17-year in situ 15N tracing reveals turnover and functional roles of residual fertilizer nitrogen in soil organic and fixed ammonium pools of a rice–wheat system

• A 17-year in situ 15 N tracing revealed the decline and transformation of residual N pools. • Over 98% of residual fertilizer N persisted in SON and fixed ammonium forms. • Long-term N fertilization accelerated the turnover of labile organic N and fixed ammonium. • Long-term N input increased crop recovery of residual N by 19 points over 17 years. Intensive nitrogen (N) fertilization has led to the accumulation of residual fertilizer N in agricultural soils, yet its long-term dynamics and functional significance remain inadequately understood. We conducted a 17-year in situ 15 N tracing and N fractionation study in a rice–wheat rotation system, where 15 N-labeled urea was applied in the initial season followed by either continued unlabeled urea applications or no further N input. Over 98 % of residual fertilizer N persisted in slow-release forms, primarily as soil organic nitrogen (SON) and fixed ammonium. Among SON fractions, amino acid N, ammonium N, and hydrolysable unknown N were the main initial sinks, collectively accounting for 85–86 % of total residual N, and functioned as active, fast-cycling pools. In contrast, amino sugar N and acid-insoluble N were less abundant but more stable, with acid-insoluble N exhibiting the slowest turnover. These active SON fractions dominated crop recovery of residual N, contributing 94–95 % of total uptake, with amino acid N alone accounting for 42–46 %. Long-term N fertilization accelerated the mineralization of amino acid N and ammonium N and enhanced fixed ammonium turnover, thereby reshaping soil N pool dynamics, shortening the half-life of residual N, and increasing cumulative crop 15 N recovery by 48 %. These findings provide novel insights into the functional partitioning and turnover of residual N pools, offering important implications for improving nitrogen use efficiency and sustainability in intensive cropping systems.