Negative effects of multiple global change factors on soil microbial diversity

Soil microbial diversity is one of the key factors affecting the structure and function of the belowground ecosystem; yet, little is known about the response of microbial diversity to multiple global change factors. Here, we conducted a global meta-analysis based on data collected from 237 published papers to explore the effect of multiple global change factors (elevated carbon dioxide (eCO₂), warming, elevated nitrogen addition (eN), wetting–drying cycle, drought, decreased precipitation (precipitation(−)), and increased precipitation (precipitation(+))) on microbial diversity (Shannon index) across different ecosystems (cropland, grassland, forest, shrubland, desert, wetland, and tundra). Global change decreased soil bacterial and fungal diversity by an average of 2.9% and 3.5%, respectively. For each global change factor, the effect sizes of precipitation(−), eN, wetting–drying cycle, and drought on soil microbial diversity were negative, whereas the effect sizes of eCO₂, warming, and precipitation(+) were positive. This phenomenon was driven by changes in mean annual temperature (MAT) and edaphic factors (especially soil pH, bulk density and organic carbon content) rather than mean annual precipitation. Moreover, the effect size of soil microbial diversity linearly declined with increasing MAT, suggesting that microbial diversity was highly dependent on climate conditions at the global scale. In addition, two- and three-way interactions of global change factors aggravated the negative effects of individual effects. We suggest that it is essential to conduct long-term, multiple-factor experiments to assess the response of soil microbial diversity to global change because multiple global change factors often occur simultaneously.