Stratifying by Vegetation and Hydrology Improves Tidal Marsh Methane Emission Accounting
Abstract Methane emissions must be directly measured or estimated using methods such as proxies when managing wetlands for greenhouse gas offset activities. Salinity is a useful proxy for tidal marsh CH4 emissions when comparing across a wide range of salinity regimes but does not adequately explain variation in brackish and freshwater regimes where variation in emissions is large. We sought to improve upon the salinity proxy in a marsh complex on Deal Island Peninsula, Maryland, USA by identifying four strata based on hydrology and plant community composition. Mean CH4 chamber-collected emissions measured as mg CH4 m-2 hr-1 ranked as S. alterniflora (1.2 ± 0.3) >> High-elevation J. roemerianus (0.4 ± 0.06) > Low-elevation J. roemerianus (0.3 ± 0.07) = S. patens (0.1 ± 0.01). Sulfate depletion generally reflected the same pattern with significantly greater in the S. alterniflora stratum (61 ± 4%) than in the S. patens stratum (1 ± 9%) with the J. roemerianus strata falling in between. We attribute the high CH4 emissions in the S. alterniflora stratum to sulfate depletion likely driven by limited connectivity to tidal waters. Low CH4 emissions in the S. patens stratum are attributed to lower water levels, higher levels of ferric iron, and shallow rooting depth. Moderate CH4 emissions from the J. roemerianus strata were likely due to plant traits that favor CH4 oxidation over CH4 production. We concluded that stratification by hydrology and plant community composition can be an effective proxy to estimate CH4 emissions at the site scale.