ORCID ID

https://orcid.org/0000-0002-4197-7884

Date Awarded

2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Iris C Anderson

Committee Member

Mark J Brush

Committee Member

Matthew L Kirwan

Committee Member

Anne E Giblin

Abstract

Salt marshes provide valuable ecosystem services to human society, but are currently under threat from accelerating sea level rise and nutrient enrichment. Carbon (C) and mineral accumulation allow salt marshes to maintain elevation above sea level and survive. Anthropogenic nitrogen (N) loading is increasing in many salt marshes, causing negative impacts on marsh resilience such as increased decomposition and decreased below-ground production. However, increasing N may also have simultaneous positive effects such as increased primary production and above-ground biomass, surface sediment accretion, and denitrification rates, which remove excess N from coastal waters. Many studies have been conducted to determine the effect of fertilization on salt marsh resilience; however, inconsistent conclusions across studies may result from varying physical and chemical characteristics across salt marsh locations that impact responses to fertilization. In this dissertation we performed experiments to determine how C cycling, C accumulation, N cycling, and microbial communities vary in both natural and fertilized salt marsh locations at Marine Corps Base Camp Lejeune, North Carolina, USA. Here we show that edge marsh with a high elevation berm had lower pore water sulfide, ammonium, dissolved organic C (DOC), and dissolved organic C (DIC) concentrations than interior marsh, which displayed longer pore water residence time and flooding duration with high pore water sulfide, ammonium, DOC, and DIC concentrations. Respiration and primary production were higher in the edge marsh compared to the interior marsh but net ecosystem CO2 exchange (NEE) was nearly balanced at all sites. Fertilization had a much greater impact on edge than interior NEE, shifting edge NEE toward net CO2 emission. Net ecosystem carbon balance (NECB), based on the mass balance of NEE, lateral C export, and sediment C deposition for edge and interior sites was calculated to examine the effect of fertilization on net C accumulation. NECB displayed a net C gain in the interior marsh but a large net C loss on the edge; fertilization stimulated more C loss on the edge than in the interior. When extrapolating NECB to the entire marsh, C loss on the edge greatly impacted the whole marsh C budget, causing the marsh to have a net loss of 53 kg C yr-1 under natural conditions and a five-fold increase in C loss with fertilization. N removal through denitrification was greater on the edge and increased with fertilization, but was not affected by fertilization at the site with highest sulfide concentrations. DNRA, which retains N in the marsh, dominated over denitrification only during summer, and varied widely across locations. Fertilization generally decreased DNRA rates. Microbial community composition was distinct on the edge vs. interior, with differences driven by the differences in pore water sulfide, ammonium, DOC, and DIC. The edge was a hotspot for nitrifying microbial communities. The processes of respiration and denitrification were positively correlated to the relative abundance of sulfate reducers and ammonia oxidizers, respectively. Thus, we conclude that fertilization had an overall negative effect on marsh resilience with especially large impacts on edge marsh.

DOI

http://dx.doi.org/10.25773/v5-dhxm-jp88

Rights

© The Author

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