Functional identity and functional diversity co-regulate litter mixture decomposition and nitrogen release in boreal riparian forest ponds

The relationship between biodiversity and litter decomposition has received considerable attention, but the complex biodiversity effects on decomposition and the underlying mechanisms are still unclear, especially in boreal riparian forest ponds. Here, we collected leaf litter from an N2-fixing species (Alnus sibirica) and two non-N2-fixing species (Betula platyphylla and Betula fruticosa) in a boreal riparian forest, northeast China. Then, we mixed leaf litter with varying composition and evenness to create a large gradient of litter functional identity and functional diversity, and used the litterbag method to examine the effect of altering plant composition on litter mass loss and nitrogen (N) release during 2 years of decomposition in forest ponds. Over 2 years of decomposition, the presence of A. sibirica in litter assemblages accelerated mass loss and N release by improving functional identity and increasing functional diversity of litter mixtures. Irrespective of the dominant species, litter mass loss generally increased with rising evenness, whereas the effect of species evenness on N release was dependent on the dominant species. When all litter mixtures were pooled, litter mixing effects on mass loss and N release correlated positively with litter functional diversity. Moreover, the decomposition dynamics of litter mixtures were regulated by litter functional diversity and the resulting mixing effects, as well as the community-weighted mean traits such as N and lignin concentrations. These findings confirm the importance of litter functional identity and functional diversity in driving decomposition processes of litter mixtures, and also indicate that increased abundance of N2-fixing species will accelerate litter decomposition and enhance N release in boreal riparian forest ponds.