Hydrogeochemical soil dynamics relative to topography for forested land units undergoing reclamation in a post-mined landscape in the Athabasca Oil Sands Region, Alberta
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Natural forest soils of the Western Boreal Forest rarely witness near-surface soil flushing events during the growing season due to the forest’s excessive evapotranspiration demands and large unsaturated zone storage capacity. This leads to the accumulation of nutrients such as Soluble Reactive Phosphorus (SRP) and Total Inorganic Nitrogen (TIN) within the surface soils, increasing along a low-relief moisture gradient transitioning through upland forests, riparian zones and wetlands, influencing vegetation communities. In the post-mined landscape, decompressed overburden produce topographically elevated hillslopes with cover soils exhibiting poor transmissivity and hydrophobic properties, which are often subject to erosion. Reclamation projects are beginning to develop entire watersheds consisting of engineered wetlands, uplands and hillslopes, varying in elevation, to ensure a hydrologic connectivity that can support resiliency to moisture deficit during periodic stresses. To avoid undesirable interactions between land units, it is important to understand their hydrogeochemical connectivity. This study focuses on the interactions between a recently (i.e. three years) reclaimed low-relief upland and three encompassing hillslopes (aged five to nine-years since reclamation), located within a constructed fen watershed. The objectives were to determine if topographically driven moisture-nutrient gradients were being formed and how this would influence vegetation colonization. No topographically driven moisture-nutrient gradient was detected within the lower-lying constructed upland, attributed to the heterogeneity of the cover soil placement and the lack of preferential flow paths, typically witnessed in newly reclaimed soils. Furthermore, the application of control release fertilizer likely hindered the detection of any topographic influence on ion mobility. Runoffs collectors suggest that fertilizer may lead to off-site movement immediately following application. Results also demonstrated that SRP is likely in excess within this system and susceptible to leaching following overland flow events. However, TIN is potentially a limiting nutrient and while immobilized at the surface, demonstrated greater susceptibility towards vertical flow, especially when groundwater recharge promoting structures are incorporated within the construction of forested land units. Sapling survival within the constructed upland appeared to be influenced by moisture stress over nutrient availability, re-examining the need for fertilizer application when reclaimed soils still lack moisture absorbing properties. The elevated hillslopes also did not demonstrate any topographically driven moisture-nutrient gradient regardless of age since reclamation. The more mature hillslope was expected to demonstrate such a gradient, however the dry growing season likely hindered subsurface interflow downslope. The two younger hillslopes still demonstrated poor transmissivity attributed to their immaturity. TIN contributions towards the constructed upland proved to be minimal, however phosphorus inputs from erosion prone areas are likely to influence SRP availability following phosphate desorption processes within the constructed upland. Although our system demonstrated positive correlations of increased SRP on native species establishment, TIN availability demonstrated increased forb and non-native species colonization. This study demonstrates how current forested upland reclamation practices might influence other land units when re-initiating hydrogeochemical connectivity throughout engineered landscapes. This study also demonstrates how contributions from topographically elevated land units might impact vegetation communities downslope, which is crucial for re-establishing the resiliency of the landscape. Current forest upland and hillslope reclamation practices will likely need to be re-evaluated when considering landscape scale hydrogeochemical connectivity.
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Tristan Gingras-Hill (2017). Hydrogeochemical soil dynamics relative to topography for forested land units undergoing reclamation in a post-mined landscape in the Athabasca Oil Sands Region, Alberta. UWSpace. http://hdl.handle.net/10012/11274