Elevated CO2 shifts soil microbial communities from K- to r-strategists

Aims: Soil microbes are key to myriad processes in terrestrial ecosystems. Elevated CO2 represents a dominant driver of global climate change; however, it remains unclear to what extent elevated CO2 impacts soil microbial communities at ecosystem and global scales. Here, we sought to address the following questions: (a) Do the compositions of microbial communities shift from K- to r-strategists under elevated CO2? (b) What is the extent of the compositional shifts of microbial communities affected by elevated CO2 concentrations, experimental duration, ecosystem types and/or background climates? (c) Are the responses of microbial communities to elevated CO2 associated with changes in soil pH and carbon and nitrogen availabilities?. Location: Global. Time period: 1998–2020. Major taxa studied: Soil microbes. Methods: We performed a global meta-analysis of 965 observations from 122 studies, which tested the effects of elevated CO2 on microbial communities. The data covered broad variations in ecosystems, climate, CO2 concentrations, experimental duration, and soil factors. Results: We revealed that elevated CO2 decreased the K- to r-strategist ratios with decreasing fungi : bacteria, Gram+ : Gram– bacteria, and Acidobacteria : Proteobacteria ratios, and increased bacterial biomass, microbial biomass carbon, Gram– bacteria, and Acidobacteria abundance. Moreover, the shifts from K- to r-strategists were more pronounced under higher CO2 concentrations and longer experimental durations. The responses of microbial attributes to elevated CO2 did not differ significantly among croplands, forests and grasslands. Furthermore, the response of microbial biomass to elevated CO2 was negatively correlated with the response of soil pH, while those of bacterial biomass and fungi : bacteria ratios were positively correlated with those of soil organic carbon and soil carbon : nitrogen ratios, respectively. Main conclusions: Our results suggest that elevated CO2 shifts soil microbial communities from K- to r-strategists, and provide supportive evidence for understanding responses of soil microbial processes to elevated CO2