Microbial Regulation of Nitrogen Retention Under Contrasting Straw‐Derived Carbon Amendments in Red Paddy Soils

ABSTRACT Excessive inorganic fertilizer use leads to nitrogen (N) losses and environmental hazards, emphasizing the need for sustainable N management strategies. While recycling crop straw into agricultural soils can regulate N dynamics, the comparative efficacy of various straw‐derived carbon amendments remains unclear. In this 15 N tracing study, we evaluated the effects of three carbon amendments—direct straw addition (ST), straw combined with microbial inoculant (IN), and straw‐derived biochar (BC)—on N transformation processes in a subtropical red paddy soil (Ultisols) from Jiangxi, China, under non‐flooded and flooded conditions. Each amendment was applied at two carbon rates: 3.9 (low) and 11.7 (high) mg C g −1 soil, with equal carbon input across amendments at each rate. Our findings showed that ST and IN markedly accelerated N turnover, increasing gross N mineralization rates by 1.8 to 8.6 times, enhancing NH 4 + and NO 3 − immobilization, and promoting dissimilatory NO 3 − reduction to NH 4 + under both water conditions. In contrast, BC exhibited limited effects, except for an increase in NH 4 + and NO 3 − immobilization under flooded conditions at high application rates. Furthermore, ST and IN suppressed autotrophic nitrification by 68%–95%, whereas BC significantly stimulated nitrification rates by 1.9–3.4 times. Partial least squares path modeling indicated that amendment‐induced changes in soil properties shifted N‐cycling functional genes, which critically mediated these distinct N transformation patterns. Over the incubation period, soil inorganic N production pathways were less stimulated than consumption pathways, and the ratio of autotrophic nitrification to NH 4 + immobilization decreased under ST and IN amendment. Collectively, these changes enhanced soil N retention and reduced the risk of NO 3 − losses. Overall, this study underscores direct straw application as a more sustainable strategy to accelerate N turnover, improve soil N retention, and mitigate NO 3 − loss risks in red paddy soils.