Sea Level Rise Explains Changing Carbon Accumulation Rates in a Salt Marsh Over the Past Two Millennia

McTigue, Nathan D.; Davis, Jenny; Rodriguez, Antonio B.; McKee, Brent A.; Atencio, Anna; Currin, Carolyn A.

doi:10.1029/2019jg005207

Cited by 28 publications

(18 citation statements)

References 52 publications

(99 reference statements)

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“…The signature of δ 13 C and its relation with C/N is similar in sediments and groundwater samples, indicating that groundwater obtains most of its carbon from soils, exporting later to the ocean. Our carbon burial rates (62 ± 32 g m −2 year −1 ) were 37% of the global average for salt marsh ecosystems (168 ± 7 g m −2 year −1 ; Wang et al 2021) but similar to rates reported in other studies in the USA (60 g m −2 year −1 ; Chmura et al 2003;Mctigue et al 2019;Wang et al 2019). Comparing carbon burial in the marsh and groundwater exports represents a challenge because these processes were quantified at different time scales.…”

Section: Carbon Burial and Origins Of The Exported Carbonsupporting

confidence: 84%

Groundwater Carbon Exports Exceed Sediment Carbon Burial in a Salt Marsh

Correa

Xiao

Conrad

et al. 2021

Estuaries and Coasts

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Salt marshes can sequester large amounts of carbon in sediments, but the relation between carbon storage and exportation remains poorly understood. Groundwater exchange can flush sediment carbon to surface waters and potentially reduce storage. In this study, we estimated groundwater fluxes and associated carbon fluxes using a radon ( 222 Rn) mass balance and sediment carbon burial rates using lead ( 210 Pb) in a pristine salt marsh (North Inlet, SC, USA). We used δ 13 C to trace carbon origins. We found that groundwater releases large amounts of carbon to the open ocean. These groundwater fluxes have the potential to export 7.2 ± 5.5 g m −2 of dissolved inorganic carbon (DIC), 0.2 ± 0.2 g m −2 of dissolved organic carbon (DOC) and 0.7 ± 0.5 g m −2 of carbon dioxide (CO 2 ) per day. The fluxes exceed the average surface water CO 2 emissions (0.6 ± 0.2 g m −2 day −1 ) and the average sediment carbon burial rates (0.17 ± 0.09 g m −2 day −1 ). The δ 13 C results suggest that groundwater carbon originated from salt marsh soils, while the sediment carbon source is derived from salt marsh vegetation. We propose that the impact of salt marshes in carbon cycling depends not only on their capacity to bury carbon in sediments, but also on their high potential to export carbon to the ocean via groundwater pathways.

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Section: Carbon Burial and Origins Of The Exported Carbonsupporting

confidence: 84%

Groundwater Carbon Exports Exceed Sediment Carbon Burial in a Salt Marsh

Correa

Xiao

Conrad

et al. 2021

Estuaries and Coasts

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“…The FC sites were within an extensive S. alterniflora –dominated marsh, while the TBC site was within a S. alterniflora zone that transitioned into Juncus roemerianius (Davis et al, 2017; McTigue et al, 2019). The tidal range averaged 0.31 m at TBC and 0.83 m at FC (Lettrich, 2011; McTigue et al, 2019). Seasonal differences in water level were greater at TBC than FC, due to the influence of freshwater inputs and wind‐driven tides (Ensign et al, 2017).…”

Section: Methodsmentioning

confidence: 99%

“…A third site was selected in the interior of a fringing marsh near the mouth of Traps Bay Creek (TBC), a tidal creek in an embayment within the New River Estuary. The FC sites were within an extensive S. alterniflora -dominated marsh, while the TBC site was within a S. alterniflora zone that transitioned into Juncus roemerianius (Davis et al, 2017;McTigue et al, 2019). The tidal range averaged 0.31 m at TBC and 0.83 m at FC (Lettrich, 2011;McTigue et al, 2019).…”

Section: Site Descriptionmentioning

confidence: 99%

The Effect of Fertilization on Biomass and Metabolism in North Carolina Salt Marshes: Modulated by Location‐Specific Factors

