Influence of tree residue retention in Mediterranean forest on soil microbial communities responses to frequent warming and drying events

In the coming decades, Mediterranean forests are likely to experience higher heat-wave frequency that may severely affect soil microbial functioning. Here, we studied the effects of warming intensification on soil microbial communities and tested whether certain types of Mediterranean plant residues favor soil biological resistance to frequent drying and warming events (FDW). To this aim, we set-up mesocosms in which soils originating from six Mediterranean forest stands (i.e, Quercus ilex and Q. pubescens and Pinus halepensis and P. sylvestris monospecific and mixed stands) were amended with corresponding coarse (crushed branches) and fine (crushed twigs with leaves/needles) woody debris. A quantity of residues corresponding to five-fold the equivalent of the quantity left on the floor after tree harvesting/logging (equivalent to 35 gC.kg−1 soil DW) was added to each pot, and mesocosms were laid out into a split-plot design and subjected to FDW. After nineteen months, soil physico-chemical properties and changes in microbial communities' diversity and structure, biomass and extracellular enzymatic activities were measured. We showed that FDW reduced the richness of the soil fungal community by ca. 11% and functional diversity by ca. 4% and increased bacterial richness by ca. 40%. We observed a drastic down-regulation of extracellular enzyme activities related with N and C cycles (respectively -44% for protease, -54% for cellulase and -31% for tyrosinase activities). We also found that residues inputs types (fine vs. coarse) only slightly mitigate those effects, while species origin did. Our study showed that residues retention could alleviate the consequences of hydric and thermic stresses on microbial communities by modulating communities' resistance in specific stands. These results suggest that tree residues retention, in specific stands, could contribute to sustain soil biological properties in the context of Mediterranean climate intensification.