Functional Diversity Accelerates the Decomposition of Litter Recalcitrant Carbon but Reduces the Decomposition of Labile Carbon in Subtropical Forests

The biodiversity of litter can regulate carbon and nutrient cycling during mixed decomposition. It is common knowledge that the decomposition rates of mixed litters frequently deviate from those predicted for these component litter species. However, the direction and magnitude of the nonadditive effects on the degradation of mixed litters remain difficult to predict. Previous studies have reported that the different carbon fractions of leaf litters responded to litter mixture differently, which may help to explain the ambiguous nonadditive effect of diversity on bulk litter decomposition. Therefore, we conducted decomposition experiments on 32 litter mixtures from seven common tree species to test the responses of different carbon fractions to litter diversity in subtropical forests. We found that the overall mass loss of the mixed litter was faster than that estimated from single species. The relative mixing effects (RMEs) of different carbon fractions exhibited different patterns to litter diversity and were driven by different aspects of litter functional dissimilarity. Soluble carbon fractions decomposed more slowly than expected from single species, while lignin fractions decayed more quickly. Moreover, we found that the RMEs of bulk litter decomposition may be determined by the lignin fraction decomposition. Our findings further support that distinguishing the response of different carbon fractions to litter diversity is important for elucidating the nonadditive effects of total litter decomposition.