Drivers of soil microbial metabolic limitation changes along a vegetation restoration gradient on the Loess Plateau, China

Soil extracellular enzymatic activity (EEA) stoichiometry could reflect the biogeochemical equilibrium between the metabolic requirements of microbial communities and environmental nutrients availability. However, the drivers of soil microbial metabolic limitation (SMML) changes remain poorly understood following vegetation restoration. We compared sites along a vegetation restoration chronosequence over a 30-year period on the Loess Plateau, China, and measured the potential activities of two C-acquiring enzymes (beta-1,4-glucosidase (BG) and beta-D-cellobiosidase (CBH)), two N-acquiring enzymes (beta-1,4-N-acetylglucosaminidase (NAG) and L-leucine aminopeptidase (LAP)), and one organic-P-acquiring enzyme (alkaline phosphatase (AP)), to quantify and compare the variations in metabolic limitations for soil microorganisms using EEA stoichiometry. The results showed constant microbial P limitation, but not N limitation, and an open downward unimodal trend in microbial C limitation; however, the microbial P limitation displayed exactly the opposite trend during vegetation restoration. Restoration age and properties of plant, soil, and microorganisms contributed to 82.9% of microbial C limitation and 84.6% of microbial P limitation, with soil presenting the highest relative effects of 76.1% and 59.6% on microbial C and P limitations, respectively. Plant productivity and species diversity decreased microbial C limitation owing to increasing plant C inputs, but increased microbial P limitation owing to plant nutrients competition with soil microorganisms. When the fungi:bacteria ratio in the soil increased, the SMML increased. Vegetation restoration increased the soil nutrients content and reduced SMML, and a decrease in the soil water content increased microbial P limitation. Thus, the effects of long-term vegetation restoration on SMML were the result of combined influences of plants, soil, and microorganisms.