Winter Climate Change Reshapes Soil Climate and Biogeochemistry in a Novel Snowmelt Experiment.

Winter climate change is outpacing our conceptual understanding of how winter conditions regulate soil biogeochemical cycling and ultimately impact vital ecosystem services like soil carbon and nutrient retention. In seasonally snow-covered ecosystems like northern temperate forests, increasingly inconsistent winters lead to less precipitation falling as snow, frequent midwinter snow melting, and the loss of a stable, insulative snowpack. These changes leave soils vulnerable to freezing, freeze/thaw cycling, and increasing dry/wet cycles from added snowmelt and rainwater. To uncover how these new winter soil climate conditions alter soil biogeochemistry, we introduce the DeFR❆ST (Determining Forest Responses to Snowmelt Treatments) experiment, a novel approach where we melt snow in situ throughout the winter and monitor changes to soil climate, gas exchange, and biogeochemical cycling. We installed DeFR❆ST in a New England temperate forest, an ecosystem that is part of the most significant global carbon sink and is also in the epicenter of winter climate change in the US. Experimental snow melting drove soil moisture fluctuations in addition to deep and persistent soil freezing. In turn, soils in melted plots exhibited blocked gas diffusion and lower soil oxygen availability. Oxygen limitation may have driven shifts in soil processes from high redox potential metabolisms like aerobic decomposition and nutrient mineralization towards low redox potential metabolisms like iron reduction and the dissolution of iron and carbon from organo-mineral associations. As these changes snowball, altered soil properties and shifts in soil microbial community structure and function could reshape forest biogeochemical cycling, both in these forests and more broadly across seasonally snow-covered ecosystems.