Abstract: Aims Large-scale Chinese fir (Cunninghamia lanceolata) plantation management has caused severe forest nutrient imbalance in subtropical regions. Transforming stands by increasing tree species diversity is an important approach to mitigate nutrient imbalance in Chinese fir plantations. However, the effects of neighbourhood tree species richness on litter nutrient concentrations and their stoichiometric characteristics remain unclear. Methods This study established gradients of neighbourhood tree species richness (1, 4, 6, 7, and 8 species) to investigate their effects on the concentrations of carbon (C), nitrogen (N), and phosphorus (P) and the stoichiometric characteristics of Chinese fir litter in subtropical plantations. Important findings The results showed that litter N and P concentrations increased by 37.1% and 45.9% at richness levels of 6 and 7 species, respectively, compared with the monoculture treatment. This increase resulted in significant reductions in litter C/N and C/P, by 26.9% and 31.5%, respectively. Soil nitrate nitrogen was positively correlated with litter N concentration and negatively with litter C/N. Soil available phosphorus was significantly positively correlated with litter P concentration, resulting in a significant negative correlation with litter C/P. The best-fitting linear mixed-effects models showed that soil available phosphorus and pH were the main factors explaining variation in litter P concentration and C/P. Therefore, these findings suggest that a moderate increase in neighbourhood tree species richness enhances litter nutrient concentrations in the target tree species and reduces its carbon-to-nutrient stoichiometric ratios, thereby promoting litter nutrient return and improving forest nutrient status in Chinese fir plantations.

Key words: Neighbourhood tree species, Plantations, Subtropical forests, Biodiversity, Nutrient cycling