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

Abstract

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.Key words: nitrogen index, chloride tracer, leaching risk, hydrologic soil group, K sat .Résumé : L'indice de l'azote (N) pour le climat tempéré humide du sud de l'Ontario (Canada), aussi appelé indice azoté de l'Ontario, tient compte de la nature de la culture actuelle et de la culture antérieure, des pratiques de gestion des engrais et du fumier ainsi que du groupe hydrologique du sol (HSG) établi d'après le type de sol et de drainage pour estimer les risques de contamination de l'eau drainée par les canalisations en terre cuite et l'eau souterraine avec les nitrates qui fuient la zone primaire des racines de la culture (couche supérieure de 60 cm du sol). L'indice azoté de l'Ontario n'a été vérifié que de façon restreinte sur le terrain aussi les auteurs en ont-ils évalué le volet lixiviation sur cinq sols (un loam sablonneux, deux loams et deux loams argileux) en utilisant comme traceur du chlorure (Cl TR ), de manière à illustrer quatre niveaux de risque fondés sur le HSG (HSG-A, risque élevé; HSG-B, risque moyen; HSG-C, faible risque; HSG-D, très faible risque). À l'automne 2007, ils ont appliqué une pulsation carrée de Cl TR à la surface du sol sous forme de KCl, puis suivi le déplacement et les pertes du traceur pendant 1-1,2 an en prélevant mensuellement des carottes dans la couche supérieure de 60 à 80 cm. Dans tous les cas, 60 à 96 % du Cl TR avaient ...

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

“…For example, a survey by Goss et al (1998) determined that 14% of 1292 Ontario farm wells contained NO 3 − concentrations in excess of the 10 mg N L −1 drinking water guideline (Fan and Steinberg 1996), which implies as a consequence that about 70 000 of Ontario's estimated 500 000 farm wells may contain excessive NO 3 − concentrations. Consistent with this result are simulation model assessments that identify Southwestern and Eastern Ontario as being "high risk" to "very high risk" regions for NO 3 − -N contamination of ground water due to intensive agriculture and the humid climate with long-term (≥30 yr) average annual precipitation between 850 and 925 mm Drury et al 2007;Reynolds et al 2016). In southern Ontario, fall-applied manures coupled with over-winter soil water drainage often causes substantial winter and spring N losses to surface and ground waters, especially when there are no cover crops and evapotranspiration losses of surplus water.…”

“…where IN T = total soil inorganic N to 60-cm depth (kg N ha Further details on soil sampling and analysis are reported in Reynolds et al (2016).…”

“…Weather data from the Harrow Research and Development Center were used for the Malden and Harrow sites (Perth and Harrow soils), from the weather station at the Eugene F. Whelan Research Farm for the Woodslee site (Brookston soil) and from the weather station at the Elora Research Station for the Maryhill and the Guelph soils (Reynolds et al 2016). However, the timing of individual precipitation events differed substantially among sites (Reynolds et al 2016).…”

“…However, it is precisely the impacts of climate, seasons, and soil hydraulic properties that are required to assess N indexes and to develop effective strategies for minimizing surface and ground water contamination by agricultural NO 3 − -N. The overall objectives of this study were to (1) characterize downward movement of NO 3 − through the profiles of five soils over a 1-yr period (four seasons) in the cool, humid regions of Southern Ontario and (2) assess the Ontario N Index by determining relationships among soil hydraulic properties (K sat ), Cl tracer movement, and N transport. This study is linked to a companion paper that examined the relationships among Cl leaching behavior, HSG categories, and K sat for the same soils and time period (Reynolds et al 2016). Reynolds et al (2016).…”

“…This study is linked to a companion paper that examined the relationships among Cl leaching behavior, HSG categories, and K sat for the same soils and time period (Reynolds et al 2016). Reynolds et al (2016).…”