Changes in soil microbial community and carbon use efficiency in freeze-thaw period restored after growth season under warming and straw return

Microbial communities and metabolic activities play key roles in carbon (C) turnover in terrestrial ecosystems, which are directly and indirectly affected by freeze-thaw cycles. However, the immediate and legacy effects of freeze-thaw periods on soil microbial community structure and C metabolic activity in agricultural ecosystems were still not fully understood. In this study, we evaluated the changes in soil microbial community structure and C metabolic activity during the freeze-thaw period and the growth season under the condition of warming and straw return. The results showed that the microbial biomass and the ratio of fungi to bacteria (F/B) were sensitive to freeze-thaw cycles and straw return. Both gradually decreased during the freeze-thaw period. The ratio of gram-positive to gram-negative bacteria (GP/GN) gradually increased during the freeze-thaw period. Then all these changes recovered during the growth season. Microorganisms have a certain self-regulating ability to deal with freeze-thaw stress. In contrast, microbial C use efficiency (CUE) did not significantly change during the freeze-thaw period, but increased during the growth season. Microbial CUE had no significant correlations with microbial biomass, the F/B, and the GP/GN, while it was negatively related with the ratio of dissoluble organic C to dissoluble total nitrogen (N) and the imbalance ratio between resources and microorganisms (C:N imbalance). These findings suggested that soil stoichiometric ratio played important role in regulating microbial CUE, instead of microbial community characteristics.