Accelerated soil nitrogen (N) loss in young Chinese fir plantation compared to natural regeneration during initial warming years

Abstract Aims To assess how +5 °C soil warming affects nitrogen (N) dynamics in early-stage subtropical forests, we compared Chinese fir plantations and natural regeneration. The hypothesis was that warming accelerates N mineralization and nitrification, increasing risks of leaching and gaseous losses where sapling N uptake is limited. Methods A warming experiment was conducted in subtropical China using buried heating cables (+5°C), incorporating treatments of Chinese fir (Cunninghamia lanceolata) plantation and natural regeneration. Results Warming significantly increased annual net N mineralization and nitrification in plantations (except in August) and in natural regeneration during January. In plantations, warming elevated DON, NO3--N, and NH4+-N in surface soil (0-10 cm), as well as NO3--N throughout the 0-60 cm profile, particularly in January. In contrast, NH4+-N decreased in natural regeneration in August. Warming also increased NO3--N concentrations in leachate and elevated N2O emissions across both natural regeneration and plantation plots. The abundance of functional genes related to N cycling declined under warming, although natural regeneration maintained higher levels of denitrification genes (narG, nirK, nosZ) and the N-fixation gene (nifH) compared to plantations. Enrichment of δ15N in N-rich plant organs further reflected enhanced soil N transformation under warming. Conclusion Warming induced a more open soil N cycle in subtropical forests, with greater N leaching in plantations than in natural regeneration during early warming years. This highlights the importance of forest management and the value of near-natural practices under climate change.