Saltwater reduces potential CO<sub>2</sub> and CH<sub>4</sub> production in peat soils from a coastal freshwater forested wetland

Abstract

Abstract. A major concern for coastal freshwater wetland function and health is the effects of saltwater intrusion on greenhouse gas production from peat soils. Coastal freshwater forested wetlands are likely to experience increased hydroperiod with rising sea level, as well as saltwater intrusion. These potential changes to wetland hydrology may also alter forested wetland structure and lead to a transition from forest to shrub/marsh wetland ecosystems. Loss of forested wetlands is already evident by dying trees and dead standing trees (“ghost” forests) along the Atlantic coast of the US, which will result in significant alterations to plant carbon (C) inputs, particularly that of coarse woody debris, to soils. We investigated the effects of salinity and wood C inputs on soils collected from a coastal freshwater forested wetland in North Carolina, USA, and incubated in the laboratory with either freshwater or saltwater (2.5 or 5.0 ppt) and with or without the additions of wood. Saltwater additions at 2.5 and 5.0 ppt reduced CO2 production by 41 % and 37 %, respectively, compared to freshwater. Methane production was reduced by 98 % (wood-free incubations) and by 75 %–87 % (wood-amended incubations) in saltwater treatments compared to the freshwater plus wood treatment. Additions of wood also resulted in lower CH4 production from the freshwater treatment and higher CH4 production from saltwater treatments compared to wood-free incubations. The δ13CH4-C isotopic signature suggested that, in wood-free incubations, CH4 produced from the freshwater treatment originated primarily from the acetoclastic pathway, while CH4 produced from the saltwater treatments originated primarily from the hydrogenotrophic pathway. These results suggest that saltwater intrusion into coastal freshwater forested wetlands will reduce CH4 production, but long-term changes in C dynamics will likely depend on how changes in wetland vegetation and microbial function influence C cycling in peat soils.

Modeling impacts of saltwater intrusion on methane and nitrous oxide emissions in tidal forested wetlands

“…Such large variations in CH 4 and N 2 O fluxes could be attributed to the within‐site differences in microtopography, soil moisture, surface vegetation cover, soil organic matter content, and soil texture, since even small differences in these site‐level conditions may cause large changes in biogeochemical responses. For example, CH 4 fluxes were significantly lower in soils of hummock than in tidal forest hollows, or small depressional areas between trees (Minick, Mitra, et al, 2019; Yu et al, 2006). Furthermore, denitrification and N 2 O flux potentials were found to be higher in hummocks with wet‐dry fluctuation than in hollows with no fluctuation (Korol & Noe, 2020).…”

“…Limited laboratory studies also showed that CH 4 production from tidal freshwater forests tends to decline with salinity treatments (0, 2, 5 psu) (e.g., Marton et al, 2012; Minick, Kelley, et al, 2019). For example, Minick, Mitra, et al (2019) found that methane production was reduced by 98% (wood‐free incubations) and by 75%–87% (wood‐amended incubations) in saltwater treatments compared to the freshwater plus wood treatments. Reduction of CH 4 emissions at low salinity forest and marsh sites along the two rivers could be attributed to the inhibition of methanogenesis due to the increased activities of sulfate‐reducing bacteria with saltwater intrusion.…”

“…Under the low sulfate concentration, other factors such as soil water level (moisture, or redox potential) and organic carbon availability likely result in the change of the direction of CH 4 emissions from reduction to no change to an increase of CH 4 fluxes. For example, it was found that additions of wood also resulted in higher CH 4 production from saltwater treatments compared to wood‐free incubations (Minick, Mitra, et al, 2019). It is also possible that increased salinity could result in increased CH 4 emission due to the reduction in humic substances that decrease CH 4 production by serving as thermodynamically favorable organic electron acceptors, thus leading to the release of methanogens from the inhibitory effects of humics (Ardón et al, 2018).…”

A case study on topsoil removal and rewetting for paludiculture: effect on biogeochemistry and greenhouse gas emissions from Typha latifolia, Typha angustifolia, and Azolla filiculoides

“…Overall, our measured CH 4 fluxes were high despite the topsoil removal and the brackish conditions, which were expected to reduce CH 4 production, due to the removal of easily degradable carbon (Harpenslager et al, 2015;Quadra et al, 2023) and the reducing effect of salinity on CH 4 production (Van der Gon and Neue, 1995;Minick et al, 2019), respectively.…”

Microorganisms in coastal wetland sediments: a review on microbial community structure, functional gene, and environmental potential

“…The abundance of nirS denitrifying bacteria is much greater than that of nrfA-DNRA microorganisms in the Chesapeake Bay watershed of United States, suggesting that denitrification is the primary nitrate reduction process (Franklin et al, 2017). The abundance of nrfA genes was low in tidal freshwater marshes in South Carolina of United States, suggesting weak DRNA process (Minick et al, 2019).…”