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 water infiltration is an important hydrological process influencing mountain ecosystems’ runoff and soil loss. Our study evaluates soil water infiltration characteristics and the underlying mechanisms under Carya cathayensis plantations with different planting years: 6 (CC 6 years), 20 (CC 20 years), and 50 (CC 50 years). Native forest was chosen as control (CC 0 year). Philip, Kostiakov, Kostiakov-lewis, and Horton models were selected to evaluate their applicability under different planting years of Carya cathayensis plantations. The results showed that: (1) Converting native forests to Carya cathayensis plantations significantly decreased soil water infiltration rates, with average infiltration rate ( AIR), initial infiltration rate ( IIR), and stable infiltration rate ( SIR) decreasing by 33.11% ~ 70.98%, 31.23% ~ 64.11%, and 40.13% ~ 75.01%, respectively；(2) Soil water infiltration rates were improved with planting years, with the highest value shown under CC 50 years；(3) The correlation and path analysis indicated that IIR and SIR was mainly affected by soil non-capillary porosity, while AIR was mainly influenced by 5-2 mm water stable aggregate fraction; (4) Kostiakov model is most suitable for modeling soil infiltration characteristics in the studied area. These findings shed insight into soil water infiltration processes and regulating factors for preventing and controlling soil erosion and restoring soil health in land use change, particularly forest cover conversion in the Dabie Mountain area.

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.