Tree diversity effects on litter decomposition are mediated by litterfall and microbial processes

Forest ecosystems are critical for their carbon sequestration potential.
Increasing tree diversity was shown to enhance both forest productivity
and litter decomposition. Litter diversity increases litter
decomposability by increasing the diversity of substrates offered to
decomposers. However, the relative importance of litter decomposability
and decomposer community in mediating tree diversity effects on
decomposition remains unknown. Moreover, tree diversity modulation of
litterfall spatial distribution, consequently, litter decomposition has
rarely been tested. We studied tree diversity effects on leaf litter
decomposition and its mediation by the amount of litterfall, litter
species richness and decomposability, and soil microorganisms in a
large-scale tree diversity experiment in subtropical China. Furthermore,
we examined how litter functional identity and diversity affect leaf
litter decomposability. Finally, we tested how leaf functional traits,
tree biomass, and forest spatial structure drive the litterfall spatial
distribution. We found evidence that tree species richness increased
litter decomposition by increasing litter species richness and the amount
of litterfall. We showed that soil microorganisms in this subtropical
forest perform 84–87% of litter decomposition. Moreover, changes in the
amount of litterfall and microbial decomposition explained 19–37% of the
decomposition variance. Additionally, up to 20% of the microbial
decomposition variance was explained by litter decomposability, while
litter decomposability itself was determined by litter functional
identity, diversity, and species richness. Tree species richness increased
litter species richness and the amount of litterfall (+200% from
monoculture to 8-species neighborhood). We further demonstrated that the
amount of species-specific litterfall increased with increasing tree
proximity and biomass and was modulated by leaf functional traits. These
litterfall drivers increased the spatial heterogeneity of litter
distribution, thus, litter decomposition. We highlighted multiple biomass-
and diversity-mediated effects of tree diversity on ecosystem properties
driving forest nutrient cycling. We conclude that considering spatial
variability in biotic properties will improve our mechanistic
understanding of ecosystem functioning.