Soil fauna accelerate litter mixture decomposition globally, especially in dry environments

More than half of the net primary production in terrestrial ecosystems returns to the soil through leaf litter fall and decomposition. In terrestrial ecosystems, litter constitutes a mixture of mainly senescent foliage from multiple species. Yet, the effect of litter mixing on litter decomposition rate remains ambiguous. Quantification of the soil fauna contribution and inclusion of their interaction with litter could remove the prevailing ambiguity, as soil fauna might influence the direction and magnitude of litter mixture effects on decomposition. We carried out a global meta-analysis to elucidate how soil fauna influenced litter mixture decomposition rate based on 55 published studies with 873 fauna inclusion/exclusion observations in field litterbag experiments. We hypothesized that soil fauna inclusion in litter mixture experiments would cause a positive change in the magnitude of mixed litter decomposition. That is, the presence of soil fauna would lead to synergistic litter mixture effects while the absence of soil fauna would lead to antagonistic litter mixture effects. Furthermore, we hypothesized that soil fauna would have a greater positive impact on the litter mixture effect in environments with low precipitation. While litter mixture had a neutral effect on decomposition on average across studies, non-additive mixture effects were common, with a key role for soil fauna, which generally enhanced decomposition rate compared to the component single-species litters. In contrast, fauna exclusion resulted in diminished decomposition rate overall. The overall synergistic soil fauna effect on litter mixture decomposition increased under low precipitation. Under high precipitation, the litter mixture effect was positive in the absence of soil fauna but non-significant in the presence of fauna. Climate imposes a great effect on litter mixture, with synergistic effects increasing towards the equator. Synthesis. These results highlight the importance of soil fauna in mixed litter decomposition, most strongly in dry environments. In this era of global climate change, where some regions are projected to become drier, soil fauna might compensate the expected slower decomposition of litter by increasingly enhancing litter mixture decomposition.