Tree species identity is the predominant modulator of the effects of soil fauna on leaf litter decomposition

Soil fauna is one of the key drivers of the litter decomposition at both local and global scales, yet the role of tree species in mediating the effects of soil fauna on litter decomposition remains elusive. We conducted a field experiment using litterbags with three different mesh sizes that allowed access of microfauna (0.1 mm), micro- and mesofauna (2 mm), and total soil fauna (5 mm) to evaluate the decomposition of foliar litter of two tree species associated with arbuscular mycorrhizal (AM) fungi and three tree species associated with ectomycorrhizal (ECM) fungi in six Danish common garden sites. We also assessed how differences in initial litter quality, soil properties, and microbial community composition among the tree species may affect litter decomposition as well as soil fauna effects on litter decomposition. Results showed that (1) litter mass loss varied significantly among different mesh sizes and tree species, with litter decomposition rates (k) ranging from 0.273 to 3.482; (2) the access of mesofauna significantly increased litter k of 0.658 for AM and 0.396 for ECM tree species with no access of soil fauna 255 and 92%, respectively, while the access of both meso- and macrofauna increased k by 265 and 108% for AM and ECM trees, respectively; (3) tree species identity, mycorrhizal association, initial litter quality, soil properties, microbial community composition, and ambient soil fauna biomass were all significantly influencing factors on litter decomposition, but tree species identity was the dominant factor regardless of litterbag mesh size; and (4) the effects of mesofauna on litter decomposition were predominantly controlled by tree species identity, initial litter Mg concentration and lignin:N ratio, while the small additional impact of macrofauna access was not well explained by any of the assessed factors. Overall, our results suggest that tree species affect litter decomposition via different stimulation of soil fauna functioning, and that AM and ECM associated tree species differ in the degree to which soil fauna stimulates litter decomposition. However, the pattern was not entirely consistent as litter decomposition rates for the ECM associated lime were stimulated to the same extent as rates for the AM associated tree species ash and maple. Overall, our results suggested that soil meso- and macrofauna communities may enhance tree species effects on litter decomposition as well as incorporation of litter C into mineral soil.