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Abstract
Direct measurement of methane emissions is cost-prohibitive for greenhouse gas offset projects, necessitating the development of alternative accounting methods such as proxies. 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 comparing emissions from four strata differing in hydrology and plant community composition. Mean CH4 chamber-collected emissions measured as mg CH4 m−2 h−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 depletion 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. Hydrology and plant community composition have significant potential as proxies to estimate CH4 emissions at the site scale.
Highlights
Methane is a potent greenhouse gas produced under the dominantly anaerobic conditions found in wetland soils
Porewater salinity in the S. alterniflora stratum was similar to other strata (Table 1) suggesting that relatively low SO42- concentrations were due to high rates of SO42- consumption
We found significantly different methane emission rates across four strata defined by hydrology and plant community composition that otherwise had similar salinity regimes
Summary
Methane is a potent greenhouse gas produced under the dominantly anaerobic conditions found in wetland soils. While the majority of CH4 emissions come from anthropogenic sources, wetlands produce most of the naturally emitted CH4 (Wang et al 1996; Solomon et al 2007) and are the most important source of uncertainty in current global CH4 budgets (Saunois et al 2020). The uncertainty introduced to greenhouse gas offset activities by CH4 emissions is especially large for coastal wetlands ecosystems with freshwaterto-brackish salinity \ 18 ppt (Poffenbarger et al 2011). The sources of this variability remain elusive as there has been relatively little research designed to partition variation. Direct monitoring of methane emissions is cost-prohibitive for most blue carbon crediting projects (Needelman et al 2018), creating a need for a better understanding of the factors that regulate coastal wetland CH4 emissions to create alternative estimation methods such as proxies and models
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