Contrasting effects of winter snowpack and soil frost on growing season microbial biomass and enzyme activity in two mixed-hardwood forests
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Winter is recognized as an important time for microbial activity that influences biogeochemical cycles. The onset of the winter snowpack in temperate hardwood ecosystems has been and will continue to be delayed over the next century. The decline in snowpack results in more soil freeze–thaw events and lower winter soil temperatures. Understanding microbial responses to varying snowpack conditions is important to understanding the effect of climate change on forest ecosystems. To this end, we removed snow to simulate a thinner, more ephemeral snowpack at two sites in the northeastern US, Harvard Forest (MA) and Hubbard Brook Experimental Forest (NH). We then measured microbial and exoenzyme activity in soils following snowmelt and three additional time points across the growing season. We found that microbial and exoenzyme activity were both positively correlated with the depth and duration of the snowpack at each site. The depth and duration of soil frost were negatively correlated with microbial biomass, exoenzyme activity and respiration, but only at Harvard Forest and not at Hubbard Brook. At both sites the changes in microbial and exoenzyme activity were transient and did not persist into the growing season past tree leaf-out. While it is possible that reductions in the snowpack and changes to microbial activity in the early spring may lead to asynchrony in the phenology of microbial relative to plant activity, it is at present uncertain whether and over what time scale this asynchrony may affect other forest ecosystem processes.
KeywordsSnow removal Soil frost Hubbard Brook Harvard Forest extracellular enzyme Winter climate change
We thank four anonymous reviewers who provided constructive criticism and helpful suggestions that greatly improved our manuscript. We are grateful to many undergraduate research assistants who participated in this study, but would specifically like to acknowledge Keita Decarlo, Emily Su, Mikayla Thistle, Cara Papakyrikos, Alice Chu, Jenny Barrs, Christina Clay, Brian Case, Kimberly Lu, Sam Pipe, Aubree Woods, Michael Kowalski, Joe Ward, Bonnie Waring, Jennifer Mceldoon, and Huong Nhan for their field and lab assistance. Other members of the Templer and Finzi labs at Boston University, including Marc Andre Giasson, Stephanie Juice, Rose Abramoff, Rebecca Sanders- DeMott, Allison Gill, Kim Spiller, and John Drake, provided generous field or lab assistance and helpful suggestions which improved earlier versions of this manuscript. Jenny Talbot also provided helpful feedback on an earlier version of this manuscript, which greatly improved our analysis. We thank Annie Socci and Andrew Reinmann for removing snow during winter at Hubbard Brook and Harvard Forest, respectively. Logistical support to complete winter field work was graciously provided by Site Manager and Research Coordinator Audrey Barker Plotkin at Harvard Forest and by Site Manager Ian Halm and Nick Green of the US Forest Service at Hubbard Brook. Funding for this study was provided by a student award from the Boston University Biogeoscience program to POS, funding from the Andrew W. Mellon Foundation, National Science Foundation (NSF DEB-1149929, 1406521, and 1114804), and the Northeastern States Research Cooperative awarded to PHT, as well as funding by the National Science Foundation (DEB-1011479, DEB-1153401, DEB—1237491) and the Office of Science (BER), U.S. Department of Energy (grant No. 10-DOE-1053) to ACF. Harvard Forest and Hubbard Brook are each Long-Term Ecological Research sites supported with funding by the National Science Foundation. This research is a contribution to the Hubbard Brook Ecosystem Study, administered by the USDA Forest Service, Northeast Forest Experiment Station, Radnor, Pennsylvania.
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