Responses of soil extracellular enzyme activities to simulated erosion and fertilization in the Chinese black soil region

Soil extracellular enzyme activities play a crucial role in soil organic matter decomposition and nutrient cycling. Soil erosion is a major threat to soil health and sustainable crop productivity worldwide, the use of chemical fertilizers is an important management practice to combat soil degradation and increase crop yields. How soil extracellular enzyme activities respond to the combined soil erosion and fertilization, and whether these enzyme activities can be used as indicators of soil health under erosion, remains unclear. Based on a long-term field simulated erosion experiment (2005-2022) in the black soil region of northeast China, 12 soil extracellular enzyme activities [ including 5 C-acquiring enzymes: β-1,4-glucosidase (BG), β-D-cellobiohydrolase (CBH), β-xylosidases (BX), α-glycosidase (α-GC) and invertase; 3 N-acquiring enzymes: β-1,4-N-acetylglucosaminidase (NAG), leucine amino peptidase (LAP) and urease; 1 P-acquiring enzyme: acid phosphatase (ACP); and 3 oxidative enzymes: catalase, phenol oxidase (POX), peroxidase (POD)] and relevant soil physicochemical and microbial properties were carried out along the simulated erosion gradient under both unfertilized and fertilized conditions during the fallow season. The results showed that: (1) Simulated erosion significantly decreased the activities of invertase, urease, ACP, and POD, and had no effect on the activities of the other 8 enzymes. Long-term fertilization significantly increased the activities of urease and NAG and decreased ACP, catalase, and POD activities. No interactions between simulated erosion and fertilization were found for all measured 12 extracellular enzyme activities. (2) The decreased activities of invertase and urease were mainly caused by the decrease in soil organic matter (SOM), no matter fertilized or not. Decreased ACP activity was mainly attributed to SOM under fertilization, but to pH and BD under no fertilization. POD activity was mainly determined by alkali-hydrolyzed N (AN) under fertilization, but none of the measured soil properties can explain the changes in POD activity under no fertilization. (3) The activities of invertase, urease, and ACP were significantly and positively correlated with multiyear average crop yield across the simulated erosion gradient under both fertilization conditions, POD activity was only positively correlated with average yield under fertilization. The results highlight that invertase, urease, and ACP activities can be used as potential indicators of erosion-induced black soil degradation, fertilizer application can improve soil health of degraded black soils partly by increasing invertase and urease activities and crop yield.