Adaptation mechanisms of soil microbial community to stoichiometric imbalances caused by forest conversion

Stoichiometric imbalances between soil resource availability and soil microbial biomass cause nutrient limitation for microbial activity, ultimately affecting soil carbon (C), nitrogen (N), and phosphorus (P) cycling. Little is known about how land use change, such as the conversion of primary natural broadleaf forests (BF) to monoculture plantations (PF) and regenerated secondary forests (SF), impacts stoichiometric imbalances and soil microbial communities. We measured soil available nutrients, microbial biomass, and potential activities of C-, N- and P-acquiring enzymes, and investigated the diversity and structure of soil microbial communities in BF, SF, and PF in subtropics. Forest conversion of BF to PF, but not to SF, increased dissolved organic carbon (DOC): available nitrogen (AN) ratio and slightly decreased microbial biomass C:P and N:P ratios, resulting in increasing C:P and N:P imbalances between soil resource availability and soil microbes. We found microbial communities to maintain stoichiometric homeostasis by increasing the threshold elements ratio of C:N (TER_C:N) and altering the stoichiometry of C-, N-, and P-acquiring enzymes in order to store scarce nutrients such as P. Higher stoichiometric imbalances of C:P and N:P in PF soils were associated with the decreases in fungal richness, α-diversity and bacterial β-diversity. Bacterial communities shifted from copiotrophs (Actinobacteria) to oligotrophs (Chloroflexi and Verrucomicrobia) with the conversion of BF to PF. This study suggests that the response of soil available nutrients (especially soil P) and soil microbial biomass to forest conversion, and associated changes in stoichiometric imbalances substantially regulate soil microbial community structure and enzyme activities with forest conversion.