Objectives Soil extracellular enzyme stoichiometry reflects nutrient availability and nutrient limitation. We explored the effect of organic materials on the characteristics of soil enzyme activity to understand soil nutrient migration and transformation in cultivated land affected by erosion.

Methods The experiment was based on a 8-year runoff observation experiment in Southwest China. The four treatments on the field were no straw and organic fertilizer control (CK), 20% nitrogen substituted by organic fertilizer (OM), 20% nitrogen substituted by straw (SW), 20% nitrogen substituted by straw and organic fertilizer (OMSW). Soil samples were collected from 0−20 cm plough layer in erosion and deposition section of each treatment, respectively. Soil enzyme activity, particle composition, and content of C, N and P were measured. The stoichiometric characteristics of soil extracellular enzyme activity, and correlation of soil nutrient stoichiometric ratio and soil erodibility factors were analyzed.

Results The activities of soil β-glucosidase (BG), β-glucosaminnidase (NAG), L-leucine aminopeptidase (LAP) and phosphatase (AKP) in the deposition section of each treatment were 52.31%, 50.60%, 16.97% and 13.11% higher than those in the erosion section, respectively. The soil extracellular enzyme activity ratio was 1∶0.79∶1.13. The soil enzyme activities of the four treatments were SW>OMSW>CK>OM. The vector angles of the four treatments were all represented as erosion area>deposition area (P<0.05), and the vector length was represented as erosion area<deposition area (P<0.05). All the vector angles of the four treatments were greater than 45°, and the vector lengths were above the 1∶1 line. The sloping farmland was generally limited by P (nutrient) and energy (carbon).

Conclusions Erosion accelerated the turnover rate of soil carbon, nitrogen and phosphorus in the sloping farmland. The activity of soil extracellular enzymes was significantly higher in deposition section than that in eroded section. The sloping farmland was generally nutrient-limited (P-limited) and energy-limited (C-limited), and the soil extracellular enzyme activity ratio deviated from 1∶1∶1. The nutrient limitation (P-limited) was more serious in erosion section while the power limitation (C-limited) was more serious in the deposition section. Straw mulch with organic fertilizer can alleviate the stoichiometric imbalance of soil extracellular enzyme activities, while straw mulching can significantly increase soil enzyme activity, alleviate soil phosphorus limitation, and promote soil nutrient cycling.