Wetlands are an important global source of methane (CH4), a potent greenhouse gas (GHG) (Ciais et al., 2013). Wetland soils are typically strong CH4 sources, however high moisture content limits the diffusive transport of CH4 from soil pores to the soil-atmosphere interface. Plants play an important role for gas exchange in wetlands by providing preferential pathways for CH4 to reach the surface, typically through the aerenchyma tissue inside vascular plant species (Turner et al., 2020). This transport mechanism is generally assumed to be positively correlated with net primary production.
Despite the role of plants in CH4 emissions, many studies are still performed using small, opaque chamber systems which cannot accommodate larger wetland vegetation and also cut off light supply to the plant, limiting photosynthesis during the course of the measurement. This case study uses the eosAC-LT automated, transparent chamber with transparent base extensions to measure wetland fluxes over various plant species. These measurements are contrasted with those taken using an eosAC small opaque chamber on the same plots to help understand the role that these wetland plants play in mediating GHG emissions.
Our objective in this case study is to examine the role that plants play in mediating CH4 fluxes from a wetland ecosystem. Based on the literature, we expect that plant photosynthesis plays an important role in this process. Photosynthesis can be characterized by measuring gross primary productivity (GPP), a measure of the total CO2 taken up from the atmosphere and converted to sugars by plants during photosynthesis. In order to calculate GPP we use “light” or transparent chamber to measure Net Ecosystem Exchange (NEE) of CO2, a measure which is the combined impact of GPP and ecosystem respiration (Reco).
NEE = GPP + Reco
We combine this measurement with a “dark” or opaque chamber to quantify ecosystem respiration (Reco) and thus are able to derive the GPP for the measurement location. Similarly, because plant productivity is hypothesized to drive plant mediated CH4 fluxes, we measure CH4 emissions both in the presence and absence of light during the measurement period.
To capture dark fluxes (Reco) we used the opaque eosAC chamber (footprint diameter of ~15 cm). For light measurements (NEE) we used the transparent eosAC-LT chamber (footprint diameter of ~52 cm). To include taller vegetation, we also used a 30 cm base extension for the eosAC-LT, which brought the total height of the chamber to ~75 cm at the top of the dome (compared to ~15 cm height for the eosAC). Both chambers used 5 cm soil collars that were installed in the soil approximately 30 minutes before measurements were taken. GHG concentrations were monitored inside the chambers using the Picarro GasScouter (G4301) which simultaneously measures CO2 and CH4. We collected measurements over a 6 hour period, powering the GasScouter on its internal battery. The eosAC-LT and eosAC were powered using a commercially available lithium ion battery pack (capacity approx. 24,000 mAh).
The wetland at Barry’s Run in Waverley, Nova Scotia was selected for this experiment. Measurements were made on May 20, 2022. The weather was sunny with nearly zero cloud cover, and the average temperature during the course of measurements was 19.8° C. We identified 5 sites of varying vegetation, summarized below. At each site between 3 and 5 measurements were made with the transparent eosAC-LT, with each measurement lasting 5 minutes followed by a 3 minute period to purge the tubing and let accumulated greenhouse gases dissipate from the chamber. Following this, approximately the same number of measurements were made with the opaque eosAC (5 minute measurement, 3 minute purge) at a randomly selected location within the same footprint as the eosAC-LT.
Saturated ground approximately 1 meter from the water’s edge. Mostly bare with small amounts of sedge grass.
Approximately 5 meters from the water’s edge. Mainly sphagnum moss with minor sedge grass.
A relatively dry hummock near the water’s edge dominated by sedge grass.
Dry soil dominated by leatherleaf approximately 2 meters from the water’s edge.
Damp soil with a large hummock of rush approximately 50 centimeters from the water’s edge.
Measurements of NEE and calculated gross primary production (GPP) followed an expected pattern based on the vegetation type, with grasses and areas with more biomass showing higher NEE and GPP. Ecosystem respiration (Reco) was highest in Site 3, which was dominantly sedge grass, and Site 4 which was the woody vegetation. These two sites also had comparatively drier soils. Conversely, Reco was low at the saturated site (Site 1) closer to the water, likely due to a combination of low oxygen concentrations available for aerobic respiration and slow diffusion of gases to surface. It should be noted that site temperature varied by approximately 8 °C during the day (Figure 1) while we measured the sites in consecutive order. We have not attempted to correct or normalize for this in the data presented here.
Methane fluxes were highest in the water saturated soil of Site 1. Sites 2 and 3 both showed positive emissions (from soil to atmosphere), but emissions were higher in the dark chamber system for both sites (Figure 2, left). Contrastingly, Sites 4 and 5 showed positive emissions in the transparent chamber, but no or negative emission (CH4 uptake) were shown in the dark chamber. When comparing the opaque chamber methane measurements to GPP measured at that location there is no clear correlation. However, when GPP and methane fluxes from the transparent chamber are plotted, there is a clear correlation where higher GPP (more negative) results in higher methane fluxes (Figure 2, right). This correlation does not hold at the heavily saturated and sparsely vegetated Site 1 which has been excluded from the plot.
The objective of this study was to measure dark and light fluxes of CO2 and CH4 from various vegetation types in a wetland ecosystem in order to understand the role of plants in mediating the exchange of greenhouse gases. While we saw some evidence of plant mediated CH4 production at Sites 4 and 5, we see that at Sites 1-3 methane emissions are higher in the dark (opaque) chamber than they were in the transparent chamber. This suggests some other process is at play. One methodological explanation could be the difference in footprint size between the opaque eosAC and transparent eosAC-LT chambers. With a larger footprint, it is possible that the eosAC-LT is able to capture or average out some spatial heterogeneity that was not measured by the smaller eosAC. In order to further understand the impact of plants on wetland emissions and to improve estimates of total wetland GHG fluxes, a combination of transparent and opaque chambers should be used.
Ciais, P., Sabine, C., Bala, G., Bopp, L., Brovkin, V., Canadell, J., et al. (2013). Carbon and other biogeochemical cycles. In Climate change 2013: The physical science basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (pp. 465–570). Cambridge: University press.
Turner, J. C., Moorberg, C. J., Wong, A., Shea, K., Waldrop, M. P., Turetsky, M. R., & Neumann, R. B. (2020). Getting to the root of plant‐mediated methane emissions and oxidation in a Thermokarst bog. Journal of Geophysical Research: Biogeosciences, 125, e2020JG005825. https://doi.org/10.1029/2020JG005825
The eosAC-T/O automated flux chamber is field-ready, compact, and ideal for long-term deployment in a wide variety of applications. The chamber’s design incorporates scientific best practices and features swappable components that allow for hassle-free repairs in the field. Seamless integration with leading gas analyzers and included flux analysis software enable simple and accurate measurements of trace gas flux.
The eosAC-LT/LO automated flux chamber is purpose-built for applications that demand a large sampling area. The chamber design incorporates scientific best practices and features swappable components that allow for hassle-free repairs in the field. Seamless integration with leading gas analyzers and included flux analysis software enable simple and accurate measurements of trace gas flux.
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