Characteristics of Erosion and Degradation for Farmlands in the Typical Water Erosion Area of the Black Soil Region, Northeast China

[Objective] Quantitatively evaluating the magnitude of soil erosion and degradation in the black soil region of Northeast China, can deliver benefits to accurately implement conservation practices in different areas. [Methods] The farmlands in the typical water erosion area of the black soil region of Northeast China were taken as the research objects, and the caesium-137 (137Cs) tracing method was applied to quantify soil erosion rate (SER) for different sampling sites. Based on the three categories of black soil degradation, i.e., thinner, poorer, and harder, seven representative indicators were selected, including SER, pH, soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP), total potassium (TK), and bulk density (BD). A soil degradation index (SDI) was developed by using total dataset (TDS) and fuzzy membership function methods. [Results] (1) The average SER for different sites displayed an order of Bin County [BX, 8 417.86 t/(km2·a)]>Hailun [HL, 7 083.96 t/(km2·a)]>Gongzhuling [GZL, 3 168.71 t/(km2·a)]>Beian [BA, 2 786.09 t/(km2·a)]>Longjiang [LJ, 2 722.46 t/(km2·a)]. According to the SER, BX and HL were classified as the intense erosion degree, whereas GZL, BA and LJ were classified as the moderate erosion degree; (2) There were significant and negative power function relationships between SER and SOM, TN, TP, but a significant and positive logarithmic relationship between SER and BD (p < 0.05), with R2 ranging from 0.222 to 0.532, which indicated that soil erosion substantially exacerbates nutrient loss and soil hardening in black soils; (3) The processes of thinner, poorer, and harder showed significant interactions in the farmlands of black soils. The SDI for different sites followed an order of BX (0.44) > HL (0.36) > LJ (0.21) > GZL (0.10) > BA (0.05). [Conclusion] The findings can provide scientific support for the diagnosis, evaluation and accurate mitigation of soil erosion and degradation in the black soil region of Northeast China.