Soil fauna community body size structure mediates litter loss responses to temperature and plant litter treatments in ecological microcosms

Litter decomposition is strongly influenced by soil communities, composed of soil biota which display large variations in body size. Body size plays a central role in metabolism and mediates the functional potential of soil biota, but the influence of soil community body size structure on litter decomposition is unresolved. Here, we conduct ecological microcosm experiments to investigate how soil fauna community body size structure mediates litter loss in response to temperature and plant litter availability. Community-weighted mean body mass (CWMBM) calculations are taken as an indicator of structural shifts in soil fauna community body size across treatments. Structural equation models revealed that CWMBM was strongly influenced by Collembola body mass and exhibited a non-linear response to temperature, with convergence at intermediate temperatures. Collembola CWMBM mediated a portion of the temperature effect on CWMBM, while Nematode responses were opposite and weakly correlated. Litter loss was jointly driven by CWMBM and plant litter input, with no direct effect of temperature. Sensitivity and mediation analyses confirmed the central role of Collembola in linking temperature to soil fauna community shifts but identified plant litter availability as the dominant driver of litter loss. Linear mixed effects models of relative litter loss, however, highlight a key role of the soil fauna community across temperature treatments when the effects of plant litter availability are controlled. Our experiment included extreme low plant litter availability (0 g) and high temperature (30°C) treatments to detect critical thresholds for the functioning of soil communities, but high variation in temperature responses between 20 and 30°C require exploration in future studies. Although our experiment did not isolate body size effects independently of treatments, our findings suggest an important role of soil fauna body size structure in soil functioning. Future work, experiments and statistical models should be designed to test the causal mechanisms driving emergent shifts in soil community structure and soil function in response to environmental perturbations. Such an understanding could guide management practices which buffer against potentially detrimental effects of environmental change.