The complexity of the carbon (C) cycle, and the potential for soil to act as both a C source and sink, have made projections of terrestrial C dynamics in light of global change difficult to determine with high confidence (Jaffe et al., 2013). The debate on whether soil is a net source or sink of C is ongoing because soil organic matter is a fundamental dynamic soil property that is capable of varying on human-time scales with changes in climate (Six et al., 2002; Janzen, 2006; West and Six, 2007; Ågren et al., 2008). If the United States is to manage and/or diminish future C emissions, scientists and policy makers must have dependable and accurate information on C stocks and fluxes (NOAA, 2013).

Of the landscapes that exist around the globe, wetland soils are one of the largest reservoirs of soil organic C (Chmura et al., 2003). Mitsch et al. (2012) estimated that wetlands store 20-30% of the earth’s terrestrial C pool; which makes them one of the landscape types under scrutiny in an attempt to mitigate the impacts of global climate change (IPCC, 2007). The primary factor controlling the quantity of C in the wetland soil reservoir is the hydrology that promotes saturation and anaerobic conditions. In soils that are saturated to the surface or inundated, (i.e. hydromorphic soils), the soil environment is anaerobic for much or all of the year. In such cases, soil organic matter (SOM) decomposition is a function of microbial activity (Borken et al., 2006). Fewer microbes are involved, and they are much less efficient at decomposing SOM into organic C compounds under anaerobic conditions than under aerobic conditions, and thus C stocks are typically greater in hydromorphic soils (Mausbach and Richardson, 2000). Carbon dioxide (CO2) is a byproduct of SOM breakdown via aerobic and anaerobic respiration, while methane (CH4) is produced via fermentation of SOM under anaerobic conditions. A secondary but important factor in SOM decomposition is soil temperature; with an increase in temperature typically leading to an increase in decomposition (Davidson and Janssens, 2006).  Significant increases in temperature have been recorded over the last couple decades and are expected to continue to increase (Rohde et al., 2013). Recent models suggest global temperature increases of 15% (approximately 3.9℃) by the next century (Brown and Caldeira, 2017), which should accelerate microbial activity and the rate of SOM breakdown in soils. The question is: How such an increase in temperature will affect C stocks in wetlands (Davidson and Janssens, 2006). One way to answer this question is to find wetlands to study with similar soils, hydrologies, and geomorphic settings but a range in temperatures.

Depressional wetlands occur worldwide. In the United States there are a range of depressional wetlands including prairie potholes, kettle holes, and Carolina Bays (Brinson, 1993). Over a short distance depressional wetlands have areas that are inundated, saturated, and unsaturated (Gala et al., 2005). The areas that are inundated and saturated change over the seasons resulting in a full range of hydrologic conditions every year. Thus, the unique hydrologic characteristics of depressional wetlands allow for a diagnostic investigation of how hydrology influences the magnitude of the biological and chemical interactions that take place in the soil such as C fluxes in all types of wetlands. Over the last four years members of the NE-1438 Multistate project have been studying the hydrology and redox processes occurring in vernal pool wetlands (seasonally wet depressional wetlands). These studies have mostly occurred across the northeast region from Massachusetts to Virginia, with one study site in the mountains of Wyoming. Even with the exclusion of the site in Wyoming, the range of soil temperatures among sites span the expected change over the next century (Table 1). Thus, these depressional wetlands represent a suite of wetlands with similar soils, hydrologies, and geomorphic settings that have a range in temperatures that can be used to understand the effects of increased temperature on wetland C stocks.

In this study, we will determine C stocks across depressional wetlands having a range of temperatures. In concert with accounting the C stored in these systems, we will measure inputs of C through litter and dead fall, rates of decomposition of these C sources, and the fluxes of C via carbon dioxide (CO2) and methane (CH4) that occur in these soils. We will make these measures in, or adjacent to, each of the two zones of these wetlands (seasonally inundated, seasonally saturated), and the adjacent uplands. Our working hypothesis is that since the multistate sites will have similar hydrologic conditions, relationships between soil temperature and soil C additions, decomposition, and losses can be identified. These relationships can be used to understand the effect of increasing temperatures on C stocks and fluxes in wetlands over the next century.