Solute dynamics and the Ontario nitrogen index: I. Chloride leaching
Reynolds, W.D., Drury, C.F., Parkin, G.W., Lauzon, J.D., Saso, J.K., Zhang, T.Q., Liu, K., Welacky, T.W., Yang, X.M., Tan, C.S., Calder, W., Oloya, T.O., et Reid, D.K. (2016 in press). « Solute dynamics and the Ontario Nitrogen Index: I Chloride leaching. », Canadian Journal of Soil Science.
Résumé en langage clair
The Ontario N Index is a rating system that indicates relative degree of risk (high, medium, low) for leaching of fertilizer or manure nitrogen below the crop root zone by downward percolating rain water or irrigation water. Although widely used in Ontario environmental farm and nutrient management plans to assess risk for nitrogen pollution of surface and ground waters, the N index has so far received only limited testing for its ability to accurately match risk level with actual nitrogen leaching rates. This study tested the Ontario N Index on five important agricultural soils in Southern Ontario, including a sandy loam and two clay loams in Essex county, and two loams in Wellington county. Nitrogen leaching was estimated by applying calcium nitrate and potassium chloride salts to the soil surfaces in the fall, and then using periodic soil sampling to trace the movement of chloride and nitrate downward through the soil profile over a 12-15 month period. The leaching measurements indicated that the N index of one loam and one clay loam should be increased from low risk to medium risk, while the N index of one loam should be reduced from medium risk to low risk. The measurements also found that chloride and nitrate leaching rates were determined primarily by the soil’s saturated permeability (Ksat), which is not measured in the current Ontario N Index. Although further research is recommended, it appears that the accuracy of the Ontario N Index system could be improved by adding measurements of soil saturated permeability (Ksat).
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 (ClTR) 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 ClTR was applied to the soil surfaces in fall 2007 as KCl, and movement and loss of ClTR 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 ClTR 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 ClTR 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 (Ksat) measured in the top 50–60 cm. ClTR leaching rate appeared to be controlled primarily by Ksat in a manner consistent with infiltration and solute transport theory. It was consequently proposed that solute leaching loss versus Ksat relationships may improve N index risk estimates for both southern Ontario and other humid temperate regions.