Plant species richness increases the relationship between soil microbial and extracellular enzyme activities and enhances soil fertility

• Plant species from the Yanchi Desert Steppe to assess plant species richness. • The microbial abundance varied, but the core microbial communities were the same among levels. • The EEAs did not show a consistent trend with the increase of species richness. • A strong correlation between C, N, and P-acquisition enzyme activities and the cellular processes. • The functional activities of microorganism influenced SOC, TN, and TP by regulating EEAs. Diverse plant species are crucial for the sustenance of soil health and the facilitation of nutrient cycling by reconstructing the soil microbial communities and improving extracellular enzyme activities (EEAs). However, it is unclear whether the effect of plant species combination on the interaction between soil microbial communities and EEAs contributes to the improvement of soil quality. Therefore, we selected three dominant and seven subdominant plant species from the northern Yanchi desert steppe of Ningxia to assess plant species richness (monoculture and 4-, 6-, 8-, and 10-species mixtures). We found the following: (1) The number of ASVs of soil bacteria and fungi in monoculture was generally higher than that in mixed communities. Although the microbial abundance varied among plant species richness levels, the core microbial communities were the same. EEAs in monoculture were higher than species mixtures, but EEAs did not show a consistent trend with the increase of species richness. (2) Phylogenetic Investigation of Communities by Reconstruction of Unobserved States predicted 5 primary functions, encompassing 25 secondary functions dominated by bacterial metabolism, and 5 primary functions, encompassing 29 secondary functions dominated by fungal biosynthesis. (3) The Mantel test results demonstrated a strong correlation between soil carbon (C), nitrogen (N), and phosphorus (P) acquisition enzyme activities and the functional cellular processes of bacteria. (4) Structural equation modeling revealed that plant species richness directly negatively affected soil C, N, and P-acquiring enzyme activities. However, the functional activities of bacterial and fungal communities positively influenced soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) by indirectly regulating EEAs. The final model explained 69% of the SOC, 61% of the soil TN, and 25% of the soil TP. This study aimed to provide valuable data to support theoretical frameworks for conserving grassland biodiversity and maintaining soil health.