Soil trophic networks are key to biogeochemical cycles, in particular
decomposition. However, few studies have yet quantified how microbial
decomposition activity along environmental gradients is jointly driven
by bacteria, fungi, and their respective consumers. Here, we quantified
these direct and indirect effects on decomposition and contrasted them
between forests and open habitats using multiple elevational gradients
in the French Alps. While environmental control on microbial
decomposition activity was comparable in the two habitats, the pathways
and strengths of biotic predictors strongly differed. The fungal channel
composition played a moderate role in forests, while the bacterial
channel composition was critical in open habitats. Importantly, we found
trophic regulation by consumers to be a key modulator of the direct
environmental effects on decomposition in open habitats. These results
highlight the need to integrate trophic regulation when predicting
future ecosystem functioning.