Optimal nitrogen management increased topsoil organic carbon stock and maintained whole soil inorganic carbon stock to increase soil carbon stock—A 15-year field evidence

Nitrogen application significantly impacts soil organic carbon (SOC) and soil inorganic carbon (SIC) stocks, both of which are crucial for soil carbon sequestration. However, the effects of nitrogen fertilizer on the dynamics of SOC and SIC stocks remain poorly understood. Over a 15-year wheat-maize rotation experiment conducted on calcareous alluvial soil in temperate continental monsoon climate, we quantitatively analyzed crop carbon return and dynamics of SOC and SIC stocks among soil layers. Compared to conventional nitrogen management (Con.N), optimal nitrogen management (Opt.N) increased crop carbon return by 130 kg C ha⁻¹ yr⁻¹. There was no significant difference in SOC stock between Opt.N and Con.N, despite Opt.N used 39.4 % less nitrogen input compared to Con.N. In the topsoil, Opt.N maintained SIC levels similar to those of the Control and significantly increased by 7.22 % compared to Con.N. The Con.N treatment reduced SIC stock by 10.1 % compared to the Control. Furthermore, Opt.N had no significant negative effect on SIC stock in subsoil, thereby maintaining the whole SIC stock. After 15 years, Opt.N achieved a soil carbon stock of 52.7 Mg C ha⁻¹, outperforming Con.N. SOC dominated the topsoil carbon stock, while SIC played a more crucial role in the subsoil. Overall, Opt.N outperformed Con.N in terms of the combined SOC and SIC stocks. Our findings suggest that Opt.N can enhance soil carbon stock with lower resource consumption, thereby contributing to global warming mitigation and promoting sustainable low-carbon agriculture.