Does forest stand density affect soil microbial communities?

Forest management aims to maintain sustainable production of quality wood while limiting increased competition between trees for light, water, and nutrients. Thinning is a widely used silvicultural practice to reduce plants competition for resources while still exploiting the wood. The investigation of the effects of forest management on stand functioning typically centers on the above-ground compartment, overlooking the alterations and influences exerted on below-ground biotic factors. Within the soil matrix, biological mechanisms are mainly governed by microbial communities. Many studies have focused on the effects of thinning on soil microbial communities (SMC), evidencing contrasted effects. Conversely, stand density effects on SMC are less documented. The aim of this study is therefore to focus on the effects of stand density (SD) on SMC biomass, gene abundance, functional diversity, and activity, according two sylvicultural practices: dynamic (low SD) and conservative (medium SD) in a temperate Quercus petraea Stand (QS) in Europe Forest. We hypothesized that dynamic silviculture (low-SD) could promote soil SMC biomass, abundance, functional diversity, and activity. Our results showed that dynamic silvicultural practices in oak forests reduced the abundances of bacteria, archaea and fungi were reduced by 43 %, 29 % and 34 %, respectively. SMC functional diversity was reduced by 10 % in dynamic forestry stands. On the contrary, dynamic silvicultural practices increased soil microbial activity by 13 to 47 %, depending on the carbon source added, compared with conservative silviculture. Our results were incremented with an extensive number of biotic and abiotic environmental variables that had contrasting effects on SMC, and there is no single factor, which alone can explain all the SMC responses. Our results seem to advocate dynamic silvicultural practices in oak forests to promote soil microbial activity. However, it remains to be seen what the long-term effects will be of the reduced abundance and functional diversity of SMCs observed jointly in low-SD.