Straw Incorporation Enhances Rice Use of Residual Fertilizer N by Regulating Labile Organic N in Upland‐Paddy Rotations

Straw incorporation regulates soil organic nitrogen transformation, while the linkage between straw‐induced changes in residual fertilizer 15 N distribution and subsequent crop nitrogen utilization remains unclear in paddy‐upland rotation systems. Based on 15 N tracer technology, this study explored the distribution characteristics of residual fertilizer 15 N and its subsequent reuse by rice. The results showed that straw incorporation significantly improved the subsequent utilization of residual fertilizer 15 N by rice, with an average increase of 31.5% in residual N uptake compared with conventional chemical fertilization. After winter crop harvest, straw incorporation reduced residual fertilizer 15 N in the 0–20 cm topsoil and altered the partitioning of residual fertilizer 15 N in soil organic N fractions. Specifically, straw incorporation facilitated the accumulation of residual fertilizer 15 N in amino acid‐N (AAN) and amino sugar‐N (ASN) fractions. The residual fertilizer 15 N distributed in AAN and ASN fractions dominated the residual N uptake of subsequent rice. This study confirms that straw incorporation alters the distribution of residual fertilizer 15 N in soil organic N fractions and consequently enhances the subsequent uptake of residual fertilizer N by rice in paddy‐upland rotation systems.