A combined approach for understanding nitrogen loading to groundwater from a field under potato production in Prince Edward Island
Environment and Climate Change Canada & Agriculture and Agri-Food Canada
Agricultural practices associated with potato production, the most important agricultural commodity in Prince Edward Island (PEI), have potential impacts on groundwater quality - the sole source of drinking water for the province. Groundwater with higher nitrogen loading can also be associated with the increased frequency of anoxic events in downgradient coastal ecosystems. In many cases, soil nitrogen cycling processes, which contribute significantly to the amount and form of nitrogen available for transport to groundwater, are neglected when nitrogen loading from agricultural fields is estimated. In this study, a preliminary model developed in Root Zone Water Quality Model (RZWQM, USDA-ARS) has been initiated for a three year potato rotation (potato-barley-red clover) implemented on a small field located at AAFC’s Harrington Research Farm, located 10 km north of Charlottetown. The nitrogen leached from the soil profile is further compared to nitrogen loading to groundwater derived based on water balance calculations, groundwater level dynamics and direct measurements of nitrogen concentrations in the aquifer. The soil at the study site is about one meter thick and is underlain by a thick (8 m) overburden derived from parental red sandstone bedrock, the dominant consolidated deposit across the province. Hydraulic conductivity in both the unsaturated and saturated zones is relatively high and previous investigations show that both the bedrock and the overburden are fractured, with fracture orientation being predominantly horizontal. The water table is about 16 m below ground and shows annual level amplitudes of 1-2 meters. In soil, the main inputs of nitrogen come from fertilizer application (150 kg N ha-1 y-1 for potatoes, 51 kg N ha-1 y-1 for barley with no fertilizer applied for clover) and mineralization of soil organic matter, which ranges between 42 and 52 kg N ha-1 y-1, depending on the rotation phase. In terms of nitrogen removal from soil, plant uptake is the most important process (126 kg N ha-1 y-1 under potatoes, 83 kg N ha-1 y-1 under barley and 23 kg N ha-1 y-1 under clover) and is followed by leaching (63.5 kg N ha-1 y-1 under potatoes, 50 kg N ha-1 y-1 under barley and 29.5 kg N ha-1 y-1 under clover), with all other losses being insignificant. The monthly dynamics of the nitrogen leaching is well reflected by the nitrogen loading calculated using a series of monitoring wells located downgradient of the study site. The loadings estimated using nitrate concentrations measured in these wells are 2-3 times lower than the amount of nitrogen leached through the bottom of the soil profile, which could be attributed to the mixing with the lower nitrogen concentrations water from the unconfined regional aquifer. The quick response of groundwater loads confirms that a portion of the nitrogen leaving the bottom of the root zone travel at a fast rate through the overburden.