Non-additive effects of different litter mixtures on soil carbon and nitrogen dynamics controlled by litter functional identity and diversity across two soils in the temperate forests of northeastern China

Abstract Background Understanding how plant litter diversity influences soil carbon (C) and nitrogen (N) dynamics is critical for explaining C and N cycling in forest ecosystems. However, the influence of litter functional traits on soil C and N dynamics across different forest soils remains unclear. In this study, we incubated 38 litter mixtures from 10 dominant tree species with contrasting functional traits in soils from natural secondary forests (NSF) and Larix olgensis plantations (LOP) in northeastern China. We measured C and N mineralization, dissolved organic carbon (DOC), and microbial biomass carbon (MBC) to quantify non-additive effects. In addition, we evaluated the effects of litter functional identity (Community-weighted mean, CWM) and functional diversity (Rao’s Q ) on non-additive effects across different soils. Results The non-additive effects of mixed leaf litter on soil C and N dynamics varied across the two soils. The proportion of non-additive effects on C and N mineralization, DOC, and MBC ranged from 17.8% to 60.7% in both soils. Non-additive effects were generally more frequent in NSF soils than in LOP soils, which may be attributed to the higher organic matter content and more diverse microbial communities. Trait-based models showed that functional diversity was the dominant predictor of non-additive effects in NSF soils, explaining 89.3% and 84.3% of the variations in C and N mineralization, respectively. However, in LOP soils, trait effects were more function-specific; functional diversity governed C mineralization, whereas functional identity had a greater influence on N mineralization. Specific traits, such as Rao’s Q of TN and TC, and CWM of TC, emerged as key regulators of non-additive effects. Conclusions Our findings underscore the importance of integrating trait-based frameworks with different forest soils to understand the non-additive effects of litter mixtures. By identifying how specific traits mediate soil C and N dynamics differently across the two soils, this study provides a theoretical basis for predicting biogeochemical responses to litter diversity, with implications for forest biodiversity conservation and soil fertility improvement strategies.