Gas transfer processes are fundamental to the biogeochemical and water quality functions of wetlands, yet there is limited knowledge of the rates and pathways of soil‐atmosphere exchange for gases other than oxygen and methane (CH4). In this study, we use a novel push‐pull technique with sulfur hexafluoride (SF6) and helium (He) as dissolved gas tracers to quantify the kinetics of root‐mediated gas transfer, which is a critical efflux pathway for gases from wetland soils. This tracer approach disentangles the effects of physical transport from simultaneous reaction in saturated, vegetated wetland soils. We measured significant seasonal variation in first‐order gas exchange rate constants, with smaller spatial variations between different soil depths and vegetation zones in a New Jersey tidal marsh. Gas transfer rates for most biogeochemical trace gases are expected to be bracketed by the rate constants for SF6 and He, which ranged from ∼10−2 to 2 × 10−1 h−1 at our site. A modified Damköhler number analysis is used to evaluate the balance between biochemical reaction and root‐driven gas exchange in governing the fate of environmental trace gases in rooted, anaerobic soils. This approach confirmed the importance of plant gas transport for CH4, and showed that root‐driven transport may affect nitrous oxide (N2O) balances in settings where N2O reduction rates are slow.
Hurricane Sandy was one of the largest
tropical storms to pass
over the Atlantic basin, causing destruction along its path as it
made landfall in Jamaica, Cuba, the Bahamas, and the United States
[Tropical Cyclone Report: Hurricane Sandy; National Weather Service,
National Oceanic and Atmospheric Administration, 2012 ()]. Hurricane Sandy passed over the Meadowlands in the midst of our
multiyear study on marsh CH4 dynamics, providing a unique
opportunity to evaluate the effects of hurricanes on CH4 cycling within an estuarine marsh. We modeled dissolved CH4 distributions in wetland sediments from 2011 (Reid, M. C.; Tripathee,
R.; Shäfer, K. V. R.; Jaffé, P. R. Tidal Marsh Methane
dynamics: Difference in seasonal lags in emissions driven by storage
in vegetated versus unvegetated sediments. J. Geophys. Res.:
Biogeosci. 2013, 118, 1802–1813)
to 2013 and estimated that Hurricane Sandy did not degas vegetated
soils but degassed between 45 and 75% of the dissolved CH4 in unvegetated sediments. Hurricanes may regularly affect coastal
wetland CH4 emissions globally, but because these wetland
sediments do not store substantial dissolved CH4 late in
the year, the degassing of these sediments by Hurricane Sandy did
not play an important role in the annual carbon emissions from this
marsh.
Biogeochemical processes in wetland soils are complex and are driven by a microbiological community that competes for resources and affects the soil chemistry. Depending on the availability of various electron acceptors, the high carbon input to wetland soils can make them important sources of methane production and emissions. There are two significant pathways for methanogenesis: acetoclastic and hydrogenotrophic methanogenesis. The hydrogenotrophic pathway is dependent on the availability of dissolved hydrogen gas (H), and there is significant competition for available H. This study presents simultaneous measurements of dissolved methane and H over a 2-year period at three tidal marshes in the New Jersey Meadowlands. Methane reservoirs show a significant correlation with dissolved organic carbon, temperature, and methane emissions, whereas the H concentrations measured with dialysis samplers do not show significant relationships with these field variables. Data presented in this study show that increased dissolved H reservoirs in wetland soils correlate with decreased methane reservoirs, which is consistent with studies that have shown that elevated levels of H inhibit methane production by inhibiting propionate fermentation, resulting in less acetate production and hence decreasing the contribution of acetoclastic methanogenesis to the overall production of methane.
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