Wetlands contribute an estimated 25% of the global annual methane (CH4) emissions. However, delineation of the processes responsible for the large variations in emission rates found within and across wetland types has been challenging. Field measurements were collected in Francis Marion National Forest in correlation with water table levels and other environmental parameters in order to quantify the contribution of bottomland hardwood wetlands. The study included three mineral sites and a peat-based site that were sampled over a two year period. Methane emission rates ranged from -7.3 to 137.8 mg C m^-2 day^-1; negative rates represent methane oxidation in the soil. The highest rates were detected from a low depression in Turkey Creek site that remained inundated for several months when soil temperature increased in spring. However, CH₄ emissions and soil temperature were strongly correlated (R²: 0.8) only when inundation persisted. When natural and agricultural soils were incubated in a laboratory experiment, the potential methane production (PMP) was poorly correlated with the soil characteristics we investigated, but the best correlation was with the chemically labile organic matter (CLOM) fraction (r: 0.467). Potential nitrous oxide production (PNOP) was also quantified and the dissolved total nitrogen (DTN) and the dissolved organic carbon (DOC) were both good indicator (r: 0.534 and r: 0.555 respectively).

The large variation in PMP and PNOP from the laboratory incubations and in fluxes from the field suggests that a given wetland must be fully characterized in order to quantify its contribution. Extrapolation of our field data to an estimate of the total annual CH₄ emission will only be accurate once the microtopography of the study sites is precisely defined. Long term studies will most likely be the only beneficial approach to investigate how environmental parameters affect methane emissions, with modeling as a companion tool.