Nitrogen loss in snowmelt runoff from non-point agricultural sources on the Canadian prairies

Nitrogen (N) is an essential element for crop growth, and many agricultural systems require N fertilizer inputs for optimal crop production. However, only a portion of the N applied as chemical fertilizer or manure is used by plants or retained in soil. Nitrogen can be lost from agricultural landscapes through volatilization (as ammonia, NH3), denitrification (in the gaseous forms N2O, NO, N2), in leaching of nitrate (NO3-N), and in rainfall or snowmelt runoff. Nitrogen is transported in runoff in a variety of forms, including soluble inorganic [NO3-N, ammonium-N (NH4-N)], and dissolved organic forms (DON), or particulate-bound organic and inorganic N (Delgado 2002). Nitrogen export to surface water in runoff from agricultural sources is a major global concern; in a worldwide study of 946 watersheds, N losses in runoff were consistently higher per unit area from agricultural than from non-agricultural land. Non-point agricultural sources of nutrients are typically more challenging to manage than point sources because of broader distribution throughout impacted watersheds and because these sources are often highly variable in time due to the role of weather in determining the frequency and intensity of runoff. To improve fertilizer use efficiency and minimize risks to downstream water quality, the pathways of N loss from agriculture must be identified, along with management practices that increase or decrease N losses.