Agricultural Cultivation Improved Soil Nitrogen Sequestration by Accelerating the Accumulation of Labile Nitrogen

Soil organic nitrogen (SON), particularly its labile fraction (LN), is pivotal in nutrient supply and soil fertility maintenance. However, the retention mechanisms of LN under low-nitrogen conditions in conservation agriculture remain unclear. This study, conducted in the Yellow River sedimentary area, applied SON functional fractionation (labile vs. stable pools) and a coupled analytical framework of soil particle composition, enzyme activity, and management practices to explore the vertical differentiation of LN under different land-use types. Results showed that LN content was greater in agricultural soils (381.36–467.85 mg/kg), primarily driven by elevated activities of invertase and alkaline phosphatase. Silt (< 50 μm) improved the efficiency of stable nitrogen (SN) sequestration through organic-mineral bonding, while sand particles (> 250 μm) led to increased LN leaching flux. Integrated fertilisation improved LN availability and SN stability through dual CN interactions and enzyme activation pathways. These findings highlight LN's ‘rapid response, high loss’ behaviour in sandy soils and offer a scientific basis for organic nitrogen regulation in fragile agroecosystems.