Constraining Marsh Carbon Budgets Using Long‐Term C Burial and Contemporary Atmospheric CO2 Fluxes

Salt marshes are sinks for atmospheric carbon dioxide that respond to environmental changes related to sea level rise and climate. Here we assess how climatic variations affect marsh‐atmosphere exchange of carbon dioxide in the short term and compare it to long‐term burial rates based on radiometric dating. The 5 years of atmospheric measurements show a strong interannual variation in atmospheric carbon exchange, varying from −104 to −233 g C m−2 a−1 with a mean of −179 ± 32 g C m−2 a−1. Variation in these annual sums was best explained by differences in rainfall early in the growing season. In the two years with below average rainfall in June, both net uptake and Normalized Difference Vegetation Index were less than in the other three years. Measurements in 2016 and 2017 suggest that the mechanism behind this variability may be rainfall decreasing soil salinity which has been shown to strongly control productivity. The net ecosystem carbon balance was determined as burial rate from four sediment cores using radiometric dating and was lower than the net uptake measured by eddy covariance (mean: 110 ± 13 g C m−2 a−1). The difference between these estimates was significant and may be because the atmospheric measurements do not capture lateral carbon fluxes due to tidal exchange. Overall, it was smaller than values reported in the literature for lateral fluxes and highlights the importance of investigating lateral C fluxes in future studies. Key Points Interannual variability in tidal salt marsh NEE was explained by variation in rainfall and salinity A significant but small difference was determined between contemporary NEE and decadal burial rates Tidal losses may be a relatively small feature of the C budget in infrequently flooded high marshes