15N Stable Isotopes to Pinpoint the Source and Quantity of Particulate Nitrogen Exported From Agricultural Fields.

The Missisquoi Bay of Lake Champlain, located in southern Quebec, Canada has become progressively eutrophic and experiences periodic cyanobacteria blooms during summer. Although phosphorous is considered to be the trigger for cyanobacteria blooms, elevated nitrogen (N) concentrations may be partially responsible for eutrophication in the Missisquoi Bay. Surrounding agricultural lands are the major source of N and other nutrients entering the Pike River, which drains into the Missisquoi Bay, due to the fact that they receive organic and mineral fertilizers. Subsurface tile drainage accounts for at least half of the water transmitted from agricultural fields to streams each year, and is likely an important pathway for nutrient export as well. Soil texture affects nutrient transport through the profile, thereby impacting the N concentration and forms in tile drains. The objective of this study was to evaluate the N source: organic vs. mineral N fertilizer, and quantify the particulate organic N (PON) in the subsurface tile drains of annually-cropped agricultural fields with contrasting soil texture (sandy vs. clayey soils) in the Pike River watershed, Quebec. Water samples were collected from tile drains in fall 2009, 2010 and spring 2011 and analysed for the natural abundance of δ15N and N forms. The δ15N values showed soil organic N as the major source of PON exported in tile drainage. Generally, the N concentration was greater in tile drains under sandy than clayey soils. NO3-N was the dominant form of N in tile drainage water, with a smaller contribution from PON. However, PON concentration was greater in clayey than sandy soils (15% vs. 10% and 22% vs. 7% of total N found in water samples collected following fall and spring rainfall events, respectively) due to more preferential flow pathways in clayey soils. In conclusion, soil organic N attached to particles is susceptible to loss through subsurface tile drainage, and the contribution of PON to total N concentration was greater after intense rainfall events (more than 5 mm of rain) that occurred in spring 2011.