2020

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The resilience of salt marshes to sea level rise depends on vertical accretion through belowground biomass production and sediment deposition to maintain elevation above sea level. Increased nitrogen (N) availability from anthropogenic sources may stimulate aboveground biomass production and sediment deposition and, thus, accretion; however, increased N may also negatively impact marsh accretion by decreasing belowground biomass and increasing net CO 2 emissions. A study was conducted in Spartina alterniflora-dominated salt marshes in North Carolina, USA, to determine how responses to fertilization vary across locations with different physical and chemical characteristics. Pore water residence time, inundation time, and proximity to tidal creeks drove spatial differences in pore water sulfide, ammonium, and dissolved carbon concentrations. Although annual respiration and gross primary production were greater at the creek edge than interior marsh sites, net ecosystem CO 2 exchange (NEE) was nearly balanced at all the sites. Fertilization decreased belowground biomass in the interior sites but not on the creek edge. Aboveground biomass, respiration, gross primary production, and net CO 2 emissions increased in response to fertilization, but responses were diminished in interior marsh locations with high pore water sulfide. Hourly NEE measured by chambers were similar to hourly NEE observed by a nearby eddy covariance tower, but correcting for inundation depth relative to plant height was critical for accurate extrapolation to annual fluxes. The impact of fertilization on biomass and NEE, and thus marsh resilience, varied across marsh locations depending upon location-specific pore water sulfide concentrations. Plain Language Summary Salt marshes provide valuable services, such as protecting the coast from storms, removing excess nutrient pollution from water, and long-term burial of carbon. Because sea level is currently rising, salt marshes need to build up elevation at the same rate as sea level rise to survive. Human-produced nitrogen pollution is rising in salt marshes, often increasing the growth of grass, which may cause the marsh to trap sediment more efficiently and build elevation faster. However, increased nitrogen may also decrease root growth and increase sediment microbial activity (which decomposes sediment organic carbon to carbon dioxide), causing elevation-building to slow down. It is unclear whether the addition of nitrogen affects the marsh's elevation-building rate in a positive or negative way. We found that the effect of nitrogen on elevation-building depends on location. Factors such as tidal inundation and residence time influence sediment water chemistry and, thereby, the response to excess nitrogen. Sulfide inhibits the uptake of nitrogen by plant roots and also microbial activity; thus, marsh locations with more sulfide have diminished responses to nitrogen pollution. This knowledge may be used for management of marshes at risk due to nitrogen pollution.

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“…The CAR measured in 59 Atlantic coast salt marsh sites averaged 126 ± 87 g C m −2 year −1 (Ouyang & Lee, 2014), similar in magnitude to the 83.5 g C m −2 year −1 NECB estimated in FC interior control plots. Traps Bay, a marsh within the New River Estuary several miles from FC, was found to have a CAR rate of 167 g C m −2 year −1 over a 4-year time scale (McTigue et al, 2019). However, CAR is measured using wide variety of methods and interpretations that are difficult to capture with a broad brush.…”

Section: Net Ecosystem Carbon Balancementioning

confidence: 99%

Net Ecosystem Carbon Balance in a North Carolina, USA, Salt Marsh

2020

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Salt marshes have among the highest carbon (C) burial rates of any ecosystem and often rely on C accumulation to gain elevation and persist in locations with accelerating sea level rise. Net ecosystem carbon balance (NECB), the accumulation or loss of C resulting from vertical CO 2 and CH 4 gas fluxes, lateral C fluxes, and sediment C inputs, varies across salt marshes; thus, extrapolation of NECB to an entire marsh is challenging. Anthropogenic nitrogen (N) inputs to salt marshes impact NECB by influencing each component of NECB, but differences in the impacts of fertilization between edge and interior marsh must be considered when scaling up. NECB was estimated for the 0.5 km 2 Spartina alterniflora marsh area of Freeman Creek, NC, under control and fertilized conditions at both interior and edge berm sites. Annual CO 2 fluxes were nearly balanced at control sites, but fertilization significantly increased net CO 2 emissions at edge sites. Lateral C export, modeled using respiration rates, represented a significant C loss that increased with fertilization in both edge and interior marsh. Sediment C input was a significant C source in the interior, nearly doubling with fertilization, but represented a small source on the edge. When extrapolating C exchanges to the entire marsh, including edge which comprised 17% of the marsh area, the marsh displayed net loss of C despite a net C gain in the interior. Fertilization increased net C loss fivefold. Extrapolation of NECB to whole marshes requires inclusion of C fluxes for both edge and interior marsh. Plain Language Summary Salt marsh ecosystems rely on carbon accumulation to increase elevation and survive sea level rise. The amount of carbon accumulated in a marsh is the net result of carbon dioxide emissions to the atmosphere, fixation of carbon by photosynthesis, export of dissolved carbon to the creek, and accumulation of organic carbon in sediments deposited on the surface. Because each component varies between edge and interior marsh, it is challenging to estimate carbon accumulation for a whole marsh system. It is not currently known how increasing nitrogen pollution impacts carbon accumulation for a whole marsh. To find out, we compared measurements of carbon accumulation in fertilized and unfertilized plots in the edge and interior of a salt marsh at Freeman Creek, North Carolina, USA. Overall, the marsh gained carbon in the interior but lost carbon on the edge, leading to a loss of about 50,000 kg C year −1 across the 0.5 km 2 marsh area. However, under fertilized conditions, Freeman Creek marsh carbon loss increased nearly fivefold overall as a result of the large increase in carbon loss from the edge marsh. This study shows that increasing nitrogen pollution in coastal waters will cause increasing net carbon loss in marshes.

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Sea Level Rise Explains Changing Carbon Accumulation Rates in a Salt Marsh Over the Past Two Millennia

Cited by 28 publications

References 52 publications

Groundwater Carbon Exports Exceed Sediment Carbon Burial in a Salt Marsh

Groundwater Carbon Exports Exceed Sediment Carbon Burial in a Salt Marsh

The Effect of Fertilization on Biomass and Metabolism in North Carolina Salt Marshes: Modulated by Location‐Specific Factors

Net Ecosystem Carbon Balance in a North Carolina, USA, Salt Marsh

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