Abstract
We evaluated the major pathways for methane emissions from wetlands to the atmosphere at four wetland sites in the Big Cypress National Preserve in southwest Florida. Methane oxidation was estimated based on the δ13C-CH4of surface water, porewater, and bubbles to evaluate mechanisms that limit surface water emissions. Spatially-scaled methane fluxes were then compared to organic carbon burial rates. The pathway with the lowest methane flux rate was diffusion from surface waters (3.50 ± 0.22 mmol m−2 d−1). Microbial activity in the surface water environment and/or shallow oxic sediment layer oxidized 26 ± 3% of the methane delivered from anerobic sediments to the surface waters. The highest rates of diffusion were observed at the site with the lowest extent of oxidation. Ebullition flux rates were 2.2 times greater than diffusion and more variable (7.79 ± 1.37 mmol m−2 d−1). Methane fluxes from non-inundated soils were 1.6 times greater (18.4 ± 5.14 mmol m−2 d−1) than combined surface water fluxes. Methane flux rates from cypress knees (emergent cypress tree root structures) were 3.7 and 2.3 times higher (42.0 ± 6.33 mmol m−2 d−1) than from surface water and soils, respectively. Cypress knee flux rates were highest at the wetland site with the highest porewater methane partial pressure, suggesting that the emergent root structures allow methane produced in anaerobic sediment layers to bypass oxidation in aerobic surface waters or shallow sediments. Scaled across the four wetlands, emissions from surface water diffusion, ebullition, non-inundated soils, and knees contributed to 14 ± 2%, 25 ± 6%, 34 ± 10%, and 26 ± 5% of total methane emissions, respectively. When considering only the three wetlands with cypress knees present, knee emissions contributed to 39 ± 5% of the total scaled methane emissions. Finally, the molar ratio of CH4emissions to OC burial ranged from 0.03 to 0.14 in the wetland centers indicating that all four wetland sites are net sources of atmospheric warming potential on 20–100 yr timescales, but net sinks over longer time scales (500 yr) with the exception of one wetland site that was a net source even over 500 yr time scales.
Highlights
Inland aquatic ecosystems actively cycle carbon from the terrestrial biosphere (Battin et al, 2009), which results in large emissions of greenhouse gases (GHGs) such as carbon dioxide (CO2) (Raymond et al, 2013; Sawakuchi et al, 2017) and methane (CH4) (Baker-Blocker et al, 1977; Matthews and Fung, 1987) from inland waters to the atmosphere
Our results show that the total flux of CH4 from cypress dome surface waters was not significantly different from non-inundated soil CH4 fluxes on the wetland edge or during periods of low water levels (Figure 5)
Our observed methane flux rates from soils and surface waters were nearly equivalent to other chamber-based measurements made in similar subtropical wetlands in Florida such as Corkscrew Swamp, FL (11.2 mmol m2 d−1 compared to 11.3 mmol m2 d−1 in this study) (Villa and Mitsch, 2014; Pereyra and Mitsch, 2018)
Summary
Inland aquatic ecosystems actively cycle carbon from the terrestrial biosphere (Battin et al, 2009), which results in large emissions of greenhouse gases (GHGs) such as carbon dioxide (CO2) (Raymond et al, 2013; Sawakuchi et al, 2017) and methane (CH4) (Baker-Blocker et al, 1977; Matthews and Fung, 1987) from inland waters to the atmosphere. The balance between carbon burial and GHG emissions in wetlands and aquatic ecosystems along the landocean continuum remain poorly constrained in the global carbon cycle and poorly represented in Earth system models (Ward et al, 2017; Ward et al, 2020). Considering the relative global warming potential of CH4, these wetlands exacerbate atmospheric greenhouse effect over short (20 years) time scales, but serve as net sinks of warming potential (i.e., produce a global cooling effect) over 100–500 years timescales since the atmospheric residence time of methane is an order of magnitude shorter than CO2 (Whiting and Chanton, 2001)
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