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 nine years members of the NE-1438 multistate project and subsequent NE-1938 multistate project have been studying the hydrology, redox processes, and carbon dynamics occurring in vernal pool wetlands (seasonally wet depressional wetlands). These studies have mostly occurred across the northeast region from Massachusetts to Virginia, with three study sites in the west (Wyoming, Kansas, and Nebraska). These depressional wetlands represent a suite of wetlands with similar hydrologies, yet vary in  temperatures, parent materials, vegetation, and other soil forming factors which leads to variation in  wetland C stocks. Further, notable amounts of recalcitrant carbon, sometimes known as “black carbon”, have been observed in some of the sites. Goldberg (1985) stated that black carbon is formed through the incomplete combustion of wood, vegetation, and fossil fuels as well as certain industrial processes. Kuhlbusch (1998) describes black carbon as a potential sink for atmospheric carbon. Black carbon is a mechanism of long-term carbon storage that was not explored in NE-1438 or NE-1938. 

Our goal is to  determine the range in C stocks across a set of 11 depressional wetlands. In concert with accounting the labile, recalcitrant, and mineral 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 measurements 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 these  multistate project sites, while exhibiting similar hydrologic conditions, will have varied carbon storage, and differing rates of soil C additions, decomposition, and losses. By quantifying C dynamics of these understudied wetland ecosystems we will gain a better understanding of the vulnerability of stored C to losses due to increasing temperatures as well as the potential for C sequestration over the next century.