Understanding the impacts of diurnal freeze-thaw cycles (DFTCs) on soil microorganisms and greenhouse gas emissions is crucial for assessing soil carbon and nitrogen cycles in the alpine ecosystems. However, relevant studies in the permafrost regions in the Qinghai-Tibet Plateau (QTP) are still lacking. In this study, we used high-throughput pyrosequencing and static chamber-gas chromatogram to study the changes in topsoil bacteria and fluxes of greenhouse gases, including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), during autumn DFTCs in the permafrost regions of the Shule River headwaters on the western part of Qilian Mountains, northeast margin of the QTP. The results showed that the bacterial communities contained a total of 35 phyla, 88 classes, 128 orders, 153 families, 176 genera, and 113 species. The dominant phyla were Proteobacteria, Acidobacteria, Actinobacteria, Chloroflexi, and Gemmatimonadetes. Two DFTCs led to a trend of increasing bacterial diversity and significant changes in the relative abundance of 17 known bacteria at the family, genus, and species levels. These were predominantly influenced by soil temperature, water content, and salinity. In addition, CO2 flux significantly increased while CH4 flux distinctly decreased, and N2O flux tended to increase after two DFTCs, with soil bacteria being the primary affecting variable. This study can provide a scientific insight into the impact of climate change on biogeochemical cycles of the QTP.

Understanding the impacts of diurnal freeze-thaw cycles (DFTCs) on soil microorganisms and greenhouse gas emissions is crucial for assessing soil carbon and nitrogen cycles in the alpine ecosystems. However, relevant studies in the permafrost regions in the Qinghai-Tibet Plateau (QTP) are still lacking. In this study, we used high-throughput pyrosequencing and static chamber-gas chromatogram to study the changes in topsoil bacteria and fluxes of greenhouse gases, including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), during autumn DFTCs in the permafrost regions of the Shule River headwaters on the western part of Qilian Mountains, northeast margin of the QTP. The results showed that the bacterial communities contained a total of 35 phyla, 88 classes, 128 orders, 153 families, 176 genera, and 113 species. The dominant phyla were Proteobacteria, Acidobacteria, Actinobacteria, Chloroflexi, and Gemmatimonadetes. Two DFTCs led to a trend of increasing bacterial diversity and significant changes in the relative abundance of 17 known bacteria at the family, genus, and species levels. These were predominantly influenced by soil temperature, water content, and salinity. In addition, CO2 flux significantly increased while CH4 flux distinctly decreased, and N2O flux tended to increase after two DFTCs, with soil bacteria being the primary affecting variable. This study can provide a scientific insight into the impact of climate change on biogeochemical cycles of the QTP.

Vast amounts of carbon are bound in both active layer and permafrost soils in the Arctic. As a consequence of climate warming, the depth of the active layer is increasing in size and permafrost soils are thawing. We hypothesize that pulses of biogenic volatile organic compounds are released from the near-surface active layer during spring, and during late summer season from thawing permafrost, while the subsequent biogeochemical processes occurring in thawed soils also lead to emissions. Biogenic volatile organic compounds are reactive gases that have both negative and positive climate forcing impacts when introduced to the Arctic atmosphere, and the knowledge of their emission magnitude and pattern is necessary to construct reliable climate models. However, it is unclear how different ecosystems and environmental factors such as drainage conditions upon permafrost thaw affect the emission and compound composition. Here we show that incubations of frozen B horizon of the active layer and permafrost soils collected from a High Arctic heath and fen release a range of biogenic volatile organic compounds upon thaw and during subsequent incubation experiments at temperatures of 10 degrees C and 20 degrees C. Meltwater drainage in the fen soils increased emission rates nine times, while having no effect in the drier heath soils. Emissions generally increased with temperature, and emission profiles for the fen soils were dominated by benzenoids and alkanes, while benzenoids, ketones, and alcohols dominated in heath soils. Our results emphasize that future changes affecting the drainage conditions of the Arctic tundra will have a large influence on volatile emissions from thawing permafrost soils - particularly in wetland/fen areas.