Multiple interactions between tree composition and diversity and microbial diversity underly litter decomposition

Litter decomposition determines carbon and nutrient cycling in terrestrial ecosystems. Prior research has addressed the importance of litter quality and microclimate on litter decomposition. However, the interacting relationships among litter chemical quality, the microenvironment, and micro-organisms - as well as their subsequent influence on litter decomposition - remain poorly understood. We sampled five forest types with varying overstory compositions in the subtropical forest of China and set up a reciprocal transplant litter decomposition experiment using a split-plot design. We used structural equation modeling (SEM) to examine the effects of litter species diversity and composition on litter decomposition rate through litter chemical properties. We also examined the effects of tree species diversity and composition on litter decomposition rate via litter moisture, microbial chemistry, and fungal diversity. Increased coniferous litter proportion restrained litter decomposition by decreasing litter N content. Litter species diversity and functional dispersion of litter C, N, and P contents had insignificant effects on litter decomposition. Overstory coniferous proportion and tree species diversity enhanced litter decomposition rates indirectly through their positive effects on litter moisture, fungal properties (a composite variable of fungal richness and squared fungal guild diversity), and microbial chemistry (a composite variable of microbial carbon and phosphorus). Our results highlighted that coniferous litter proportion and overstory coniferous proportion had opposite effects on litter decomposition via litter nitrogen content and the microenvironment, respectively. Furthermore, fungal community plays a key role in how overstory species composition and microclimate affect litter decomposition.