Fate and transport of subsurface nitrate through an agricultural field and forested riparian zone

Nitrogen (N) is an essential nutrient for plant growth and is applied to agricultural cropland in the form of both organic and inorganic fertilizers. However, excess N, primarily in the form of nitrate-N (NO3-N), within soil systems can be transported to surface water via various subsurface pathways such as groundwater recharge and tile drainage systems. Once in surface water, NO3-N can promote eutrophication, and impact the oxygen carrying capacity of aquatic organisms. Establishing effective Beneficial Management Practices (BMPs) for reducing NO3-N loading to surface water systems requires an understanding of the primary modes of transport. The objectives of this project are to determine: (i) the relative contribution of the two primary subsurface pathways (groundwater recharge and tile systems) to NO3-N loading, (ii) temporal patterns in NO3-N export from agricultural landscapes typical of the Annapolis Valley, and (iii) the role forested riparian buffer zones play in attenuating groundwater NO 3-N loads.;Temporal patterns in NO3-N concentrations and loads were observed in the groundwater, tile water and surface water, with NO3-N concentrations and mass loads being higher during the dormant season (November to May) than during the growing season (June to October). Results indicate that NO 3-N is being attenuated in the riparian buffer zone. The riparian buffer zone removed, on average, 73% of the NO3-N exported from the field. Results from isotopic analysis provided evidence that denitrification is occurring in the riparian buffer zone. However, an analysis of the chloride concentrations along the groundwater flow path indicated that dilution could be contributing to approximately 45% of the NO3-N concentration reductions. Results of this study have shown that annual NO3-N loading rates through the groundwater system and the tile drainage system are of similar order of magnitude. Establishing and maintaining forested riparian buffer zones, and implementing BMPs to mitigate losses of NO3-N through tile drainage systems, would both help reduce NO3-N loading to the Thomas Brook.;The research was conducted in the Thomas Brook watershed located in Kings County, Nova Scotia. The watershed is a 760 ha catchment consisting of agricultural, forested and residential land. A network of groundwater monitoring wells was installed in the forested riparian buffer zone and cropped area of a 9.6 ha corn field. The field is systematically tile drained and receives both organic (semi-solid dairy manure) and inorganic fertilizers. Monitoring of NO 3-N export through both the tile system and groundwater was initiated in May, 2007. Results show that NO3-N concentrations in the field groundwater wells range from approximately 4--9 mg L-1 while the concentrations in the riparian zone wells range from 0--3 mg L-1, indicating the forested riparian zone is attenuating groundwater NO3-N. The NO3-N concentrations from tile drainage discharge range from 1 mg L-1 to 34 mg L -1. The annual loading rates from the groundwater system and the tile drainage system were of similar order of magnitude. The total annual mass NO3-N loading through the groundwater system was 418 kg and the annual mass NO3-N loading from the tile drainage system was 292 kg.