Iron (Fe) minerals possess a huge specific surface area and high adsorption affinity, usually considered as rust tanks of organic carbon (OC), playing an important role in global carbon storage. Microorganisms can change the chemical form of Fe by producing Fe-chelating agents such as side chains and form a stable complex with Fe(III), which makes it easier for microorganisms to use. However, in seasonal frozen soil thawing, the succession of soil Fe-cycling microbial communities and their coupling relationship with Fe oxides and Fe-bound organic carbon (Fe-OC) remains unclear. We characterized changes in the Fe phase, Fe-OC, Fe-oxidizing bacteria (FeOB), and Fe-reducing bacteria (FeRB) in the subsoil and analyzed the microbial mechanism underlying Fe-OC changes in alpine grassland by constructing a composite structural equation model (SEM). We found that the Fe(III) content consistently exceeded that of Fe(II). Among the three types of Fe oxides, organically complex Fe (Fe-p) decreased from 2.54 to 2.30 gkg(-1), whereas the opposite trend was observed for poorly crystalline Fe (Fe-o). The Fe-OC content also decreased (from 10.31 to 9.47 gkg(-1); p < 0.05). Fe-cycling microorganisms were markedly affected by the thawing of frozen soil (except FeRB). Fe-p and Feo directly affected changes in Fe-OC. Soil moisture (SM) and FeOB were significant indirect factors affecting Fe-OC changes. Freeze-thaw changes in the subsoil of alpine grassland in Central Asia significantly affected FeOB and Fe oxides, thus affecting the Fe-OC content. To the best of our knowledge, this was the first study to examine the influence of Fe-cycling microorganisms on the Fe phase and Fe-OC in the soil of alpine grassland in Central Asia. Overall, our findings provide scientific clues for exploring the biogeochemical cycle process in future climate change.

In the context of climate change, the variation of seasonal frozen soil affects the eco-hydrological process in a water tower, which has been widely noted by scientists worldwide. However, the latitudinal characteristics of the temporal and spatial variation of seasonal freezing depth and their response to changing climatic factors need to be strengthened. Therefore, Changbai Mountain, a typical high-latitude water tower, was chosen to analyze the change in temporal and spatial variation of freezing depth and the influence of climatic factors by the modified Mann-Kendll trend test and Generalized Additive Model (GAM) methods. Results showed that the higher the latitude, the greater the freezing depth, the longer the freeze-thaw cycles, the earlier the freeze onset and the later the thaw onset. However, the frozen soil had a clear degradation trend during the period 1960-2018. There was also a significantly latitudinal characteristic. The higher the latitude, the greater the degradation of frozen soil. The downward trend was the largest with-0.35 cm/yr in the high latitude, followed by-0.24 cm/year in the middle-high latitude and the smallest with-0.10 cm/yr in the low latitude. In addition, due to climate change, the period of freeze-thaw cycles has been shortened, the freeze onset was delayed, and thaw onset has been advanced. According to the response of monthly average freezing depth (MAFD) to climatic factors, there is a strong correlation between MAFD and climatic factors in different months. When the soil started to freeze and thaw, temperature was the main factor influencing the change in freezing depth (p < 0.05). It is interesting to note that the air temperature contributed more strongly to the change in MAFD than surface temperature. When the frozen soil was in stable freezing period (from December to March of the following year), the snow cover gradually became the main influencing factor. Snow depth and snow pressure had the greatest contribution to the degradation of frozen soil. The higher the latitude, the longer the duration of influence of snow on frozen soil (explained difference = 20-61%). In addition, wind speed was also an important influencing factor on the change of MAFD in each month. Especially during the thaw period in April and May, wind speed was the most important influencing factor in the high latitude region. This study would be beneficial for the protection of the ecohydrological cycle in cold region and would provide a basis for the study of seasonal frozen soil.