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

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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.