Nitrate and dissolved organic carbon mobilization in response to soil freezing variability
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Reduced snowpack and associated increases in soil freezing severity resulting from winter climate change have the potential to disrupt carbon (C) and nitrogen (N) cycling in soils. We used a natural winter climate gradient based on elevation and aspect in a northern hardwood forest to examine the effects of variability in soil freezing depth, duration, and frequency on the mobilization of dissolved organic carbon (DOC) and nitrate (NO3−) in soils over the course of 2 years. During a winter with a relatively thin snowpack, soils at lower elevation sites experienced greater freezing and especially variable freeze/thaw cycles, which in turn led to greater leaching of DOC from the organic horizon during the following growing season. In contrast to several previous field manipulation studies, we did not find changes in soil solution NO3− concentrations related to soil freezing variables. Our results are consistent with a soil matrix disturbance from freezing and thawing which increases leachable C. These results build upon previous laboratory experiments and field manipulations that found differing responses of DOC and NO3− following soil freezing, suggesting that mobilization of labile C may suppress NO3− losses through microbial immobilization of N. This research highlights the importance of studying natural variation in winter climate and soil freezing and how they impact soil C and N retention, with implications for surface water runoff quality.
KeywordsClimate change Winter Snow Soil water Northern hardwood forest Soil frost
We thank Don Buso, Tammy Wooster, Sam Werner, and Afshin Pourmokhtarian for assistance with field work. Chris Johnson, David Chandler, and Laura Lautz provided helpful comments on earlier versions of the manuscript, and two anonymous reviewers made suggestions to further improve it. The HBEF is administered by the U.S. Department of Agriculture Forest Service, Northern Forest Research Station, Newtown Square, PA. Hubbard Brook is a National Science Foundation supported Long-Term Ecological Research site. Support for this project was provided by the National Science Foundation (Grants DEB 0949664 - Ecosystem Studies and DEB 1114804 – Long-Term Ecological Research) and by the Department of Civil and Environmental Engineering at Syracuse University. Colin Fuss was supported by the Wen-Hsiung and Kuan-Ming Li Fellowship from the Department of Civil and Environmental Engineering, Syracuse University.
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