@article {12756, title = {Winter soil freeze-thaw cycles lead to reductions in soil microbial biomass and activity not compensated for by soil warming}, journal = {Soil Biology and Biochemistry}, volume = {116}, year = {2018}, month = {2018/01/01/}, pages = {39 - 47}, abstract = {Air temperatures are rising and the winter snowpack is getting thinner in many high-latitude and high-elevation ecosystems around the globe. Past studies show that soil warming accelerates microbial metabolism and stimulates soil carbon (C) and nitrogen (N) cycling. Conversely, winter snow removal to simulate loss of snow cover leads to increased soil freezing and reductions in soil microbial biomass, exoenzyme activity, and N cycling. The Climate Change Across Seasons Experiment (CCASE), located at Hubbard Brook Experimental Forest, NH (USA) is designed to evaluate the combined effects of growing season soil warming and an increased frequency of winter soil freeze-thaw cycles on a northern forest ecosystem. Soils were collected from CCASE over two years (2014 and 2015) and extractable C and N pool sizes, as well as microbial biomass, exoenzymes, and potential net N mineralization and microbial respiration were measured. Soil warming alone did not stimulate microbial activity at any sampling time. Extractable amino acid N and organic C, proteolytic and acid phosphatase activity, and microbial respiration were reduced by the combination of warming in the growing season and winter soil freeze-thaw cycles during the period following snowmelt through tree leaf out in spring. The declines in microbial activity also coincided with an 85\% decline in microbial biomass N at that time. Growing season warming and winter soil freeze-thaw cycles also resulted in a two-fold reduction in phenol oxidase activity and a 20\% reduction in peroxidase activity and these declines persisted throughout the snow-free time of the year. The results from this study suggest that positive feedbacks between warming and rates of soil C and N cycling over the next 100 years will be partially mitigated by an increased frequency of winter soil freeze-thaw cycles, which decrease microbial biomass and rates of soil microbial activity.}, keywords = {Exoenzymes, Hubbard Brook Experimental Forest, Microbial biomass, Soil freeze-thaw cycle, Soil warming}, isbn = {0038-0717}, doi = {10.1016/j.soilbio.2017.09.026}, url = {http://www.sciencedirect.com/science/article/pii/S0038071717302213}, author = {Sorensen, Patrick O. and Finzi, Adrien C. and Giasson, Marc-Andr{\'e} and Reinmann, Andrew B. and Rebecca Sanders-DeMott and Pamela H. Templer} } @article {12465, title = {Contrasting effects of winter snowpack and soil frost on growing season microbial biomass and enzyme activity in two mixed-hardwood forests}, journal = {Biogeochemistry}, year = {2016}, month = {2016/03/19/}, isbn = {0168-2563, 1573-515X}, url = {http://link.springer.com/10.1007/s10533-016-0199-3}, author = {Sorensen, Patrick O. and Pamela H. Templer and Finzi, Adrien C.} } @article {12484, title = {Reduced snow cover alters root-microbe interactions and decreases nitrification rates in a northern hardwood forest}, journal = {Ecology}, volume = {97}, year = {2016}, month = {2016/12/01/}, pages = {3359 - 3368}, abstract = {Snow cover is projected to decline during the next century in many ecosystems that currently experience a seasonal snowpack. Because snow insulates soils from frigid winter air temperatures, soils are expected to become colder and experience more winter soil freeze-thaw cycles as snow cover continues to decline. Tree roots are adversely affected by snowpack reduction, but whether loss of snow will affect root-microbe interactions remains largely unknown. The objective of this study was to distinguish and attribute direct (e.g., winter snow- and/or soil frost-mediated) vs. indirect (e.g., root-mediated) effects of winter climate change on microbial biomass, the potential activity of microbial exoenzymes, and net N mineralization and nitrification rates. Soil cores were incubated in situ in nylon mesh that either allowed roots to grow into the soil core (2~mm pore size) or excluded root ingrowth (50~μm pore size) for up to 29~months along a natural winter climate gradient at Hubbard Brook Experimental Forest, NH (USA). Microbial biomass did not differ among ingrowth or exclusion cores. Across sampling dates, the potential activities of cellobiohydrolase, phenol oxidase, and peroxidase, and net N mineralization rates were more strongly related to soil volumetric water content (P~<~0.05; R2~=~0.25{\textendash}0.46) than to root biomass, snow or soil frost, or winter soil temperature (R2~<~0.10). Root ingrowth was positively related to soil frost (P~<~0.01; R2~=~0.28), suggesting that trees compensate for overwinter root mortality caused by soil freezing by re-allocating resources towards root production. At the sites with the deepest snow cover, root ingrowth reduced nitrification rates by 30\% (P~<~0.01), showing that tree roots exert significant influence over nitrification, which declines with reduced snow cover. If soil freezing intensifies over time, then greater compensatory root growth may reduce nitrification rates directly via plant-microbe N competition and indirectly through a negative feedback on soil moisture, resulting in lower N availability to trees in northern hardwood forests.}, keywords = {Hubbard Brook, nitrification, phenol oxidase, root production, snow, Soil frost, sugar maple (Acer saccharum)}, isbn = {1939-9170}, url = {http://onlinelibrary.wiley.com/doi/10.1002/ecy.1599/abstract}, author = {Sorensen, Patrick O. and Pamela H. Templer and Lynn M. Christenson and Duran, Jorge and Timothy J. Fahey and Melany C. Fisk and Peter M. Groffman and Morse, Jennifer L. and Finzi, Adrien C.} } @article {12370, title = {Correction factors for dissolved organic carbon extracted from soil, measured using the Mn(III)-pyrophosphate colorimetric method adapted for a microplate reader}, journal = {Soil Biology and Biochemistry}, volume = {78}, year = {2014}, month = {2014/11//}, pages = {284 - 287}, abstract = {Oxidizable dissolved organic carbon (DOC) is regularly measured in environmental samples using a colorimetric method with Mn(III)-pyrophosphate as the oxidizing agent. It is simpler to use and has a much higher throughput than the commonly used dichromate oxidation and combustion methods. Here, we demonstrate that the method often leads to an underestimation or overestimation of the concentration of common organic compounds in solutions. To our knowledge, no published study has taken this fact into account when analyzing DOC data. Hence, we compared Mn(III)-pyrophosphate-based results with measurements performed with a total organic carbon combustion analyzer for samples of organic and mineral soil horizons of two temperate deciduous forests, of organic soil horizon of a primary-growth hemlock stand, and of a peatland located in New England, USA. The Mn(III)-pyrophosphate method consistently underestimated DOC concentration in soil extracts. We present correction factors for the different types of soil studied. By employing correction factors, we find the method can be an inexpensive, accurate, and high throughput tool to measure DOC in environmental samples.}, keywords = {Colorimetric method, DOC, Forest soils, Mn(III)-pyrophosphate, Organic carbon}, isbn = {0038-0717}, url = {http://www.sciencedirect.com/science/article/pii/S0038071714002892}, author = {Giasson, Marc-Andr{\'e} and Averill, Colin and Finzi, Adrien C.} }