The Mitigation of Microbial Carbon and Nitrogen Limitations by Shrub Encroachment: Extracellular Enzyme Stoichiometry of the Alpine Grassland on the Qinghai-Tibetan Plateau

Shrub encroachment changes the patterns of nutrition allocation in the below- and aboveground soil. However, influence of shrub encroachment on microbial carbon (C) and nitrogen (N) limitations remains unclear. Using the extracellular enzyme stoichiometry model, microbial nutrition limitations in bulk and rhizosphere soils at various soil layers were investigated at non-shrub alpine grasslands (GL) and shrub-encroached alpine grasslands including Spiraea alpina lands (SA), Caragana microphylla lands (CM) and Potentilla fruticosa lands (PF) on the Qinghai-Tibetan Plateau. We determined C-acquisition (β-1,4-glucosidase (BG); β-D-fibrinosidase (CBH)), N-acquisition (β-1,4-N-acetylglucosaminidase (NAG); leucine aminopeptidase (LAP)) and phosphorus (P)-acquisition (acid phosphatase (AP)) enzyme activities. The contents of soil organic carbon (SOC) in top- and subsoils significantly increased following shrub encroachment. Interestingly, (LAP + NAG) activities in subsoil increased following shrub encroachment. EC:N in subsoil decreased following shrub encroachment. Microbial C and N limitations were found in shrub-encroached and non-shrub alpine grasslands. Furthermore, microbial C and N limitations in bulk topsoil layers decreased following shrub encroachment. Microbial N limitations in subsoil decreased following shrub encroachment. This result indicates that shrub encroachment mitigated microbial C and N limitations. The limitations were gradually mitigated following shrub encroachment, which led to the decrease of the decomposition rate of organic carbon by microorganisms, indicating shrub encroachment might potentially contribute to SOC storage. In addition, the structural equation modeling (SEM) showed that increases of SOC and NH4+–N in top- and subsoils under shrub encroachment could mitigate microbial C and N limitations, respectively. This study provides available information on the environmental variables affecting the stoichiometry of extracellular enzymes following shrub encroachment, and the theoretical basis for the study of C and N cycling in alpine grasslands.