Positive effect of long-term nutrient additions on litter decomposition depends on the stage coordination between litter quality and soil environment in a subtropical plantation

<title>Abstract</title> <p> <bold>Background and aim</bold> Numerous studies have explored how litter decomposition responds to nutrient supply in forest ecosystems. However, the temporal mechanisms underlying the regulation of litter decomposition affected by long-term nitrogen (N) and phosphorus (P) addition via functional coordination between altered litter quality and soil environment remain poorly elucidated. <bold>Methods</bold> A one-year in-situ litter decomposition experiment was implemented in a Chinese fir ( <italic>Cunninghamia lanceolata</italic> ) plantation receiving 10 years of continuous nutrient addition, comprising four treatments: control, +N, +P, and N + P. We measured mass loss, quality, nutrient release of decomposing litters, and corresponding soil general properties, enzyme activities at four sampling times (90, 180, 270, 360 days). <bold>Results</bold> All N and/or P additions significantly promoted litter mass loss across the whole decomposition period, with N + P treatment exhibiting the strongest synergistic effect. Litter quality, particularly the release of litter cellulose, explained 64%-81% of the variation in litter mass loss, far exceeding the 19%-36% explained by soil environment. Soil nutrient availability and enzyme activities exerted positive and stage-dependent effects on litter mass loss. N and P addition exhibited distinctly temporal patterns in regulating litter decomposition, with N addition dominating the process at 90 and 180 days by increasing soil N availability and litter cellulose release, while P addition acted as the primary driver at 270 and 360 days via elevated soil available P and sustained litter cellulose release. <bold>Conclusion</bold> Long-term N and/or P addition exerts positive effects on litter decomposition, with pronounced stage-specific coordination between altered litter quality and soil environment. These findings provide novel insights into the dynamic regulation of litter decomposition under global nutrient enrichment and offer a theoretical basis for nutrient management in subtropical forests. </p>