Dynamics of soil microbial C:N:P stoichiometry and its driving mechanisms following natural vegetation restoration after farmland abandonment.

Vegetation restoration after farmland abandonment has increased greatly and is commonly used to improve soil fertility and ecosystem service. Knowledge of soil community-level elemental homeostasis following natural vegetation restoration is specially limited for the abandoned farmlands. This study examined the changes in soil microbial biomass stoichiometry and homeostasis with a chronosequence of 3, 8, 13, 18, 23 and 30 years following natural vegetation restoration since farmland abandonment on the Loess Plateau, China. Vegetation communities, soil properties, microbial communities, and enzyme activities were analyzed to study the drivers on soil microbial C:N:P stoichiometry. The results showed that soil microbial biomass C: N ratios had little change following natural vegetation restoration since farmland abandonment, natural vegetation >23 years had significantly enhanced the microbial biomass C:P and N:P ratios by 26.1%-133.9% and 31.7%-67.4%, respectively. However, microbial biomass C:N, C:P and N:P ratios were constrained following natural vegetation restoration. Vegetation restoration for 30 years enhanced urease and alkaline phosphatase activities by 125.4% and 42.9%, respectively, which showed synchronous changes with N and P contents in microbial biomass. Soil fungi, urease and alkaline phosphatase were the drivers to the changes in microbial C:N:P stoichiometry. The results suggest that long-term vegetation restoration (>23 years) will aggravate microbial P limitation, however, soil microorganism maintained the homeostatic regulation of stoichiometric ratios to mitigate P limitation. Fungi played a strong role in shaping microbial community-level elemental homeostasis and nutrient cycling through releasing N-converting and P-converting enzymes into soil following natural vegetation restoration.