Tree leaf and root traits mediate soil faunal contribution to litter decomposition across an elevational gradient

Plant litter decomposition is key to carbon and nutrient cycling in terrestrial ecosystems. Soil fauna are important litter decomposers, but how their contribution to decomposition changes with alterations in plant composition and climate is not well established. Here, we quantified how soil mesofauna affect decomposition rate interactively with climate and leaf and root traits. We conducted an in situ decomposition experiment using eight dominant tree species per forest site across four elevations (50, 400, 600 and 1,000 m a.s.l.) in northern Japan. We used litterbags with different mesh sizes to control litter accessibility to soil mesofauna. We found stronger effects of plant litter quality on both decomposition rates and faunal contribution thereto, and perhaps of local variation in soil nutritional and moisture regimes, than climatic effects of elevation. This suggests that changing climate likely alters forest litter decomposition rates indirectly through shifts in tree community composition more than directly through changing abiotic regimes. Considering both leaves and roots as litter resources enlarged the overall contribution of variation in litter quality to decomposition rates and faunal effects thereupon, because litter quality and decomposition rate varied more between leaves and roots overall than among leaves within and across elevations. The contribution of mesofauna to litter decomposition was larger in nutrient-rich litter than in recalcitrant litter across the elevational gradient, suggesting amplification of the effect of litter traits on decomposition through preference of soil fauna for their food resources. Our findings highlight the importance of considering synergistic influences of soil faunal activities with litter traits of both leaves and roots for better understanding biogeochemical processes across environmental gradients over space or time.