OPP-PRF: Freeze-thaw effect on Biogeochemistry and Nutrient Cycling in Arctic Soils

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Warming air temperatures are thawing frozen ground (permafrost) in the Arctic at a rapid rate. As frozen ground thaws it undergoes temperature fluctuations above and below freezing, known as freeze-thaw cycles. Freeze-thaw can impact the availability of important nutrients such as nitrogen and phosphorus, the activity of microorganisms, soil moisture and associated changes in oxygenation, and the release of greenhouse gases from thawing ground. The implications of freeze-thaw in thawing permafrost are amplified in the Arctic, which stores just under half of all soil carbon across the globe. This research will determine how exposure of previously frozen ground to freeze-thaw alters soil function and nutrient availability. This project supports one postdoctoral scholar and will fund freeze-thaw research and the development of an educational R/RStudio module for an Arctic dataset.Freeze-thaw is a disruptive, temperature driven process with direct impacts to both abiotic and biotic soil properties and their coupled interactions. Rising air temperatures expose previously unavailable compounds to plant uptake, microbial activity, and mineral sorption and precipitation while also altering biogeochemical processes within thawed soil. Freeze-thaw has been recorded to influence the migration of soil solutes (such as P, N, and Fe) through cryosuction in non-permafrost soils. In permafrost-affected soils, freeze-thaw triggered solute migration may coincide with nutrient pulses to the soil environment following microbial cell lysis and death or soil aggregate instability and breakage. In alpine and grassland systems, cryosuction during freeze-thaw can increase anoxic soil conditions with varying levels of Fe reduction and result in subsequent impacts to soil aromatics and dissolved organic carbon concentrations. The number of freeze-thaw cycles, freezing rate, and spatial distribution of freeze-thaw may vary, altering the degree of transport during cryosuction as well as informing microbial adaptation and resiliency. The proposed work will combine field data with freeze-thaw experiments to investigate temporal and spatial variations in freeze-thaw and the direct impacts to critical processes of soil biogeochemistry across a hillslope gradient in Alaskan tundra permafrost at the Toolik Field Station. The investigator will test the hypothesis that permafrost soil chemistry will be altered during the first freeze-thaw cycle, resulting in continuous changes in redox and nutrient availability throughout subsequent freeze-thaw. The study aims to restructure our understanding of permafrost thaw to include responses of the soil nutrient regime, redox chemistry, and microbial communities to freezing rate, repeated freeze-thaw, and spatial variation of freeze-thaw cycling.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.