Macro-detritivore identity and biomass along with moisture availability control forest leaf litter breakdown in a field experiment

Forests are structurally rich ecosystems with strong spatial variation in microclimate. Local temperature and soil moisture are important drivers of leaf litter breakdown, a key ecosystem process vital for forest functioning. Additionally, detritivore species composition and activity are equally dependent on microclimate, rendering changes in microclimate key to understand leaf litter breakdown. We investigated the interaction between microclimatic variables (i.e. temperature and moisture) and different combinations of macro-detritivores (a drought sensitive i.e. Oniscus asellus vs. a drought tolerant species i.e. Glomeris marginata) on litter breakdown of easily decomposable (high quality) Acer litter and decomposition resistant (low quality) Quercus litter in a full factorial microcosm field experiment in a temperate forest in Belgium. We hypothesize litter breakdown to be faster for high quality litter and macro-detritivore biomass and dependent on macro-detritivore identity, mediated by forest microclimate e.g. faster with higher soil moisture and warmer temperatures. We found high quality litter breakdown to be reduced by decreasing moisture availability, while it was not affected by temperature. There was no effect of moisture and temperature on litter breakdown of low quality litter. The effect of detritivore biomass on the breakdown of Quercus litter depended on detritivore identity: elevated millipede biomass increased Quercus litter breakdown, which was not the case for woodlice. There was a positive effect of macro-detritivore biomass but no effect of macro-detritivore identity on leaf litter breakdown of high quality litter. In addition, the relative consumption rates were equal between the drought sensitive (woodlouse) and the drought tolerant (millipede) species for high quality litter, but different for low quality litter. The woodlouse species was more efficient in the breakdown of low quality litter compared to our tested millipede species. Relative consumption rate was not influenced by the moisture or temperature treatments. Combining both detritivore taxa in a single microcosm had additive (non-synergistic) effects on litter breakdown, indicating that they are not complementary in their resource use. We conclude that mainly differences in moisture availability in forest ecosystems are important for litter breakdown and that detritivore identity is critical for the breakdown of especially low quality litter