Deciphering the mechanisms underlying soil fauna-microbe interactions

Soil fauna and microbial communities are key drivers of soil organic matter turnover and nutrient cycling, but we are still far from unraveling the mechanisms underlying the full complexity of their interactions. While soil fauna is generally hypothesized to release microbes from bottom-up resource limitations, they could also exert a strong top-down control by either direct feeding or by shifting the stoichiometric balance of the soil solution, thus constraining microbial growth. To try filling this knowledge gap we report novel data on a microcosm incubation experiment in which we controlled the presence of meso and macrofauna and measured the flows of carbon (C), nitrogen (N) and phosphorus (P) from litter to the different soil pools and tracked their effects on a comprehensive set of microbial functioning variables which included growth rates, stoichiometry, enzyme activity, substrate degrading capacity, and rates of N and P mineralization and consumption. Additionally, we evaluated changes in the microbial community composition through 16S, ITS and 18S DNA marker sequencing. Soil macrofauna boosted the release of C, N and P from the litter pool. This led to a strong increase in dissolved organic C and a moderate increase in free amino acids, ammonium and phosphate concentration, thus resulting in a sharp increment of the C:N and C:P ratios in the soil solution. Microbial C and growth were greater in the microcosms with meso and macrofauna, but their C-use efficiency did not change. Macrofauna presence boosted the microbial gross production and consumption of amino acids, ammonium and nitrate, but P mobilization and uptake rates remained equal across treatments. The activity of beta-glucosidase also increased with macrofauna while N and P mining enzyme activities did not change. Overall, soil macrofauna strongly up regulated microbial communities by releasing them from C limitation.