0.52	0.70	1.00	0.60	0.06	0.41	0.70
A6	2.34	3.27	2.81	2.34	3.74	1.21	0.93	0.88	0.90	0.93	0.85	1.00
A7	97.70	66.40	37.00	57.10	11.12	66.60	1.00	0.66	0.34	0.56	0.05	0.66
A8	99.80	72.20	87.20	91.20	92.25	96.80	1.00	0.55	0.79	0.86	0.88	0.95

¹ A1—Soil erosion rate (g m⁻² h⁻¹), A2—Runoff (L), A3—Soil organic matter content (g kg⁻¹), A4—Soil total nitrogen (g kg⁻¹), A5—Soil total phosphorus (g kg⁻¹), A6—Energy value (sej), A7—Runoff reduction ratio (%), A8—Soil erosion reduction ratio (%).

,

Table 4. Original data and standardized values of each indicator of soil and water conservation measures at the 100 mm h⁻¹ rainfall intensity.

Indicator Layer 1	Original Data (g m−2 h−1)						Standardized Value
	Contour Ridge	Wide Ridge along Hillslope	Deep Application of Straw	Smashed Straw Turnover	No-Tillage	No-Tillage + Straw Mulch	Contour Ridge	Wide Ridge along Hillslope	Deep Application of Straw	Smashed Straw Turnover	No-Tillage	No-Tillage + Straw Mulch
A1	16.09	1941.94	321.73	197.42	148.69	89.66	1.00	0.00	0.84	0.91	0.93	0.96
A2	5.21	43.77	28.22	22.04	41.13	15.56	1.00	0.00	0.40	0.56	0.07	0.73
A3	24.18	37.20	25.90	25.90	25.52	30.85	0.00	1.00	0.13	0.13	0.10	0.51
A4	1.27	1.30	1.25	1.31	1.27	1.28	0.00	0.00	0.00	0.00	0.00	0.00
A5	0.52	0.55	0.51	0.46	0.49	0.52	0.70	1.00	0.60	0.06	0.41	0.70
A6	2.34	3.27	2.81	2.34	3.74	1.21	0.93	0.88	0.90	0.93	0.85	1.00
A7	97.70	56.80	36.60	50.60	7.11	64.90	1.00	0.55	0.33	0.48	0.00	0.64
A8	99.10	66.90	82.60	89.30	91.96	96.80	1.00	0.34	0.66	0.80	0.85	0.95

¹ A1—Soil erosion rate (g m⁻² h⁻¹), A2—Runoff (L), A3—Soil organic matter content (g kg⁻¹), A4—Soil total nitrogen (g kg⁻¹), A5—Soil total phosphorus (g kg⁻¹), A6—Energy value (sej), A7—Runoff reduction ratio (%), A8—Soil erosion reduction ratio (%).

and

Table 5. Assessment values of the effectiveness and suitability of soil and water conservation measures on hillslopes and their categorization for the Pressure–State–Response model.

Soil and Water Conservation Measure	50 mm h−1					100 mm h−1
	Pressure	State	Response	Total Value	Categorization of Effectiveness and Suitability	Pressure	State	Response	Total Value	Categorization of Effectiveness and Suitability
Contour ridge	0.26	0.12	0.35	0.73	Relatively effective and suitable	0.25	0.09	0.36	0.70	Relatively effective and suitable 1
Wide ridge along hillslope	0.00	0.37	0.25	0.62	Relatively effective and suitable	0.00	0.25	0.22	0.46	Moderately effective and suitable
Deep application of straw	0.16	0.09	0.24	0.50	Moderately effective and suitable	0.16	0.09	0.23	0.48	Moderately effective and suitable
Smashed straw turnover	0.20	0.16	0.28	0.64	Relatively effective and suitable	0.19	0.02	0.27	0.48	Moderately effective and suitable
No-tillage	0.13	0.10	0.21	0.44	Moderately effective and suitable	0.13	0.06	0.21	0.40	Moderately effective and suitable
No-tillage + straw mulch	0.22	0.22	0.31	0.76	Very effective and suitable	0.21	0.15	0.32	0.68	Relatively effective and suitable

¹ Categorization of effectiveness and suitability is also divided into four classes, which are ineffective and unsuitable (<0.25), moderately effective and suitable (0.25–0.5), relatively effective and suitable (0.5–0.75), and very effective and suitable (>0.75) class [66].

). Then, the effectiveness and suitability of different soil and water conservation measures were clearly demonstrated. According to the findings, assessment values of the effectiveness and suitability of soil and water conservation measures were 0.40–0.76, or 0.44–0.76 and 0.40–0.70 under 50 mm h⁻¹ and 100 mm h⁻¹ rainfall intensity, respectively. The rationale was that increased rainfall intensity led to increased hillslope runoff at higher velocities [9], which increased soil erosion and impacted the effectiveness and suitability of measures [75]. Therefore, with an increase in rainfall intensity from 50 to 100 mm h⁻¹, assessment values of the effectiveness and suitability of soil and water conservation measures generally decreased, particularly for wide ridge and smashed straw turnover, where assessment values declined by 25–26%. Nonetheless, assessment values of no-tillage + straw mulch and contour ridge still continued to display a high level.

