What controls forest litter decomposition? A coordinated distributed teabag experiment across ten mountains

Litter decomposition in mountainous forest ecosystems is an essential process that affects carbon and nutrient cycling. However, the contribution of litter decomposition to terrestrial ecosystems is difficult to estimate accurately because of the limited comparability of different studies and limited data on local microclimatic and non-climatic factors. Here, we designed a coordinated experiment within subtropical and tropical forests across ten mountains to evaluate variation in litter decomposition rates and stabilization. We tested whether elevations, soil microclimate, soil physiochemistry, tree species diversity, and microhabitat affect decomposition rates and stabilization by using the Tea Bag Index as a standardized protocol. We found that the associations of decomposition rates and stabilization with elevation and each environmental factor varied between mountains. Elevation significantly affected decomposition rates and stabilization in the western mountains, where soil microclimate also played a dominant role due to relatively cold environments. Across all mountains, decomposition rates decreased while stabilization increased with increasing elevation. In terms of microclimate, decomposition rates increased with increasing soil temperature and temperature variation during the growing season, whereas stabilization decreased with increasing soil temperature and moisture variation. In terms of non-climatic factors, decomposition rates increased with increasing tree species diversity, whereas stabilization decreased with soil pH and slope. Our findings enhance the general understanding of how different factors control forest litter decomposition, highlighting the dominant role of soil microclimate in controlling carbon and nutrient cycling in cold environments and high elevations.

Methods
Environmental data and litter decomposition data were collected from a coordinated experiment across ten mountains in the tropical and subtropical regions of China. We used standardized litter bags "red tea" and "green tea" as decomposing materials. The calculated Tea bag index included decomposition rate (k) and stabilization (S). We modeled k and S on the function of each environmental variable or multiple variables depending on the research hypotheses.