Responses of soil microbial activity and nutrient mineralization to moisture changes across grassland types in the arid Helan Mountain region, China

Soil microorganisms are key drivers of nutrient cycling and ecosystem functioning, particularly in arid regions where low moisture availability constrains their activity. This study investigated the effects of increasing soil moisture to 50% of water-holding capacity (WHC) on dissolved nutrients, microbial biomass, extracellular enzyme activities, and nutrient mineralization in three grassland types in the Helan Mountains, Inner Mongolia, China: temperate desert (TD), temperate desert steppe (TDS), and temperate steppe (TS). Compared with TD, soil organic carbon, total nitrogen, and total phosphorus were 192–469%, 192–388%, and 75–205% higher, respectively, in TDS and TS. TS had the greatest microbial biomass, whereas TDS generally showed higher dissolved nutrient concentrations and nutrient mineralization rates. Water addition significantly enhanced microbial biomass by 7.4%–97.7%, while nutrient mineralization responses varied across grassland types. Specifically, the carbon and nitrogen mineralization rates in TS and TDS exhibited an increase ranging from 9.2% to 83.6%, while the phosphorus mineralization rate in TD increased by 28.1–119%. Partial least squares path modeling indicated that, under 50% WHC, dissolved nutrients and microbial biomass were positively associated with nutrient mineralization, whereas the relative abundances of Basidiomycota and Chytridiomycota were negatively associated with nutrient mineralization. The measured extracellular hydrolases were not the dominant statistical predictors of short-term nutrient mineralization under the present incubation conditions. These findings highlight the roles of moisture availability and microbial communities in regulating nutrient cycling in arid grasslands, providing insights into nutrient mineralization mechanisms.