In the northeastern U.S., climate is changing more rapidly in winter than in summer. The impacts of winter climate change on ecosystems are greatly complicated by effects on snow depth and soil freezing. Snow is important as an insulator of the soil, and many northern hardwood forest soils normally remain unfrozen during the winter. A lack of snow can result in soil freezing, which is a significant disturbance to forest ecosystems, potentially killing tree roots and microorganisms and disrupting nutrient cycling processes leading to losses of nutrients to water and air. These losses can decrease the productivity of the forest ecosystem and lead to air and water pollution. In this project investigators will use a landscape-scale approach to evaluate three aspects of the effects of changes in snow depth on soil freezing and the cycling of carbon and nitrogen in the northern hardwood forest at the Hubbard Brook Long Term Ecological Research site (HBR) in New Hampshire. First, the research will address uncertainty about the occurrence of colder soils in a warmer world, and whether this pattern will increase losses of nitrogen, an important plant nutrient and source of water and air pollution, from the northern hardwood forest at HBR. Uncertainty about the extent and effects of soil freezing in a warmer world is rooted in limitations in our understanding of where two "tipping points" occur across a forest landscape experiencing climate change; where snowpacks are too shallow to insulate the soil, and where air temperatures are too warm to freeze the soil. This uncertainty will be addressed by measuring and modeling snow depth, and soil climate across the entire ~3000 ha Hubbard Brook Experimental Forest (HBEF), which encompasses the range of climate variability that has been predicted for the northeastern U.S. over the next 50-100 years. Measurements of soil temperature, moisture and frost, and nitrogen losses to water and air will be made at 20 experimental field sites that experience a broad range of long-term snowpack regimes to explore critical uncertainties surrounding soil freezing events and their effects on nitrogen cycling. Second, the proposed research will address how winter climate change affects microbial and soil invertebrate processes and resultant changes in carbon flow during winter, testing the hypothesis that carbon flow is the key integrative regulator of winter microbial activity. To test this hypothesis, the movement of isotopically labeled carbon and nitrogen will be traced from sugar maple detritus into and through the soil ecosystem in six intensive study sites that represent the full range of variation in winter climate at HBEF. Third, the proposed research will address if soil freezing alters hydrologic controls of ecosystem nitrogen processing at snowmelt and subsequent export of nitrogen to receiving waters. Snowmelt dynamics will be tested by adding isotopically labeled nitrogen to the snowpack and tracing its movement into and through the soil ecosystem. Effects of soil freezing on export to receiving waters will be tested by analyzing nitrogen export and solutes that serve as natural tracers of hydrologic flowpaths on small watersheds that differ in winter climate/soil freezing.
The project will include education and policy activities that explore the effects of winter climate change in the Northeast U.S. on recreation, timber harvesting, biomass energy production, and other ecosystem services. The project will connect ecosystem researchers with relevant stakeholders and interest groups, including loggers and foresters, ski-area operators, maple sugar producers, recreational snowmobile users, conservation-oriented NGOs, citizen scientists, and public and private land managers through a "Science Links" program. There will also be two pilot educational initiatives: a winter field course for undergraduates and creation of a teaching guide for middle and high school teachers. Finally, the research will be well integrated into ongoing HBR research on modeling the effects of climate change, fostering extension of current climate change modeling studies to depict the hydrologic and biogeochemical response to soil freezing under future climate change scenarios.
Intellectual Merit: Nitrogen and carbon are essential elements for plants and soil micro-organisms and therefore the nitrogen and carbon cycles are among the most widely studied biogeochemical cycles. The majority of controls on the nitrogen and carbon cycles have been examined during the growing season when a significant amount of biological activity occurs. Winter has long been considered to be a relatively insignificant period for biogeochemical cycling in northeastern U.S. forest ecosystems, but recent studies suggest otherwise with biological activity of plants and microbes having been shown to continue in the winter months. A large proportion of land at northern latitudes and higher elevations within the United States experiences soil freezing each year. Soils that are not covered by an insulating layer of snow may freeze when air temperatures get below freezing. Many past studies examining the role of winter climate change in nutrient cycling of northern hardwood forests have been carried out using manipulative experiments that rely on snow removal to mimic reductions in the winter snowpack. Past studies show that a shallower snowpack and greater depth and duration of soil freezing in winter leads to increased movement of nitrate from northern hardwood forests into nearby streams. In this study, we took advantage of a natural gradient in snow depth and soil frost to examine the effects of soil frost on nitrogen and carbon retention and loss and the role that plant-microbial interactions have in these relationships. We established 20 sites along an elevation gradient at the Hubbard Brook Experimental Forest in Woodstock, NH. At the three highest (low soil frost) and three lowest (high soil frost) elevation sites, we installed eight litterboxes (0.5 X 0.5 m2) in each of three plots in May 2011. In three litterboxes within each plot, we installed root ingrowth (2 mm mesh) and root exclusion (50 and 250 micron mesh) cores, which allow us to examine microbial activity in the presence and absence of roots, respectively. These cores incubated in the field and a subset was harvested on three dates (July and October 2012 and October 2013) to quantify the effect of soil freezing on root and microbial biomass, microbial activity, and retention of carbon and nitrogen in plants, microbes and soils. We found greater root biomass in the ingrowth than exclusion cores, but no effect of soil frost depth or duration on root or microbial biomass. We found that phenol oxidase activity, a measure of microbial enzyme activity, decreased significantly with increasing depth and duration of soil freezing, suggesting that decomposition of recalcitrant soil organic carbon could decrease with a smaller snowpack and increased depth and duration of soil freezing. We found that rates of nitrification decreased with greater amounts of root growth, suggesting that the presence of plant roots inhibits net production of nitrate in these soils. We also found that at low amounts of soil freezing, the presence of roots stimulated rates of net mineralization, but in contrast, the presence of roots diminished rates of net mineralization at sites with greater depth and duration of soil freezing. These results show that the positive effect of roots on net mineralization diminishes with greater freezing. Together, these results show that the relationship between plant roots and soil microbes vary with changes in winter climate and that the projected increase in soil freezing for the future could result in diminished availability of soil nitrogen. Results of this work demonstrate how plant microbial interactions influence the relationship between soil freezing and nutrient cycling in northern hardwood forest ecosystems. Broader Impacts: We coordinated our education and outreach activities with the Hubbard Brook Research Foundation. For example, we convened a series of Roundtable Discussions at the Hubbard Brook Experimental Forest. Participants included a diverse group of ecosystem scientists, winter forest users (e.g. snow mobile users), and land managers. We discussed how winter climate in the northeastern United States is changing, how both work and recreation will be affected by these changes in climate, and the effects on ecosystem services for this region.
