查看更多>>摘要：? 2021 The Author(s)Freeze-thaw (FT) events exert a great physiological stress on soil microorganisms and hence impact biogeochemical processes in soils. As numerous environmental factors affect microbial and chemical responses to FT, a better understanding of the leverage factors that regulate the responses to FT events is required. To date, FT-induced shifts and transformations in microbial and resource stoichiometry have received particularly little attention. We exposed fifteen Luvisol soils with different time after restoration and corresponding differences in soil organic C contents from a postmining agricultural chronosequence to a single FT event and analysed changes in soil chemistry and microbial stoichiometry one hour and eighteen hours after thawing. FT considerably altered soil biochemical attributes within the first hours of thawing. Microbial biomass C declined substantially after FT, and its relative losses were positively correlated with enhanced dissolved organic C contents. Thus, microbial cell lysis likely led to the significant increase of dissolved organic C. Moreover, microbial biomass C losses were disproportionally higher in C-rich soils, suggesting that soil microorganisms in high-C soils might be particularly prone to FT stress. Microbial biomass N marginally decreased one hour after thawing, yet returned to initial levels eighteen hours after thawing. The alternating responses of microbial biomass C and N caused a strong stoichiometric reduction of the microbial C:N ratio. The resulting microbial oversaturation with N relative to C is likely the first step in the chain of processes that generally lead to the high N losses commonly recorded in agricultural soils in the aftermath of FT events. Metabolic activity of the soil microbial community increased with the relative decline of the microbial biomass C:N ratio eighteen hours after thawing, suggesting increased levels of microbial metabolic expenditure due to stoichiometric shifts. The strength of the FT-driven biochemical responses was strongly dependent on soil organic C content, indicating that high-C soils might be especially vulnerable to initial C and N losses due to shifts in microbial stoichiometry.
查看更多>>摘要：? 2021 Elsevier B.V.Humin (HM), an insoluble humic substance that occurs as an organo-mineral complex, mediates extracellular electron transfer (EET) among microorganisms in the geosphere. Although sulfur (S)-containing functional groups have been posited as possible redox-active groups in HM, there has been no direct evidence in this regard. Here, we characterized the redox-active S in HM from different natural sources using synchrotron radiation-based X-ray absorption spectroscopy in partial fluorescence yield mode. The S content in HM is less than 0.9%, with the oxidation states ranged widely from ?2 to +6. About 9% to 19% of total S in HM was redox variable, corresponding to 1.2 × 10?5 to 17.1 × 10?5 Eq/g-HM redox capacity. The redox capacity of HM driven by S is comparable to or larger than the electron donating capacity of HM observed in anaerobic microbial dechlorination of pentachlorophenol, suggesting that S is one of the significant redox-active elements in HM. These findings provide new insights into the role of S cycling in biogeochemical processes and microbial energy networks.
查看更多>>摘要：? 2021 Elsevier B.V.Rhizosphere processes play a critical role in soil organic carbon (SOC) cycling that is primarily regulated by temperature. Understanding the response of rhizospheric SOC decomposition to global warming, which is called temperature sensitivity (Q10), is pivotal for predicting the feedback of SOC cycling to global warming. However, the rhizosphere effects (REs) on Q10 and their underlying mechanisms in forest ecosystems remain unclear. Here, the REs on Q10 for Cunninghamia lanceolata and three understory ferns (e.g., Woodwardia japonica, Parathelypteris glanduligera and Microlepia marginata) in a subtropical forest were explored using a novel incubation procedure with periodically changing temperatures based on the mean annual temperature. Our results showed that the positive REs on Q10 were observed for all plant species, which ranged from 33% to 88%, and P. glanduligera exhibited higher REs on Q10 than C. lanceolata. The positive REs on Q10 were associated with the rhizospheric nitrogen (N) availability and microbial properties. The REs on N component (i.e., the REs on total N, NH4+ and NO3? along the first PCA axis), which is the most important driver, had a positive direct effect on the REs on Q10. Furthermore, the rhizospheric microbial biomass and the REs on microbial residues were also positively related to the REs on Q10. Overall, these findings highlight that plant-covered soils have high risks of C emissions under planetary warming, underscore the importance of root-soil interactions for accurately predicting SOC dynamics and reveal that rhizospheric nutrients and microbial properties drive the feedback of the root-associated SOC cycle to global warming.
查看更多>>摘要：? 2021 The Author(s)Climate warming is responsible for many environmental changes in the Arctic, which lead to the decomposition of soil organic matter (SOM) and emissions of greenhouse gases from the soil into the atmosphere. Soil minerals play a crucial role in SOM stabilization. However, little is known about the occurrence and stability of organo-mineral associations in permafrost-affected soils in the Arctic. The main aims of this study were: 1) to determine the potential occurrence of SOM within interlayers of swelling clay minerals in the permafrost-affected soils in central part of Spitsbergen (Svalbard, High Arctic) and 2) to determine resistance of the intercalated SOM within the clay minerals against thermal and chemical oxidation. The obtained results indicate that 10% to 15% of organic carbon and 30% to 45% of total nitrogen occurring in the clay fraction of the Arctic permafrost-affected soils are intercalated within the swelling clay minerals. We also report that this part of SOM is highly resistant to both chemical and thermal oxidation. These findings should be taken into consideration in the refinement of climate models and in studies concerning the thawing of permafrost, SOM mineralization, and emissions of greenhouse gases from the soil into the atmosphere.
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