Soil fertility shifts the relative importance of saprotrophic and mycorrhizal fungi for maintaining ecosystem stability.

Soil microbial communities are essential for regulating the dynamics of plant productivity. However, how soil microbes mediate temporal stability of plant productivity at large scales across various soil fertility conditions remains unclear. Here, we combined a regional survey of 51 sites in the temperate grasslands of northern China with a global grassland survey of 120 sites to assess the potential roles of soil microbial diversity in regulating ecosystem stability. The temporal stability of plant productivity was quantified as the ratio of the mean normalized difference vegetation index to its standard deviation. Soil fungal diversity, but not bacterial diversity, was positively associated with ecosystem stability, and particular fungal functional groups determined ecosystem stability under contrasting conditions of soil fertility. The richness of soil fungal saprobes was positively correlated with ecosystem stability under high-fertility conditions, while a positive relationship was observed with the richness of mycorrhizal fungi under low-fertility conditions. These relationships were maintained after accounting for plant diversity and environmental factors. Our findings highlight the essential role of fungal diversity in maintaining stable grassland productivity, and suggest that future studies incorporating fungal functional groups into biodiversity-stability relationships will advance our understanding of their linkages under different fertility conditions.