Decoupling of soil organic carbon and nutrient mineralization across plant communities as affected by microbial stoichiometry

To investigate the effects of shrubification—the global phenomena of an increase of shrubs in grasslands—on C, N, and P cycles, the changes in soil organic C, N, and P mineralization in an alpine meadow were compared across five plant communities: grasses and four shrub species–dominated patches. The nutrient content and stoichiometry (C:N:P) of leaves, litter, microbial biomass, and soil organic matter (SOM) were analyzed during vegetation season. Net rates of N and P mineralization were measured in situ in the top 20 cm of soil throughout the growing season, and organic C mineralization was determined under controlled conditions. Microbial C:N, C:P, and N:P ratios in the top 20 cm generally decreased with increasing plant size (height combined with crown diameter), associated with greater input of litter with lower C:nutrient ratios under shrubs. The net N and P mineralization rates in soil under shrubs were about 3- to sevenfold and 4- to 15-fold faster, respectively, compared with those under grasses. The increase in organic C mineralization under shrubs compared with that under grasses was much smaller than the increase of N or P mineralization under shrubs. This indicates faster turnover of nutrients than C leading to decoupling of organic C and nutrient mineralization across plant communities by shrubification. The C:N, C:P, and N:P ratios of organic pools mineralized in soil decreased with increasing plant size, but increased with respective microbial C:N, C:P, and N:P ratios across plant communities. This indicates that specific SOM pools were mineralized depending on plant communities and microbial stoichiometry in soil. Consequently, the decoupling of organic C and nutrient mineralization across plant communities is driven by microbial stoichiometry and increases by shrubification.