Monitoring Greenhouse Gas Emissions from Re-Constructed Wetlands in the Athabascan Oil Sands Region

Naturally occurring, highly saturated wetlands are an important player in the global carbon cycle; acting as net carbon sinks over a geological time scales. As land use changes, with increased urbanization and industrial development, wetlands are disappearing. Measuring greenhouse gas (GHG) fluxes in both natural and restored wetlands has become prevalent in ecological research in order to provide a greater understanding of the role of wetlands in the global GHG balance. A study was conducted by Nwaishi et al. (2016) to observe the differences in GHG emissions from a constructed wetland, in this case a fen, compared to a naturally-occurring wetland. The wetland of interest was re-constructed within an oil sands landscape. Constructed wetlands act as either GHG sinks or sources, depending on edaphic properties such as hydrology, vegetation, soil/water chemistry, soil temperature, humidity, etc. Constructing peatlands on post-mining landscapes gives the dual benefit of sequestering more GHG while reclaiming land use.

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Figure 1: Sites of constructed fen and reference fen

In the Athabasca oil sands region, fens were the predominant wetland type prior to surface mining. In an effort to reclaim these wetlands, the Alberta Environmental Protection and Enhancement Act requires post-mining wetland re-construction. The re-constructed fen was located 50 km north of Fort McMurray, AB, with an area of 3 hectares. A naturally-occurring fen located further from industrial activity was used as a reference fen (Figure 1). The fen was re-created by draining peat layers from a larger wetland, however, this presents further issues as each layer in the wetland supports different microbial processes which may not transfer over to the newly constructed wetland (Krab et al., 2010). This location receives high volumes of rainfall and snowmelt runoff from surrounding slopes, and nutrient input to promote re-vegetation.

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Figure 2: Example of a closed-chamber apparatus. Photo by D. Risk, 2016

Emissions of CO2, CH4, and NO2 were measured in-situ using a closed chamber apparatus similar to those shown in Figures 2 and 3, however, using a transparent shell to allow photosynthesis during measurement. CO2 concentrations were measured on a portable infrared gas analyzer, and CH4 and NO2 concentrations were both determined using a Fourier Transform Infrared- Gas Analyzer (FTIR-GA).

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Figure 3: Schematic of a transparent closed-chamber apparatus. Retrieved from: Olssen et al. 2014

The goal of the study was to quantify GHG measurements of a re-constructed wetland as compared to a reference wetland. A significant uptake of CO2 was observed after the first growing season of the re-vegetated wetlands, due to the introduction of vascular plants and mosses. Two years following the wetland construction, the re-vegetated plots were proven to be more efficient GHG sinks than the plots without vegetation. Methane fluxes were substantially lower in the re-constructed wetland compared to the reference wetland, likely caused by the drainage of the donor peatlands prior to re-construction, which increases oxidation of organic matter and suppresses methanogenesis. Lower N2O fluxes than predicted were identified in the re-constructed wetland, caused by a diminished nitrate ion (NO3) concentration, which acts as the electron acceptor for N2O production. Through identifying trends in the fluxes of the three main GHG contributors, Nwaishi et al. were able to draw conclusions about the regulation of GHGs through wetland reconstruction.

Nwaishi et al. state that continuous monitoring is necessary to obtain a complete understanding of GHG fluxes in the long-term. Environmental conditions change with time, causing modifications in gas fluxes; as fluxes depend on water chemistry, the source of the donated peatland, edaphic contributions, etc. Re-vegetation of the fen increased the sequestration of CO2 while reducing N2O emissions, especially observed in the plots containing vascular plants and mosses. Methane emissions did not appear to be affected by re-vegetation. Overall it was found that the wetlands acted as a GHG sink until the terminal electron acceptors were consumed.