[Effects of Converting Farmland into Forest and Grassland on Soil Nitrogen Component and Conversion Enzyme Activity in the Mountainous Area of Southern Ningxia].

The effects of the policy of converting farmland to forest and grassland on soil nitrogen content and conversion enzyme activity were studied. Caragana intermedia, Prunus davidiana, Medicago spp., and Stipa bungeana and a corn control were examined to determine the concentrations of seven soil nitrogen components and the activity of two nitrogen conversion enzymes. The main factors affecting soil nitrogen distributions and transformation was also explored using redundancy analysis (RDA). The results showed that:① Compared with the corn soil, the content of particulate organic nitrogen, light fraction organic nitrogen, microbial biomass nitrogen, and ammonium nitrogen in the Stipa bungeana soil increased by 35.3%, 83.3%, 64.2%, and 110.0%, respectively; soluble organic nitrogen and ammonium in the Medicago spp. soil increased by 0.7% and 67.5%, respectively; the asparaginase and protease activities in the Stipa bungeana soil increased by 360% and 144.8%, respectively, indicating that conversion of farmland to forest and grassland has a promoting effect on nitrogen components and conversion enzyme activity; ②The content of organic nitrogen, light organic nitrogen, particulate organic nitrogen, and soluble organic nitrogen in the Prunus davidiana soil was 3.7%, 133.3%, 70.6%, and 28.1% higher than that of the corn soil, respectively. The light fraction organic nitrogen content of the Caragana intermedia soil and microbial biomass nitrogen content of the Prunus davidiana soil was 16.7% and 49.6% higher than that of the corn soil, respectively. Protease activity in the Caragana intermedia and Prunus davidiana soils was higher than in the corn soil, further indicating that the conversion of farmland to forest and grassland promotes the accumulation of nitrogen components and enhances conversion enzyme activity; ③ The RDA and environmental factor explanation rate results indicated that soil water content, pH, and soil organic carbon were the key factors affecting nitrogen distribution and transformation in the mountainous area of southern Ningxia. Overall, the results show that converting farmland into forest and grassland has changed conversion enzyme activity and has promoted the accumulation of nitrogen components in soils. This provides a theoretical basis for ecological restoration and soil quality management in the Loess Plateau.