Response of soil biogeochemistry to freeze-thaw cycles and implications for the soil respiration-climate feedback: A process-oriented experimental approach

冻融循环是影响中高纬度地区土壤的主要气候强迫。土壤的冻结和解冻会影响其物理性质、生物地球化学过程、微生物群落以及碳和营养收支。虽然过去几十年的研究已经揭示了冻融循环对温室气体排放和土壤化学浸出的巨大影响，但潜在的水文、物理、地球化学和微生物机制及其相互作用仍然知之甚少。其中一个原因是缺乏时间序列数据来捕捉冻融循环期间土壤系统内相关的运输和反应过程。因此，我们目前在预测土壤过程对当前和未来气候变化导致的冻融事件的频率、幅度和持续时间变化的响应方面受到严重限制。这项发现基金研究的中心目标是揭示土壤中耦合的物理、水文和生物地球化学过程，这些过程共同调节温室气体的产生，以及冻融循环期间微生物活动和孔隙水组成的变化。该方法结合了新开发的土壤柱系统中综合物理、化学和微生物数据的采集，该系统生成了与时间相关的垂直温度曲线，模拟了冻融条件下真实土壤的温度分布。该研究将被分解为四个高度创新的相互关联的（子）项目。项目1将使用多光纤光电微传感器记录经历连续冻结和解冻时期的土壤中氧气（O2）浓度的地下变化。在项目2中，将进行现场气相和水相化学的详细测量，以描绘冻融循环期间土壤微生物呼吸的途径并量化其速率。项目3将重点研究冻融引起的土壤物理和水力特性的变化，以及微生物群落结构的变化。项目4将把项目1-3的实验结果整合并联系到一个反应性输运模型框架中，该模型框架将能够解释冻融条件下的土壤生物地球化学过程。该项目的成果加强了对中高纬度土壤温室气体排放的野外测量结果的解释，并使人们能够在气候变化情景下对这些气体排放的变化以及地下水质量进行知情预测。

Freeze-thaw cycles represent a major climate forcing acting on soils at middle and high latitudes. Freezing and thawing of soils affect their physical properties, biogeochemical processes, microbial community, and carbon and nutrient budgets. Although research during the last few decades has revealed the large effects freeze-thaw cycles have on greenhouse gas emissions and chemical leaching from soils, the underlying hydrological, physical, geochemical and microbial mechanisms, and their mutual interactions, remain poorly understood. One reason is the paucity of time-series data that capture the relevant transport and reaction processes within the soil system during freeze-thaw cycles. As a consequence, we are currently seriously limited in our ability to predict the response of soil processes to changes in the frequency, magnitude and duration of freeze-thaw events driven by ongoing and future climate change. The central aim of this Discovery Grant research is to unravel the coupled physical, hydrological and biogeochemical processes in soils that, together, regulate the production of greenhouse gases and changes in microbial activity and pore water composition during freeze-thaw cycles. The proposed approach combines the acquisition of integrated physical, chemical and microbial data in a newly-developed soil column system in which time-dependent, vertical temperature profiles are generated that mimic temperature distributions in real soils under freeze-thaw conditions. The research will be broken down in four highly innovative interrelated (sub-)projects. Project 1 will document the subsurface changes in oxygen (O2) concentrations in a soil experiencing successive periods of freezing and thawing, using multi-fiber optode microsensors. In Project 2, detailed measurements of in situ gas-phase and aqueous phase chemistry will be carried out, in order to delineate the pathways and quantify the rates of soil microbial respiration during freeze-thaw cycles. Project 3 will focus on the changes in soil physical and hydraulic properties, as well as the changes in microbial community structure, caused by freezing and thawing. Project 4 will integrate and link the experimental results of Projects 1-3 into a reactive transport model framework that will be able to account for soil biogeochemical processes under freeze-thaw conditions. The outcomes of the project enhance the interpretation of field-scale measurements of greenhouse gas emissions from middle and high latitude soils, and enable informed predictions about the changes in these gas emissions, as well as groundwater quality under scenarios of climate change.