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

冻融循环是作用于中高纬度土壤的主要气候强迫因素。土壤的冻结和融化影响其物理性质、生物地球化学过程、微生物群落以及碳和养分收支。尽管过去几十年的研究揭示了冻融循环对温室气体排放和土壤化学淋溶的巨大影响，但其潜在的水文、物理、地球化学和微生物机制及其相互作用仍然知之甚少。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的相关运输和反应过程。因此，我们目前在预测土壤过程对目前和未来气候变化驱动的冻融事件的频率、幅度和持续时间变化的反应方面的能力受到严重限制。这项探索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成分的变化。该方法结合了在新开发的土柱系统中综合物理、化学和微生物数据的获取，在该系统中，生成了随时间变化的垂直温度分布，模拟了真实土壤在冻融条件下的温度分布。这项研究将分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型传感器记录经历连续冰冻和融化的土壤中氧气(O2)浓度的地下变化。在项目2中，将进行详细的现场气相和水相化学测量，以确定冻融循环期间土壤微生物呼吸的途径和速率。项目3将侧重于冻融引起的土壤物理和水力性质的变化，以及微生物群落结构的变化。项目4将把项目1-3的实验结果纳入一个能够解释冻融条件下土壤生物地球化学过程的反应性运移模型框架。该项目的成果加强了对中高纬度土壤温室气体排放的实地测量的解释，并使人们能够对这些气体排放的变化以及气候变化情景下的地下水质量作出明智的预测。

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.