Microbial nutrient limitations in soil during afforestation in the Mu Us Desert, China: insights from extracellular enzyme stoichiometry

Abstract

Afforestation is a key ecological restoration measure for combating desertification in arid and semi-arid regions. But the changes and drivers of nutrient limitations faced by soil microbes during long-term afforestation are unclear. This study investigated soil ecosystems of different-aged stands (planted 10, 30, 50, and 70 years ago) formed by the sand-fixing Caragana korshinskii shrub in China’s Mu Us Desert, using extracellular enzyme stoichiometry to infer the dynamics of microbial nutrient limitation. Results showed that establishing C. korshinskii sand-fixing vegetation alters the carbon (C), nitrogen (N), phosphorus (P) stoichiometry of soil microbes and their extracellular enzymes. With increasing stand age, soil C: N, C: P, N: P, microbial biomass C: P, and enzyme N: P ratios all increased significantly, peaking at 70 years. However, enzyme C: N peaked much earlier, at 10 years, after which it declined, while enzyme C: P reached its peak at 10 years and then stabilized afterwards. Microbial C- and P- use efficiency both decreased, whereas N-use efficiency initially improved but later weakened. Vector analysis indicated that microbial C limitation intensified during the first decade of shrub stand development (0–10 years), and then stabilized after 10 years, while P limitation declined (0–70 years). Both multiple regression models for VA and VL identified the enzyme C: P as a closely associated variable with microbial nutrient limitations. This work provides scientific insights into how C and N are coupled mechanistically in soil during afforestation of desertland from the perspective of soil–microbe–extracellular enzyme ecological stoichiometry, offering theoretical support for greening desert-like ecosystems and promoting sustainable development in arid and semi-arid regions.