Nitrogen enrichment alters carbon fluxes in a New England salt marsh

Geoghegan, Emily K.; Caplan, Joshua S.; Leech, Francine; Weber, Paige; Bauer, Caitlin; Mozdzer, Thomas J.

doi:10.1080/20964129.2018.1532772

Cited by 17 publications

(12 citation statements)

References 66 publications

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“…Therefore, in all cases when NO3− was directly added to marsh sediments, both to the rhizosphere and to the overlying water in intact cores, there was a rapid increase in microbial respiration of NO3− through denitrification. In situ measurements of ecosystem respiration (Geoghegan et al 2018 ) and soil respiration (Wigand et al 2018 ) were also significantly higher in the nutrient enriched creeks, providing further evidence of NO3− stimulated microbial processes. This suggests that salt marsh sediments have populations of microbes capable of using NO3− within hours of it being added and implies that there is a sufficient supply of biologically available salt marsh organic matter to support this NO3− respiration.…”

Section: Both Field and Controlled Laboratory Experiments Support Micmentioning

confidence: 90%

Not All Nitrogen Is Created Equal: Differential Effects of Nitrate and Ammonium Enrichment in Coastal Wetlands

et al. 2020

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Excess reactive nitrogen (N) flows from agricultural, suburban, and urban systems to coasts, where it causes eutrophication. Coastal wetlands take up some of this N, thereby ameliorating the impacts on nearshore waters. Although the consequences of N on coastal wetlands have been extensively studied, the effect of the specific form of N is not often considered. Both oxidized N forms (nitrate, NO3−) and reduced forms (ammonium, NH4+) can relieve nutrient limitation and increase primary production. However, unlike NH4+, NO3− can also be used as an electron acceptor for microbial respiration. We present results demonstrating that, in salt marshes, microbes use NO3− to support organic matter decomposition and primary production is less stimulated than when enriched with reduced N. Understanding how different forms of N mediate the balance between primary production and decomposition is essential for managing coastal wetlands as N enrichment and sea level rise continue to assail our coasts.

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Section: Both Field and Controlled Laboratory Experiments Support Micmentioning

confidence: 90%

Not All Nitrogen Is Created Equal: Differential Effects of Nitrate and Ammonium Enrichment in Coastal Wetlands

et al. 2020

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“…This loss of 62 mol C m −2 year −1 along the fertilized edge is significant and could result in a decrease in overall accretion rates and elevation. Other salt marsh studies have similarly concluded that fertilization increases both GPP and R (Anisfeld & Hill, 2012;Caplan et al, 2015;Geoghegan et al, 2018;Martin et al, 2018;Morris & Bradley, 1999;Wang, Zhu et al, 2013); however, studies are less congruent about the effect of fertilization on NEE, showing variation between no detectable effect (Geoghegan et al, 2018) and increasing CO 2 emissions (Caplan et al, 2015;Wang, Zhu et al, 2013). A decrease in soil carbon content has been observed with fertilization (Morris & Bradley, 1999) but may be a result of increased sediment input changing sediment composition rather than loss of carbon in sediment (Morris et al, 2002).…”

Section: Vertical Carbon Fluxes 431 Metabolic Ratesmentioning

confidence: 95%

“…BGB has been observed to either increase (Morris et al, 2013;Wigand et al, 2015) or decrease (Darby & Turner, 2008;Deegan et al, 2012) in response to fertilization. Several studies found that R and GPP (Wigand et al, 2009;Anisfeld & Hill, 2012;Caplan et al, 2015;Geoghegan et al, 2018;Morris & Bradley, 1999;Wang, Zhu et al, 2013, Bulseco et al, 2019 increased in response to fertilization, but the results for NEE were mixed with either no detectable response (Geoghegan et al, 2018) or an increase in net CO 2 emissions (Caplan et al, 2015;Wang, Zhu, et al, 2013).…”

Section: Introductionmentioning

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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“…Czapla et al in a companion paper in this issue pointed out that fertilization effects on C fluxes differ across marsh sites due to location‐specific physical and chemical characteristics, and this heterogeneity in responses to fertilization may complicate the relationship between salt marsh N availability and NECB. While several studies have reported an increase in AGB and net CO 2 emissions with fertilization, other studies have reported no effect on AGB (Davis et al, 2017; Johnson et al, 2016) and NEE (Geoghegan et al, 2018) at certain sites. Belowground biomass (BGB) has been reported to either increase (Morris, Sundberg, & Hopkinson, 2013; Wigand et al, 2014), decrease (Darby & Turner, 2008; Deegan et al, 2012; Graham & Mendelsshon, 2016), or remain the same (Davis et al, 2017) in response to fertilization.…”

Section: Introductionmentioning

confidence: 96%

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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Nitrogen enrichment alters carbon fluxes in a New England salt marsh

Cited by 17 publications

References 66 publications

Not All Nitrogen Is Created Equal: Differential Effects of Nitrate and Ammonium Enrichment in Coastal Wetlands

Not All Nitrogen Is Created Equal: Differential Effects of Nitrate and Ammonium Enrichment in Coastal Wetlands

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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