Net ecosystem carbon exchange and the greenhouse gas balance of tidal marshes along an estuarine salinity gradient

• Published in: Biogeochemistry Vol. 120; no. 1-3; pp. 163 - 189
• Main Authors: Weston, Nathaniel B, Neubauer, Scott C, Velinsky, David J, Vile, Melanie A
• Format: Journal Article
• Language: English
• Published: Cham Springer-Verlag 01.08.2014; Springer; Springer International Publishing; Springer Nature B.V
• Subjects: Accretion; Animal and plant ecology; Animal, plant and microbial ecology; Aquatic ecosystems; Aquatic plants; Atmosphere; Biogeochemistry; Biogeosciences; Biological and medical sciences; biomass; Carbon; Carbon dioxide; carbon sequestration; carbon sinks; Climate change; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; ecosystem respiration; Ecosystems; Engineering and environment geology. Geothermics; Environmental Chemistry; Environmental impact; Estuaries; Exact sciences and technology; Fresh water; Fresh water ecosystems; freshwater; Freshwater ecosystems; Freshwater resources; Fundamental and applied biological sciences. Psychology; Greenhouse gases; Life Sciences; Marine and continental quaternary; Marshes; Methane; Methane production; Methanogenesis; Mineralization; Monoculture; net ecosystem exchange; net ecosystem production; Organic matter; Plant biomass; Plant production; Plants; Pollution, environment geology; Saline water; Salinity; Salt marshes; saltwater intrusion; soil; Spartina alterniflora; Sulfates; Summer; Surficial geology; Synecology; Tidal marshes; Water; Wetlands; Zizania aquatica; United States; Delaware River; USA, Delaware R; Tidal freshwater marsh; Methane; Salt marsh; Climate change; Salt-water intrusion; Accretion; Greenhouse gas; Carbon; atmosphere; respiration; sulfates; North America; methanogenesis; greenhouse gas; wetlands; Gramineae; carbon; dating; estuaries; soils; Spartina alterniflora; climate change; carbon dioxide; models; salinity; biomass; salt-water intrusion; Plantae; marshes; carbon cycle; mineralization; angiosperms; salt marshes; Monocotyledoneae; ecosystems; organic materials; methane; Spermatophyta
• ISSN: 0168-2563; 1573-515X
• DOI: 10.1007/s10533-014-9989-7

Tidal wetlands are productive ecosystems with the capacity to sequester large amounts of carbon (C), but we know relatively little about the impact of climate change on wetland C cycling in lower salinity (oligohaline and tidal freshwater) coastal marshes. In this study we assessed plant production, C cycling and sequestration, and microbial organic matter mineralization at tidal freshwater, oligohaline, and salt marsh sites along the salinity gradient in the Delaware River Estuary over four years. We measured aboveground plant biomass, carbon dioxide (CO₂) and methane (CH₄) exchange between the marsh and atmosphere, microbial sulfate reduction and methanogenesis in marsh soils, soil biogeochemistry, and C sequestration with radiodating of soils. A simple model was constructed to estimate monthly and annually integrated rates of gross ecosystem production (GEP), ecosystem respiration (ER) to carbon dioxide ([Formula: see text]) or methane ([Formula: see text]), net ecosystem production (NEP), the contribution of sulfate reduction and methanogenesis to ER, and the greenhouse gas (GHG) source or sink status of the wetland for 2 years (2007 and 2008). All three marsh types were highly productive but evidenced different patterns of C sequestration and GHG source/sink status. The contribution of sulfate reduction to total ER increased along the salinity gradient from tidal freshwater to salt marsh. The Spartina alterniflora dominated salt marsh was a C sink as indicated by both NEP (~140 g C m⁻² year⁻¹) and ²¹⁰Pb radiodating (336 g C m⁻² year⁻¹), a minor sink for atmospheric CH₄, and a GHG sink (~620 g CO₂₋ₑq m⁻² year⁻¹). The tidal freshwater marsh was a source of CH₄ to the atmosphere (~22 g C–CH₄ m⁻² year⁻¹). There were large interannual differences in plant production and therefore C and GHG source/sink status at the tidal freshwater marsh, though ²¹⁰Pb radiodating indicated modest C accretion (110 g C m⁻² year⁻¹). The oligohaline marsh site experienced seasonal saltwater intrusion in the late summer and fall (up to 10 mS cm⁻¹) and the Zizania aquatica monoculture at this site responded with sharp declines in biomass and GEP in late summer. Salinity intrusion was also linked to large effluxes of CH₄ at the oligohaline site (>80 g C–CH₄ m⁻² year⁻¹), making this site a significant GHG source (>2,000 g CO₂₋ₑq m⁻² year⁻¹). The oligohaline site did not accumulate C over the 2 year study period, though ²¹⁰Pb dating indicated long term C accumulation (250 g C m⁻² year⁻¹), suggesting seasonal salt-water intrusion can significantly alter C cycling and GHG exchange dynamics in tidal marsh ecosystems.