Temperature and Moisture Modulate the Contribution of Soil Fauna to Litter Decomposition via Different Pathways

Soil fauna are crucial decomposers in terrestrial ecosystems, but how the role of soil fauna varies among climatic conditions and litter substrates remains poorly understood. Here, we conducted a four-year litter decomposition experiment along an elevational gradient (453 m, 945 m, 3058 m and 3582 m) in southwestern China. Two dominant tree species with contrasting leaf traits (coniferous vs. broadleaf) were used for field incubation at each site. Litterbags with two mesh sizes (3 vs. 0.04 mm) were used to permit and exclude the presence of soil fauna. The changes in elevation caused corresponding shifts in temperature and precipitation but did not affect the abundance and diversity of soil fauna communities. Soil fauna increased annual decomposition rates (k) by 14.5–28.7% across all litter types. Our structural equation models indicated that increasing temperature reduced while increasing moisture increased soil fauna effects on decomposition. Moreover, temperature and moisture modulated the contribution of soil fauna to litter decomposition via different mechanisms: (1) the reduced soil fauna contribution was driven by the increased temperature through increasing the litter C/N and fauna density (possibly because higher densities were associated with smaller organisms) and (2) the increased soil fauna effects were driven by increased moisture that increased the diversity of soil fauna. These results demonstrate that the effects of soil fauna are sensitive to changes in climate and litter quality across elevation gradients, and environment factors (i.e., temperature and moisture) may mediate the contribution of soil fauna to litter decomposition in opposite directions.