Solute dynamics and the Ontario nitrogen index: I. Chloride leaching

The nitrogen (N) index for humid temperate southern Ontario, Canada (Ontario N index) incorporates previous and current crop type, fertilizer and (or) manure management, and hydrologic soil group (HSG) to estimate risk for contamination of tile drainage water and groundwater by nitrate leached below the primary crop root zone (top 60 cm of soil). The Ontario N index has received limited ground-truthing, and the leaching component was assessed using chloride tracer (Cl-TR) on five soils (one sandy loam, two loams, and two clay loams) representing four HSG-based risk levels (HSG-A, high risk; HSG-B, medium risk; HSG-C, low risk; HSG-D, very low risk). A square-wave pulse of Cl-TR was applied to the soil surfaces in fall 2007 as KCl, and movement and loss of Cl-TR was tracked over 1-1.2 years using monthly soil core samples collected from the top 60-80 cm. For all five soils, 60-96% of Cl-TR was leached out of the primary crop root zone (below 60 cm depth) during the noncropping period (October 2007 to March 2008 inclusive), and > 80% was leached out of the root zone within 1 year. The percentage of Cl-TR that leached did not correlate with precipitation or HSG designation, but produced significant (P < 0.05) power function regressions with minimum and harmonic mean saturated soil hydraulic conductivity (K-sat) measured in the top 50-60 cm. Cl-TR leaching rate appeared to be controlled primarily by K-sat in a manner consistent with infiltration and solute transport theory. It was consequently proposed that solute leaching loss versus K-sat relationships may improve N index risk estimates for both southern Ontario and other humid temperate regions.