Quantifying greenhouse gas emissions on a peat stockpile in the northern Alberta Oil Sands Region

Abstract

The Oil Sands Region (OSR) in Alberta, within Canada’s boreal forest, is a combination of three recognized oil deposits. The boreal forest in the OSR can reach peatland coverage of over 50%. Peatlands are natural carbon sinks through a net uptake of atmospheric carbon dioxide (CO2). The removal of peat for mining operations and the subsequent stockpiling of the peat under aerated conditions promote organic matter decomposition, potentially emitting significant amounts of carbon into the atmosphere. Peat stockpiles in the oil sands can stand for decades, often with actively introduced vegetation on the surface, leaving the actual decomposition rates largely unknown. Some studies suggest that all carbon is lost from peat in stockpiles during the initial stripping and stockpiling process, and final use as reclamation soil. This research aims to improve our accounting of wetland soil carbon losses for national emissions reporting by evaluating the carbon emissions on a peat stockpile at an in-situ oil sands lease across different vegetation treatments, locations, and soil moisture contents. To evaluate peat decomposition and carbon emissions, a laboratory incubation with peat samples from the stockpile was conducted over 5 weeks, and in-field carbon fluxes were measured weekly over 4 months using the closed chamber technique for both CO2 and methane (CH4) exchange. The incubation study included peat samples from bare organic, fen, and grass locations on the stockpile. Different soil moisture contents, as well as the presence of roots, were analyzed. The interaction between soil moisture and root presence moderately affected respiration from the peat samples, with the fen sample respiring the most. However, CH4 flux was weakly affected by the same interaction. All wet samples (high soil moisture treatment) produced more CH4 than their dry counterparts. These results give some insight into how the in-field processes of soil moisture and vegetation presence may affect decomposition and respiration. To confirm these results, in-field flux measurements were conducted on the stockpile. Fluxing collars with intact vegetation, clipped vegetation, and trenching were set up along a transect to compare soil respiration and net ecosystem exchange. Each collar treatment was set up in four elevated, dry locations dominated by shrubs and grasses, and four depression locations, with high soil moisture and with mainly sedges present. Biomass was collected to determine the total addition of new organic matter. The net carbon balance of the stockpile treatments was estimated as the difference between soil respiration, CH4 flux, and carbon accumulation in biomass to determine how large a carbon source the stockpile has become. Results suggest no significant difference between stockpile locations (depressions vs elevations) for ecosystem respiration, gross ecosystem production, or net ecosystem exchange, but CH4 emissions were significantly higher at depressions on the top of the stockpile. Depression locations had a greater range in net ecosystem exchange, gross ecosystem production, and ecosystem respiration due to the dense vegetation at these locations and optimal soil moisture contents. The significant increase in CH4 flux at these wet locations, and the greater range in CO2 produced and consumed, align with the findings in the incubation experiment. However, the effects were more pronounced in the in-field data. Utilizing the flux data and the biomass data, the net carbon balance of the stockpile is approximately 539 g C m-2 yr-1. This value indicates that even after a standing duration of 6 years, the stockpile is still a large source of carbon to the atmosphere. All carbon was not lost during the initial construction processes, and serves to be a continuous carbon source in the future. Therefore, large peat stockpiles on oil sand leases have substantial implications for carbon emissions, and accurate emission reporting is needed. This study highlights the importance of studying peat stockpiles since they are large carbon sources and understudied overall. This data will be utilized for annual estimates of peat stockpile-related carbon losses, which will add to our limited knowledge of national-scale estimates of carbon losses from organic soils disturbed by oil sands operations and other infrastructure.