Soil degradation is an essential environmental problem that threatens agricultural productivity and sustainability in the typical black soil region in northeast China. Assessing the degradation status and risks of sloping cropland is necessary to restore and reconstruct degraded soil. This study aimed to map soil degradation status based on a newly developed soil degradation index (SDI) and assess the uncertainty associated with degradation risk mapping using Ordinary Kriging and Sequential Gaussian simulation (SGS). The results showed that SDI ranged from 0.013 to 0.864, with a mean value of 0.445. The realization with 200 times by SGS was the best, with an increasing pattern of SDI from north to south in the study area. Mildly degraded and moderately degraded sloping cropland accounted for 35.3% and 52.4% of the total area of sloping cropland, respectively. High-risk areas were mainly distributed in the Greater Xingan Mountains and Changbai Mountains to the Songnen Plain. Proactive measures are necessary to control and mitigate further degradation by developing soil management and weakening the interaction between slope gradient and ridge-slope angle. The developed risk assessment maps provide baseline information for regional sustainable development.

Soil infiltration is a hydrological process dramatically affected by land use/cover changes. Taking the miscellaneous woody forest (MWF), a natural vegetation cover in the Dabie mountainous area, China, as the reference (control), this study aimed to evaluate the effect of litter, root traits, and soil properties on soil infiltrability and quantify the characteristics of soil infiltration processes of four intensively-managed sloping economic forests, including Castanea Mollissima forest (CMF), Castanea Mollissima-tea inter-planting forest (CMTF), Camellia Oleifera forest (COF), and Camellia Oleifera-tea inter-planting forest (COTF). The results showed that significant differences in soil water infiltration under different land use and management types (p < 0.05). Soil initial infiltration rate (IIR), average infiltration (AIR) and steady infiltration rate (SIR) decreased 39.63%-60.88%, 46.11%-67.39% and 49.88%-72.8%, respectively after the native forests were developed to economic forests, following the order: MWF > CMTF > COTF > CMF > COTF, but no significant difference was found between CMTF and COTF (p > 0.05). Soil water infiltration characteristics measured by IIR, AIR, and SIR were positively correlated to litter coverage, litter thickness, the maximum water-holding capacity of the litter, effective water-retention capacity, maximum water-retention capacity, root traits, capillary porosity, non-capillary porosity, total porosity, soil organic matter, and clay and silt content (p < 0.05), but negatively correlated with the bulk density, and sand and gravel content (p < 0.05). Variation partitioning analysis showed that soil water infiltration was more susceptible to soil properties than litter characteristics or root traits. The redundancy analysis explained 99.97 % of the variation to explore the relationship between litter characteristics, root traits, soil properties, and soil water infiltration characteristics, and the clay content is the main factor affecting soil water infiltration. The structural equation model suggested that land use and management indirectly affect soil infiltrability by modulating soil properties through affecting litter and root traits, with clay content having a higher contribution.

Clearing the status of sloping cropland degradation in typical black soil region is the basis for restoring and reconstructing degraded black soil. To date, the spatial distribution pattern of degree of soil degradation is lacking. In this paper, we predicted the spatial distribution of degree of soil degradation based on the soil degradation index (SDI). We compared the accuracy of Ordinary Kriging (OK) and Sequential Gaussian Simulation (SGS) with different simulation times. SGS was also used to evaluate the degradation risk of sloping cropland in typical black soil region of Northeast China. The results showed that: SDI ranged from 0.013 to 0.864, with a mean value of 0.445. The realization with 200 times by SGS was the best, with an increasing pattern of SDI from north to south in the study area. Mildly degraded soil, moderate degraded soil, severe degraded soil, and extremely severe degraded soil accounted for 35.3%, 52.4%, 12.1%, and 0.2% of total sloping cropland, respectively. The potential degradation risk and high-risk areas were about 53.4% and 23.1% of the total sloping cropland, respectively. The result of risk assessment could serve for making decisions by relevant departments.

Soil microorganisms are the main driving force in soil ecological and biogeochemical processes, such as carbon mineralization, humus formation, and nutrient cycles. Soil erosion may dramatically alter microbial properties. However, it is still unclear how soil microbial characteristics affected by soil erosion. In hilly areas, soil erosion is highly attributed to topographic characteristics such as slope position and slope gradient. Herein, we compared the soil microbial characteristics between different slope gradients and positions in the farmland in the typical black soil area of Northeast China. The relative abundances of major microbial phyla are illustrated. Proteobacteria, Acidobacteria, Gemmatimonadetes, Planctomycetes, and Actinobacteria accounted for 83% of the total bacterial sequences, and Ascomycota, Basidiomycota, and chytridiomycosis accounted for 78% of total fungal sequences in the studied area. Microbial alpha-diversity (Bacteria and Fungi) was similar among the slope gradients or slope positions. The bacterial and fungal community composition was only affected by slope gradient, which was significantly lower in the highest slope gradient level than in others. This study suggests that slope gradient but not slope gradient affecting microbial community composition in the cropland of black soil region.

A comprehensive evaluation indicator system is needed to provide a integrated assessment of the degree of sloped cropland degradation. We employed bibliometrics to perform statistical analysis on research studies involving cropland degradation. Frequency analysis was then used to identify high-frequency indicators with which to construct a total index set (TIS) for evaluation of the degree of sloped cropland degradation in black soil region. In addition, soil measurement data from sloped cropland in Baiquan and Keshan Counties, Heilongjiang province, China, were used as a basis to construct a minimum index set (MIS). The TIS included A-horizon thickness, clay content, organic matter content, pH, slope gradient, ridge-slope angle, gully density, bulk density, large water-stable aggregate content, soil cation exchange capacity, and crop yield. The first six of these were included in the MIS. In the studied area, undegraded soil, mildly degraded soil, and moderately degraded soil and above accounted for 7%, 48% and 45% of investigated sloped croplands, respectively. Slope gradient is one of the main factors affecting soil degradation. Soil degradation mainly presented as worsening of soil physicochemical characteristics. In addition, downslope and small-angle ridge cultivation are benefit for soil organic matter maintenance and the soil structure and nutrient retention capacity is better than soil with contour or large-angle ridge cultivation. The reason might be that downslope and small-angle ridge cultivation are usually employed on soil with small slope. The study results provide a scientific basis for improving the quality and productivity of sloped cropland in black soil region.