Regulation Mechanism of Nitrogen Fertilizer on Soil Organic Carbon in Global Saline‐Alkaline Soils

ABSTRACT Saline‐alkaline soils are prone to land degradation due to high salinity and alkalinity, yet they possess both carbon sequestration capacity and the potential of reserved arable land, and soil organic carbon (SOC) plays a crucial role in their amelioration. Nitrogen (N) application is an effective approach to increasing SOC, yet the mechanism by which N application regulates SOC in such soils remains unclear. This study used 861 global surface soil samples (0–10 cm) from saline‐alkaline soils. It analyzed the potential controlling factors of SOC by integrating SOC data and environmental variables, and explored the effects and driving mechanisms of different N application levels—CK (0 kg N/ha), Group 1 (0–20 kg N/ha), Group 2 (20–40 kg N/ha), and Group 3 (> 40 kg N/ha)—on SOC in saline‐alkaline soils via the structural equation model (SEM) and Mantel test. Compared with CK (1.46 kg C/m 2 ), N application significantly increased SOC. The Mantel test further revealed that the correlation between SOC and environmental variables varied by N application levels: in Group 1, soil particle composition and NDVI were closely correlated with SOC; in Group 2, all environmental variables except precipitation were significantly associated with SOC; and in Group 3, soil particle composition, NDVI, temperature, and precipitation were significantly correlated with SOC. SEM revealed that different N application levels regulated SOC through both direct and indirect pathways. Furthermore, whereas specific regulatory mechanisms differed under different N application levels, the regulation of SOC by N application—through its effect on plant cover—stood out as a particularly critical pathway. Additionally, N application affected soil pH by altering soil particle composition, which might be an important potential pathway for N application to regulate SOC. This study clarified the threshold effect of N application on SOC increase, provided novel insights into the regulatory mechanism, enhanced the application of the traditional carbon–nitrogen coupling theory to special ecosystems, and provided a scientific basis for increasing SOC stocks in saline‐alkaline soils and contributing to global carbon neutrality through optimizing N application levels.