Optimized farming management enhances carbon sequestration and nitrogen retention in an intensive wheat–maize cropping system of the North China Plain

Soil organic matter (SOM) dynamics in intensively managed agroecosystems are strongly influenced by nitrogen (N) fertilization and organic inputs; however, their coupled effects on soil organic carbon (SOC) sequestration and N retention, particularly in subsoil layers, remain insufficiently understood. To address this, a 12-year field experiment involving a winter wheat–summer maize rotation in the North China Plain was conducted to evaluate the effects of optimized farming management involving reduced mineral N input, organic amendments (manure and crop residues), and water-saving irrigation (OPT) on SOC and N dynamics in comparison with those under conventional farming (CON). OPT maintained comparable crop yields while reducing fertilizer N input by 41.4% and irrigation water by 25%. OPT significantly increased the SOC contents in the 0–20 cm and 20–40 cm layers by 28.6% and 23.2%, respectively, relative to those in the CON treatment. These increases were accompanied by higher C conversion efficiency and N retention efficiency, indicating a simultaneous SOC accumulation and lower N losses under optimized farming management. A substantial increase in the SOC stock was also observed in subsoil layers, highlighting the potential for long-term SOC sequestration. These findings demonstrated OPT as a robust strategy to achieve high productivity with reduced resource inputs while increasing soil SOC sequestration and C use efficiency. This study further advances the current understanding by revealing the critical role of C–N coupling and subsoil processes in regulating long-term SOC sequestration in the intensive agricultural systems.