Responses of Soil Organic Matter and Minerals to Winter and Growing Season Climate Change

ABSTRACT Changes to soil organic matter (SOM) in response to warming temperatures and a shrinking snowpack may exacerbate climate change due to carbon (C) cycle feedbacks. Winter is warming more rapidly than the growing season, resulting in declines in snowpack quantity and duration and leaving the soil without insulation from freezing air temperatures. Frozen soil can induce root and microbial mortality and create a barrier to O 2 diffusion, which can impact SOM inputs, SOM storage mechanisms, and soil CO 2 production. At the Climate Change Across Seasons Experiment (CCASE) at the Hubbard Brook Experimental Forest, we examined SOM responses to winter freeze–thaw cycles and growing season soil warming. After 8 years of the seasonal treatments, we sampled bulk soil and soil gas from organic and mineral soil layers. We quantified C and nitrogen in bulk, particulate, and mineral‐associated organic matter and examined organo‐mineral associations using selective metal dissolutions and Mössbauer spectroscopy. Soils subjected to the combination of winter freeze–thaw cycles and growing‐season warming had significantly lower bulk C concentrations in the organic and mineral horizons, lower rates of growing‐season CO 2 production from the organic horizon, and altered organo‐mineral associations compared to control soils and soils warmed during the growing season. Had the experiment only subjected the soils to growing season warming, we would have missed the unique role winter climate plays in controlling soil ecosystem function. These results suggest that winter freeze–thaw cycles reduce the soil's capacity to store organic matter, and thus, this forested ecosystem's ability to buffer against climate change.