Effects of biochar produced at different pyrolysis temperatures on nitrogen transformation, microbial functional genes, and enzyme activities in saline soils

Introduction The application of biochar is generally considered to enhance soil nutrient availability and improve soil fertility, particularly in saline environments. However, the mechanisms by which biochar pyrolysis temperature influences nitrogen (N) availability and transformation in soils with varying degrees of salinity remain unclear. Methods In this study, a laboratory incubation experiment was conducted to investigate the effects of biochar produced at different pyrolysis temperatures under three salinity levels (0.78, 1.92, and 2.79 dS m −1 ). Five treatments were applied: control (CK), conventional nitrogen application (N1), and N1 combined with 1% (w w −1 ) biochar produced at 300 °C (N1C1), 500 °C (N1C2), and 700 °C (N1C3). Soil microbial biomass, enzyme activities, nitrogen-cycling functional genes, and nitrogen mineralization processes were measured. Results (1) Under low and moderate salinity, the N1C2 treatment significantly increased soil microbial biomass as well as urease and acid protease activities, whereas under high salinity, N1C3 performed better. Biochar addition also elevated the relative abundance of nitrogen-cycling functional genes, indicating enhanced net nitrogen transformation capacity. (2) Increasing salinity significantly suppressed nitrogen mineralization, while biochar application alleviated this inhibitory effect. Under low and moderate salinity conditions, the N1C2 treatment exhibited higher net nitrification and nitrogen mineralization capacities, whereas under high salinity conditions, the N1C3 treatment showed the most pronounced effect. (3) Soil microbial biomass and the activities of urease and acid protease were key factors influencing nitrogen mineralization across different salinity levels. With increasing salinity, the nitrogen mineralization pathway shifted from one dominated by aerobic mineralization/nitrification to more complex mechanisms involving facultative or anaerobic processes. Discussion These findings provide valuable insights into nitrogen management in saline soils and underscore the importance of tailoring biochar applications according to pyrolysis temperature and salinity conditions.