Meta-analysis of the impact of freeze–thaw cycles on soil microbial diversity and C and N dynamics

Reduction in snow cover is a prominent aspect of global change. Freeze–thaw cycles (FTCs) of different amplitudes and durations in soil due to insufficient thermal insulation may alter microbial diversity and key ecological functions mediated by microorganisms. These changes could then further alter the cycling of material and energy in the ecosystem. Yet despite many assessments, the impact of FTCs upon microbial diversity remains poorly understood. Here, 546 observations from 61 published studies were collected for a global meta-analysis with the objective to explore how soil microbial diversity and C and N dynamics it drives respond to FTCs. The results showed that: in general, FTCs did not lead to a reduction of microbial α-diversity, but they did reduce levels of soil microbial biomass carbon, microbial biomass nitrogen, and phospholipid fatty acid by 7%, 12%, and 11%, respectively; they also significantly changed the microbial community structure. FTCs did not significantly affect the α-diversity of bacteria and fungi, but community structures of both were changed significantly, with that of the bacteria more sensitive to FTCs. FTCs were responsible for a 6% decrease in functions related to C, N cycling, which could be explained by the changes found in microbial biomass and community structure. FTCs could also indirectly impact microbial biomass via changed pH and soil water content (SWC). The response of microbial community to FTCs was related to the FTC frequency, freezing temperature and sampling time. FTCs had a large effect on the C and N pool components and fluxes in soil. It is particularly noteworthy that FTCs drove a 137% increase in N2O emission. Further, the changes in pH and SWC directly affected the C and N pool components and fluxes. The results of current meta-analysis deepen the comprehensive understanding of the effects of FTCs on the soil microbial community and C and N dynamics it mediated, and provide a reference for subsequent research in terms of experimental scheme and scientific issues requiring close attention.