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

Abstract

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.

Lateral Marsh Edge Erosion as a Source of Sediments for Vertical Marsh Accretion

“…We draw on additional studies conducted in the Plum Island Sound ecosystem to put the results of this study in perspective. Measures of metabolism using the eddy covariance approach in the marshes adjacent to Plum Island Sound show net ecosystem exchange (NEE) to average 168 gC m −2 yr −1 (Forbrich et al, ), indicating the potential for accumulation of refractory root and rhizome material produced in situ. If all the NEE is associated with belowground production, then in situ production can provide 147% of the organic matter required to support historic rates of marsh elevation gain of 2.8 mm yr −1 (Forbrich et al, ).…”

“…While we calculated budgets for both mineral and organic matter, our analysis approach is most appropriate for mineral matter because we found earlier that there is sufficient marsh NEE attributable to the accumulation of undecomposed root and rhizome material to supply in excess of 100% of organic matter accretion needs (Forbrich et al, ). Further work will be required to more fully understand the production, respiration, transport, export, and burial of organic carbon produced in Plum Island Sound estuary marshes and to balance the overall estuarine organic and inorganic carbon budgets.…”

“…Deposition of organic matter onto the marsh surface in conjunction with mineral deposition as well as edge erosion need to be incorporated. These two fluxes are roughly 10-20% of recent measures of marsh NEE (Forbrich et al, 2018) and will likely increase substantially in the future as the rate of SLR accelerates and river sediment export decreases. Current model estimates of marsh gross primary production, ecosystem respiration, net ecosystem production, and burial (e.g., Bauer et al, 2013) underestimate the exchange with adjacent systems when not factoring in marsh surface organic matter deposition and marsh edge erosion.…”

Hydrologic Export Is a Major Component of Coastal Wetland Carbon Budgets

“…Lateral exports of dissolved organic C (DOC) from coastal marshes have been estimated more frequently than DIC (Bergamaschi et al, 2011;Bergamaschi, Fleck, et al, 2012;Bouillon, Borges, et al, 2008;Cai, 2011;Dittmar et al, 2001Dittmar et al, , 2006Downing et al, 2009;Santos et al, 2019), and marsh-estuary OC exchanges may be several times greater than riverine inputs in some regions (Cai, 2011), but not in others (Jassby et al, 1993). Comprehensive budgets combining OC and DIC exports over annual or longer timescales are near absent (Santos et al, 2019;Wang et al, 2016), but large overall lateral C exports have been inferred by indirect estimates subtracting vegetation net C uptake from soil C burial (Feijtel et al, 1985;Forbrich et al, 2018;Troxler, 2013;Webb et al, 2018). Ultimately, direct and simultaneous measurement of both DIC and OC at a high temporal resolution and across complete annual cycles is required to better define the magnitude and chemical makeup of coastal wetland lateral C exports.…”

Tidal Wetland Gross Primary Production Across the Continental United States, 2000–2019

“…To estimate the effect of salinity and the error induced by excluding it from the BC model, we assessed the differences in the parameterization at the US‐SKR site (Barr et al, ) and the US‐PHM site (Forbrich et al, ). We found no added measurable benefit to including salinity into the framework when predicting GPP (Table ; only US‐SKR data shown).…”