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The first three years of net ecosystem exchange of carbon dioxide (NEE) for a wetland constructed on bitumen mine tailings show its progression towards becoming an annual carbon dioxide sink. • A reclaimed wetland was a source of CO 2 for the first 2 years after construction. • In year 3 the wettest region absorbed 77 g C m−2, similar to natural peatlands. • Drier regions remained net CO 2 sources due to greater rates of soil respiration. Northern peatlands contain up to 20% of the ∼3000 Pg of global soil organic carbon. Carbon-rich peatlands cover upwards of 65% of the landscape in northern Canada where resource extraction activities disturb both the carbon pools and the future carbon sequestration capacity of the landscape. Previous estimates of the carbon losses from this disturbance predict a complete loss of the region's peatland carbon pool. Mining industries operating in these sensitive environments have recently begun constructing closure landscapes which are intended to develop carbon cycle processes similar to undisturbed northern peatlands. This study investigates eddy covariance fluxes of carbon dioxide (CO 2) at one of Canada's first fully constructed boreal plains watersheds, the Sandhill Fen Watershed. During the first three years since inception, only the lowland region had an annual net ecosystem exchange of CO 2 (NEE) indicative of increasing CO 2 sink potential. The lowland region was characterized by saturated salvaged peat soils, standing water, thriving communities of Typha and Carex spp. and was a net CO 2 sink of 77 g C m−2 in the third year. At the same time the upland and the midland regions characterized by moist salvaged peat soils and a mix of herbaceous, shrub and planted Picea glauca and Pinus banksiana remained net sources of CO 2. Despite similar rates of gross primary production, ecosystem and plot-level respiration rates in the lowland were significantly lower than in the midland region, likely due to very low reduction potentials within the lowland's saturated soils. With no other significant outflows of carbon, the lowland of the Sandhill Fen Watershed may be in the early stages of organic matter accumulation. Due to limited oxidation of the salvaged peat substrate in the lowland region, wetland reclamation employing these techniques may reduce the disturbance loss of the carbon pool in the boreal plains. [ABSTRACT FROM AUTHOR]

Oil sand mining in the boreal planes of Canada has been predicted to release stockpiled carbon from disturbed peatlands and permanently degrade the carbon sequestering capacity of the landscape (Rooney et al. 2012). The Sandhill Fen Watershed (SFW) is a pilot watershed built on soft tailings in an infilled pit mine to reclaim lost peatland services. It was constructed between 2009 and 2012 and is composed of 60 ha of mixed upland and lowland ecosystems. This analysis presents the first six years of carbon cycling using a mix of DOC pore-water samples, eddy covariance techniques and chamber measurements. At first, the lowland region rapidly shifted to an annual sink of CO2, having an annual Net Ecosystem Exchange of 77 g C-CO2 m-2 by the third year, while maintaining comparable rates of GPP to the reclaimed upland regions. Unlike the uplands however, the lowlands had much lower rates of heterotrophic respiration. This suggests that the design of the ecosystem maintained the salvaged peat in sufficiently anoxic conditions to limit decomposition. However, shifts in the vegetation towards marshland associated species altered the seasonality of photosynthesis has resulted in delayed spring uptake and the lowlands returning to an annual net source of carbon to the atmosphere. Methane emissions have continued to be suppressed over all six years despite organic material in anoxic conditions. The suppressed methane is likely due to competition from sulphate- and metal-reducing bacteria. Over the same period, DOC is increasing in the SFW pore water, but with limited outflow pathways to the surrounding landscape, it is likely not a major carbon flux out of the ecosystem. [ABSTRACT FROM AUTHOR]

Plant communities and water chemistry properties were characterized at Sandhill Wetland; an experimental reclamation site constructed above a backfilled open pit mine in northeastern Alberta, Canada. Observations from the reclamation site were compared to 12 mature reference wetlands (10 fens and 2 marshes) to determine the type of natural wetland the site most closely resembles at its current successional stage. From this survey of 20 research plots distributed across Sandhill Wetland, 44 plant species were encountered (35 vascular plants and 9 bryophytes). Using multivariate approaches, diversity indices, and coefficient of conservatism values, results indicate areas with water levels above the soil surface at Sandhill Wetland are most comparable to regional marshes, with Typha latifolia and Carex aquatilis exhibiting the highest cover. Across the wetland periphery where water levels are several centimeters below the soil surface, plant communities were quite dissimilar from the reference sites and dominated by the grass Calamagrostis canadensis. Based on principle components analysis, we show water chemistry at Sandhill Wetland is in a state that is between marshes and saline fens. Sandhill Wetland overall possesses a unique set of structural and chemical features in the sixth year of development. In areas where water levels are above the soil surface there is potential for development into a slightly brackish marsh analogue. The successional trajectory of low water table position areas is somewhat uncertain as the wetland periphery exhibited mixed characteristics resembling those of wet meadow, riparian, and upland environments, which we did not survey. • Sandhill Wetland plant communities resemble regional marshes in some areas • Plants have sorted out into spatially distinct zones due to water level preference • Sodium concentrations have increased annually at Sandhill Wetland since inception • Sodium-tolerant plants are present at Sandhill Wetland, albeit at low abundance • Further monitoring should continue to track potential community shifts [ABSTRACT FROM AUTHOR]