Stratified Nitrogen Application Enhances Subsoil Carbon Sequestration via Enzyme-Mediated Pathways in Straw-Incorporated Croplands of North China Plain

Nitrogen (N) fertilization critically regulates the storage and availability of soil carbon (C) and N pools. However, the internal mechanism through which stratified N application affects soil organic carbon (SOC) sequestration and soil quality index (SQI) remains unclear. To investigate the effects of stratified N application on C sequestration and SQI in both topsoil and subsoil, this study established six treatments (N0:0, N1:0, N4:1, N3:2, N2:3, N1:4) and analyzed soil biochemical indicators. The results showed that compared to N1:0, stratified N fertilization did not significantly improve soil C and N content in the 0–20 cm layer. In contrast, the N2:3 and N1:4 treatments even led to a significant reduction in soil C and N pools in the topsoil. In the 20–40 cm, compared to N1:0, stratified N fertilization increased SOC, TN, labile C fractions, N fractions (particulate organic N and microbial biomass N), enzyme activity and C pool management index (CPMI), increasing by 0.52–7.94%, 2.05–8.42%, 4.77–42.59%, 14.46–56.01%, 6.34–45.82%, and 31.26–51.93%, respectively. In 0–20 cm, compared to N0:0, N application increased SQI by 24.84–45.77%, and N2:3 and N1:4 treatments were lower SQI than N1:0. Furthermore, N2:3, N3:2, and N1:4 treatments in 20–40 cm were higher than other treatments. N fertilizer application drives the synergistic changes in C and N fractions by regulating enzyme activity and stoichiometric ratio, thus affecting CPMI and SQI. Thus, the 3:2 stratified N fertilization (0–20 cm:20–40 cm) method achieves synergistic dual-layer enhancement-maintaining surface C and N pools while boosting subsoil C sequestration and quality-through enzyme-mediated precision regulation of C/N stoichiometry. The study provides a scientific foundation for integrated C emission reduction and cropland quality enhancement in the North China.