Solute dynamics and the Ontario nitrogen index: II. Nitrate leaching

Nitrogen (N) leaching from soil into surface and ground waters is a concern in humid areas of Canada. As a result, N management protocols, including the Ontario N Index, are widely used to identify N leaching risk, although field assessment remains limited. Nitrogen fertilizer and chloride (Cl) tracer were fall-applied to five agricultural soils in Ontario with different textures and hydrologic soil groups (HSG) to assess the Ontario N Index and characterize inorganic N movement over 1 yr. The treatments included three N rates (0, 100, and 200 kg N ha(-1)) plus Cl tracer and 200 kg N ha(-1) rate without Cl. After spring thaw, N loss from the crop root zone (top 60 cm) ranged from 68% for Brookston clay loam to 99% for Harrow sandy loam. A strong linear relationship between apparent N recovery and apparent Cl recovery indicated that N loss from the root zone occurred primarily by downward leaching. Leaching was controlled by the minimum measured saturated hydraulic conductivity (K-sat), and good estimates of N leaching were obtained using a quasi-theoretical relationship between N loss and K-sat. We concluded that Ontario N Index estimates of N leaching risk might be improved by including site-specific measurements of K-sat.