Soil specific enzyme stoichiometry reflects nitrogen limitation of microorganisms under different types of vegetation restoration in the karst areas

Knowledge about resource limitations faced by soil microorganisms is crucial for understanding ecosystem functions and processes. In recent decades, vegetation restoration has been carried out in the degraded karst areas, leading to the alteration in the status of microbial resource limitation (MRL). However, mechanisms underlying MRL in different karst ecosystems remain poorly understood. Here we investigated MRL based on the theory of soil extracellular enzyme stoichiometry. Soil carbon (C), nitrogen (N), and phosphorus (P) acquiring enzyme activity (glucosidase, cellobiohydrolase, leucine aminopeptidase, urease and alkaline phosphatase) per unit microbial biomass carbon (MBCE) and per unit soil organic carbon (SOCE) of four main vegetation types (natural community, NC; ecological forest, EF; abandoned cropland, AC and economic plantation, EP) at 0–20 cm depth were measured in the karst areas in southwestern China. Significantly higher MBCE levels were found in EP than the other three vegetation types (p < 0.05), with EF having significantly higher SOCE than NC (p < 0.05), suggesting the highest metabolic activity and soil organic carbon (SOC) utilisation efficiency in EP and EF, respectively. C- and N-specific enzyme activity (MBCE and SOCE) were strongly negatively correlated with microbial biomass and soil C, N contents (p < 0.05), suggesting microbial resource requirement promotes the production of specific enzyme in resource-deficient ecosystems. Homeostasis analysis of microbial biomass C∶N and resources C∶N for all communities showed no homeostasis, indicating the microorganisms may be autotrophic to meet their N demands. In addition, the specific enzyme C∶N ratios were less than 1, the N∶P ratios were greater than 1, and vector angles were all less than 45° in all four types of vegetation restoration, indicating clear N limitation. A homeostasis analysis, as well as extracellular enzymatic stoichiometry and vector analysis, all suggested that soil microorganisms in the four vegetation types were nitrogen-limited, with NC most severely affected. Overall, we suggest that nitrogenous fertilisers should be added to restore the balance of elements while recovering the karst ecosystems.