The following is the ranking of assessment values for the effectiveness and suitability of soil and water conservation measures: no-tillage + straw mulch > contour ridge > smashed straw turnover > wide ridge > deep application of straw > no-tillage (Table 5). The reason was that the former measures could clearly boost infiltration and control runoff to prevent its collection in contrast to the latter ones [17,43]. The effectiveness and suitability of soil and water conservation measures were also categorized into four classes, which are ineffective and unsuitable (<0.25), moderately effective and suitable (0.25–0.5), relatively effective and suitable (0.5–0.75), and very effective and suitable (>0.75). By comparing the categorization, for the 50 mm h⁻¹ rainfall intensity, a very effective and suitable measure was no-tillage + straw mulch. Relatively effective and suitable measures included contour ridge, smashed straw turnover, and wide ridge. Moderately effective and suitable measures included deep application of straw and no-tillage. For the 100 mm h⁻¹ rainfall intensity, there was no very effective and suitable measure because greater rainfall intensity with greater rainfall erosivity and runoff erosivity was difficult to control [9]. Relatively effective and suitable measures included contour ridge and no-tillage + straw mulch. Moderately effective and suitable measures included smashed straw turnover, deep application of straw, wide ridge, and no-tillage. The categorization of the effectiveness and suitability of soil and water conservation measures showed different results because sloping farmland was more sensitive to changes in rainfall intensity [43]. This indicated that the same measure might fall into different classes depending on rainfall intensities [75,76]. Thus, reference to the categorization standard of this study is helpful for choosing soil and water conservation measures.

3.2.3. Coordination Degree Analysis of PSR Model

After calculating the coordination degrees of the PSR model [77] of various soil and water conservation measures (

Table 6. System coordination degrees of soil and water conservation measures for the Pressure–State–Response model at 50 and 100 mm h⁻¹ rainfall intensities.

Rainfall Intensity (mm h−1)	Contour Ridge	Wide Ridge along Hillslope	Deep Application of Straw	Smashed Straw Turnover	No-Tillage	No-Tillage + Straw Mulch
50	1.62	1.38	1.52	1.58	1.54	1.71
100	1.56	1.41	1.53	1.45	1.58	1.66

), the weight analysis of the coordination degree was performed using the same methodology as the categorization of assessment values based on the quartile method [66]. According to the assessment, the soil and water conservation measures fell into four categories: ineffective and unsuitable (coordination degree < 1.32), moderately effective and suitable (coordination degree 1.32–1.53), relatively effective and suitable (coordination degree 1.53–1.64), and very effective and suitable (>1.64) class.

By comparing the coordination degrees of the PSR model, it was discovered that when rainfall intensity was relatively low, a very effective and suitable measure was no-tillage + straw mulch; relatively effective and suitable measures included contour ridge, smashed straw turnover, and no-tillage; moderately effective and suitable measures included deep application of straw and wide ridge (Table 5). When rainfall intensity was relatively high, a very effective and suitable measure was still no-tillage + straw mulch; relatively effective and suitable measures included no-tillage, contour ridge, and deep application of straw; moderately effective and suitable measures included smashed straw turnover and wide ridge. There was no ineffective and unsuitable measure also because of greater rainfall erosivity and runoff erosivity [9].

The coordination degrees of the PSR model generally declined in the following order: no-tillage + straw mulch > contour ridge > no-tillage > smashed straw turnover > deep application of straw > wide ridge (Table 5). The smashed straw turnover, no-tillage + straw mulch, and contour ridge measures showed a decrease in coordination degrees of 2.9–8.2%. However, the no-tillage, wide ridge, and smashed straw turnover showed an increase in coordination degrees of 0.7–2.6% when rainfall intensity was increased from 50 to 100 mm h⁻¹. The results demonstrated improved coordination between the three subsystems [77], as indicated by the coordination degrees of the PSR model. This finding might be a reflection of the practical and scientific assessment of the effectiveness and suitability of the hillslope soil and water conservation measures.

In terms of assessment values of the effectiveness and suitability of soil and water conservation measures and coordination degrees of the PSR model, no-tillage + straw mulch and contour ridge were the most effective and suitable soil and water conservation measures on hillslope for treatments with different rainfall intensities. It was necessary to assess the effectiveness and suitability of the other soil and water conservation measures in light of the particular circumstances, as they did not manifest an obvious difference.

4. Conclusions

This study focused on soil and water conservation measures on hillslopes in the black soil region to assess their effectiveness and suitability by using the comprehensive index method and the PSR model. Soil erosion reduction ratio, gradation changes of soil erosion intensity, energy value, and the proportion of cultivated land occupied were chosen to compute comprehensive indexes. The findings revealed that the mean soil erosion reduction ratio was 84.1%, with ranges from 34.5 to 99.6%. The gradation changes of soil erosion intensity were typically 2–3 levels. The energy values decreased in the order of bench terrace, ridge belt, ridge cultivation in low hilly areas, straw mulch, conventional ridge tillage, and agricultural protection forest. The proportions of cultivated land occupied were roughly 6% for the ridge belt and plant hedge and 13–20% for the bench terrace. By comparing the comprehensive indexes and categorization of the effectiveness and suitability of various soil and water conservation measures, very effective and suitable measures included no-tillage + straw mulch and ridge belt. Additionally, an assessment index system was developed based on the PSR model. Its assessment values of the effectiveness and suitability ranged from 0.40 to 0.76, and they were arranged as follows: no-tillage + straw mulch > contour ridge > smashed straw turnover > wide ridge > deep application of straw > no-tillage. The coordination degrees of the PSR model, which ranged from 1.38 to 1.71, might be indicative of the findings of the scientific and practical assessments. The two methods corroborated each other’s categorizations of the effectiveness and suitability of soil and water conservation measures. Thus, appropriate soil and water conservation measures might be chosen based on the categorizations of effectiveness and suitability. In future studies, the effectiveness and suitability should be further assessed in allocation of hillslope soil and water conservation measures according to different geomorphic or soil erosion types.