Regulation of straw-derived nitrogen dynamics by its quality: Implications for N2O mitigation

Straw incorporation improves soil fertility and regulates nitrogen (N) cycling, with straw quality serving as a key driver of N transformation. However, the mechanisms by which straw quality regulates straw-derived N dynamics remain unclear. Here, we used ¹⁵N-labeled maize straw with contrasting contents of carbon, N, phosphorus, lignin, hemicellulose, and cellulose to trace ¹⁵N incorporation into ¹⁵NH₄⁺, ¹⁵NO₃⁻, and ¹⁵N₂O during early (14-day) and late (84-day) incubation. On day 14, 3.3 %, 0.7 %, and 0.01 % of straw-derived ¹⁵N had been transformed into ¹⁵NH₄⁺, ¹⁵NO₃⁻, and ¹⁵N₂O, respectively. By day 84, ¹⁵NH₄⁺ declined to 2.1 %, whereas ¹⁵NO₃⁻ and ¹⁵N₂O increased to 1.1 % and 0.04 %, respectively, resulting in a cumulative loss of 0.1 %–0.3 % of straw-derived ¹⁵N as ¹⁵N₂O. During the early stage, decomposition of labile carbon fractions (hemicellulose and cellulose content) stimulated tyrosine aminopeptidase activity and the abundance of N-mineralizing genes, driving ¹⁵NH₄⁺ formation. However, strong microbial N demand, reflected by elevated NH₄⁺ assimilation and low nitrification gene abundance, promoted microbial immobilization and limited ¹⁵NO₃⁻ and ¹⁵N₂O production. In the late stage, the shift toward recalcitrant carbon reshaped microbial communities and increased α-diversity, thereby suppressing further N mineralization but enhancing nitrification of previously immobilized N via increased amoA, amoB, and hao gene abundances, leading to greater ¹⁵NO₃⁻ accumulation and ¹⁵N₂O emissions. Random forest analysis identified hemicellulose and cellulose content as the dominant regulators of straw N transformation, with higher hemicellulose content consistently associated with reduced ¹⁵N₂O emissions. These findings reveal temporal shifts in microbial N processing mediated by straw quality and suggest that optimizing straw C/N ratios, together with microbial inoculants or nitrification inhibitors, could improve synchronization of straw N release with crop demand while mitigating gaseous N losses.