The Role of the Upper Tidal Estuary in Wetland Blue Carbon Storage and Flux

Abstract

Carbon (C) standing stocks, C mass balance, and soil C burial in tidal freshwater forested wetlands (TFFW) and TFFW transitioning to low‐salinity marshes along the upper estuary are not typically included in “blue carbon” accounting, but may represent a significant C sink. Results from two salinity transects along the tidal Waccamaw and Savannah rivers of the U.S. Atlantic Coast show that total C standing stocks were 322–1,264 Mg C/ha among all sites, generally shifting to greater soil storage as salinity increased. Carbon mass balance inputs (litterfall, woody growth, herbaceous growth, root growth, and surface accumulation) minus C outputs (surface litter and root decomposition, gaseous C) over a period of up to 11 years were 340–900 g C · m−2 · year−1. Soil C burial was variable (7–337 g C · m−2 · year−1), and lateral C export was estimated as C mass balance minus soil C burial as 267–849 g C · m−2 · year−1. This represents a large amount of C export to support aquatic biogeochemical transformations. Despite reduced C persistence within emergent vegetation, decomposition of organic matter, and higher lateral C export, total C storage increased as forests converted to marsh with salinization. These tidal river wetlands exhibited high N mineralization in salinity‐stressed forested sites and considerable P mineralization in low‐salinity marshes. Large C standing stocks and rates of C sequestration suggest that TFFW and oligohaline marshes are considerably important globally to coastal C dynamics and in facilitating energy transformations in areas of the world in which they occur.

Modeling Soil Porewater Salinity Response to Drought in Tidal Freshwater Forested Wetlands

“…The Waccamaw middle forest site will continue to experience increased periods of salinity of 0.5–2 psu, thus facing the threat of potential reduction of plant growth and potential change in the composition of species in the community (Krauss, Duberstein, et al, ). However, stress on this site will not necessarily come from saltwater intrusion, rather from hydrology regime changes, as flooding could become the limiting factor on this site in the future (Conner et al, ; Cormier et al, ; Krauss et al, ; Thomas et al, ). For this forest site, flood frequency, duration, and timing may be more important than slight increases in porewater salinity on maintaining forest productivity (e.g., litterfall) and soil N dynamics (e.g., soil total N, foliar N, litter C/N ratio) during drought (Cormier et al, ).…”

“…Our modeling simulation showed that under a nondrought year, mean porewater salinity would be 2.1 ± 1.4 psu at the Waccamaw site and 3.6 ± 2.4 psu at the Savannah site. Mean annual soil porewater salinities during 2005–2016 were 3.1 ± 0.5 psu at the Waccamaw marsh site and 4.9 ± 0.5 psu at the Savannah marsh site, respectively (Krauss et al, ). These data indicate that the two marsh sites will be unable to maintain their current oligohaline marsh (<2–5 psu) species composition and associated productivity and soil biogeochemical processes in the future with climate change and SLR.…”

“…Modeling results suggest that the lower forest and marsh sites along the two river systems would experience even worse salinity stress under drought‐induced saltwater intrusion. With the increase in soil salinity, forest sites may shift the anaerobic respiration balance toward greater C mineralization via sulfate reduction from saltwater intrusion, thus resulting in reduction of soil surface elevation, and potentially causing more saltwater intrusion (Krauss et al, ; Stagg et al, ). Soil salinity was found to be the primary driver of change in the C budget (Krauss et al, ).…”

“…The soil porewater salinity model was calibrated using monthly field observed salinity at these sites during 2008–2016 (Cormier et al, ; Krauss, Doyle, & Howard, ; Noe et al, , ; Krauss et al, ). At each site, monthly mean and variation in salinity data among the four wells were used to evaluate model performance during calibration to obtain the best values in modeling (Table ) and verify the model's capability in describing the trend and variability of soil salinity at these sites.…”

“…For example, the combination of dredging (deepening) the navigation channel and the tidal gate along the Savannah River and its tributaries in the southeastern United States have affected flow paths and resulted in increased river salinity upstream, which led to shifts in vegetation structure (Jones et al, ; Krauss, Doyle, & Howard, ). It was found that soil porewater salinity increased from average salinity 1.2 ± 0.2 psu (range: 0.49–2.38) at a moderately impacted TFFW site to average salinity 4.3 ± 0.3 psu (range: 2.53–6.63) at a highly salt‐impacted TFFW site along the Savannah River (Krauss et al, ). The increase in river and soil porewater salinity has led to the conversion of freshwater wetlands to brackish marsh (Pearlstine et al, ).…”