Seasonal freeze-thaw processes regulate and buffer the distribution of microbial communities in soil horizons

The freeze-thaw process alters microbial communities in alpine soils. However, soil microbial responses to seasonal freeze-thaw processes have been overlooked under climate change, especially in deep soil. In this study, microbial samples were collected from three alpine meadows on the northeastern Qinghai-Tibet Plateau during four seasonal freeze-thaw periods (TP: thawed period; UFP: unstable freezing period; FP: frozen period; and UTP: unstable thawing period), and the taxa and biomass of living microbes were determined by the phospholipid fatty acid (PLFA) method. The results showed that the microbial biomass in the TP was significantly lower than that in other seasonal freeze-thaw periods. The biomass of fungi and gram-positive bacteria (GP) was high in the UFP and FP, and the biomass of bacteria and gram-negative bacteria (GN) was high in the UTP. The soil mi-crobial biomass decreased with soil depth, and the difference in soil microbial biomass between soil horizons in the FP was smaller than those in the UTP, FP and UFP. The values of GN/GP and fungi/bacteria (F/B) decreased with soil depth during all seasonal freeze-thaw periods and were lower in the UFP and FP than in the TP and UTP. The main controlling factors of the total microbial biomass of the alpine meadow were soil temperature, soil organic carbon (SOC) and total nitrogen (TN) during the seasonal freeze-thaw periods. TN was the critical soil variable affecting the total microbial biomass in the TP. SOC was the critical soil variable affecting the total microbial biomass in the UFP and UTP. With global warming (especially in winter), soil microbial biomass can be expected to decrease in alpine regions. These results could provide a deep understanding of biodiversity and the prediction of carbon cycle in alpine regions.