Time and litter species composition affect litter-mixing effects on decomposition rates

Litter decomposition is a fundamental process of biogeochemical cycles. Mixing litter of different species can induce non-additive effect (NAE) on decomposition processes. A better understanding of the factors influencing the direction and magnitude of NAE is important for quantification of ecosystem carbon and nutrient cycling. Litter mixing effects on leaf litter decomposition were examined using two parallel experiments (each with leaf litter of four tree species combined into four diversity levels) conducted in Chinese temperate forests for 2 years. A significant diversity effect in both experiments demonstrated NAEs of litter mixing on the mass remaining. Separating the diversity effect into species richness and composition showed that composition was statistically significant in both experiments whereas an effect of species richness was only detected in Experiment 2. In both experiments, the NAE on decomposition changed with incubation time and showed a declining trend during decomposition process. However, a slight difference of the NAE in the two experiments was observed in our study. Decomposition in the litter mixtures differed from the predictions based on single species, and the presence/absence of poplar, pine and oak in the litter mixture significantly influenced the direction and magnitude of the NAE. Presence of coniferous species litter significantly reduced decomposition rates even within the same species richness level. The coefficient of variation of remaining litter mass was lower in the treatments with higher number of litter species. Our findings showing of changes in the direction and magnitude of NAE with decomposition time and litter species composition indicate that tree species composition and decomposition duration should be considered in predicting biodiversity effect on biogeochemical cycling in temperate forests.