Objective This paper explores the effects of different thinning intensities on decomposition rates and nutrient release dynamics of needles and fine root litter in Larix gmelinii var. principis-rupprechtii plantations to provide a reference for sustainable plantation management.

Method The study was carried out based on long-term permanent plots established in 2019 at Saihanba Mechanical Forest Farm, Hebei Province of northern China, totally 12 fixed monitoring plots (30 m × 30 m each). These plots included a control (CK, no thinning), light thinning (LT, 20%), moderate thinning (MT, 39%), and heavy thinning (HT, 60%). An in-situ decomposition experiment was carried out for one and a half years using the litterbag method starting in November 2023. Soil temperature and water content were monitored synchronously. The initial total carbon, total nitrogen, and total phosphorus contents of the litter were measured, as well as mass residual rates and nutrient release dynamics during decomposition. The Olson exponential decay model was used to calculate decomposition rates, and key influencing factors were analyzed through the mixed-effects model and correlation analysis.

Result (1) Thinning treatments significantly increased soil temperature and water content at a 5 cm depth, with the largest increase observed in HT. Meanwhile, LT significantly increased initial nitrogen and phosphorus contents of needles and fine root litter and decreased the initial C/N and C/P ratios, thereby improving litter quality. (2) All thinning treatments promoted the needle litter decomposition, with decomposition rates following the order of LT > MT > HT > CK. In contrast, the response of fine root decomposition to thinning intensity was inconsistent. LT and MT significantly promoted fine root decomposition, while the decomposition rate under HT was lower than that under CK. (3) The decomposition rate of needle litter was significantly positively correlated with soil temperature and initial nitrogen and phosphorus contents of litter itself, and significantly negatively correlated with initial C/N and C/P ratios. The decomposition rate of fine roots was mainly significantly correlated with initial chemical properties of litter itself (nitrogen and phosphorus contents, C/N and C/P ratios), with no significant correlation with soil temperature and water content during monitoring period.

Conclusion LT and MT effectively promote the decomposition of needles and fine root litter and nutrient cycling by improving soil microenvironment and litter quality. Although HT accelerates needle decomposition, it inhibits fine root decomposition. Litter decomposition is jointly driven by microenvironment and substrate quality, but the dominant factors vary. Overall, light to moderate thinning is appropriate management intensity for optimizing litter decomposition and nutrient return in L. gmelinii var. principis-rupprechtii plantations.