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.
Wang et al. (Fri,) studied this question.