Influence of hydrological connectivity and ditch substrate on nutrient transformations and export during peat harvesting phases in a sub-humid, glaciated, boreal landscape

Horticultural peat harvesting is expanding in Canada with the potential to negatively impact downstream water quality. Previous studies have reported variable responses in outflow nitrogen (N) and phosphorus (P) concentrations associated with peat harvesting operations, which may be due to unaccounted differences in biogeoclimatic setting or harvesting phase. Within a given peatland, major changes occur to its hydrological and physicochemical properties as it transitions through sequential harvesting phases: from a natural peatland, to an extraction field, and finally to a restored peatland. The linkage between hydrology and nutrient export, and the impact on water quality associated with each phase, have not been studied in relation to geology, relief, and climate across the Canadian boreal. This knowledge is crucial to account for the variability in exported nutrient concentrations, accurately determine the relative risk to downstream waterways, and direct best management practices. The objective of this study was to understand the linkages between peatland hydrological connectivity, ditch substrate, and peat harvesting phase on nutrient mobility at two peatlands in the sub-humid, glaciated, boreal region in Alberta, Canada. Water level, volumetric flow rate, ice and aeration depth, electrical conductivity (EC) and pH were measured at natural, newly opened, extracted, and restored peatlands. Chemical analyses of dissolved and particulate N and P were assessed in surface water, groundwater, and at outflow locations approximately once per month from March through October in 2019 and 2021. In situ ion availability was measured in surface peat layers, alongside surface and below ground temperature, soil moisture, and peat aeration in 2021. Compared to natural peatland areas, the results show that harvesting activities greatly decreased natural water storage capacity, encouraged ice formation, and increased spring runoff in a summer runoff dominated landscape. Drainage ditches further increased hydrological connectivity and outflow from extracted peatlands throughout the year. When ditches reached underlying mineral sediments, EC and pH values differed drastically from natural peatland drainage waters. The effect of extraction and ditching on in situ moisture and aeration in extracted surface layers was minimal compared to the natural peatland, despite lowered water tables. In contrast, N and P values varied drastically between the three harvest phases. During extraction, surface peat had elevated ammonium and nitrate availability compared to natural and restored peatlands. Nitrate remained available in the surface peat throughout the year; however, exports were only detected at the outflow when the peat fields were frozen or following large rainfall events. Nitrate was not detected if the hydrological connectivity between the peat field and its outflow was severed. Extracted and restored peatlands had elevated particulate P at field outflow locations compared to natural sites, likely from ditch clearing or loose peat in the process of settling. Clearly, ditch substrate has a major influence on water quality. Further, hydrological connectivity differs with each harvest phase, resulting in contrasting nutrient transformations and export that will continue to evolve as additional peat fields are opened, extracted, and restored.