Altered soil microbial properties and functions after afforestation increase soil carbon and nitrogen but not phosphorus accumulation

Afforestation can substantially influence terrestrial carbon (C), nitrogen (N), and phosphors (P) stocks by changing soil microbial properties and functions, but the direction and magnitude of the effects of afforestation on soil microbial properties and functions at global scales remain unknown. Here, we conducted a meta-analysis of 1292 paired observations from 217 studies to explore the effects of afforestation on soil microbial communities and their linkages with the dynamics of soil C, N, and P stocks. Afforestation increased total plant biomass and litterfall by 314% and 643%, respectively, and increased soil organic C (SOC) and N stocks by 37% and 29%, respectively, but did not significantly affect soil P stock. Afforestation increased soil bacterial and fungal biomass by 36% and 49%, respectively, and fungi: bacteria ratio by 20%, suggesting that microbial groups shifted; i.e., the biomass of r- strategists decreased and the biomass of K- strategists increased. Moreover, afforestation increased the activities of beta-glucosidase (59%), cellulase (136%), beta-1,4-N-acetylglucosamnidase (123%), urease (59%) and acid phosphatase (66%), and increased microbial biomass C (MBC, 85%), N (MBN, 109%), and the MBC:SOC ratio (27%), but decreased the microbial metabolic quotient (13%). The beneficial effects of afforestation on soil microbial properties and functions differed depending on the initial land cover type, stand age, stand type, plantation species, and climate change. The increases in soil C and N stocks following afforestation were indirectly associated with the increases in fungal biomass, MBC, MBN, and directly related to the increases in the activities of cellulase and beta-1,4-N-acetylglucosaminidase and the decreases in the microbial metabolic quotient besides the increased plant biomass and litterfall. Overall, these results suggest that the preservation of soil functions following afforestation should be considered for C and N sequestration to mitigate climate change.