NM8005 - Refining the N Index to determine the risk of nitrate contamination of surface and groundwater for Ontario agricultural soils

The ministry funded this project through the Nutrient Management Joint Research Program (2006-2007). The program supported the development of environmentally effective and scientifically robust management practice options for land application of agricultural and non-agricultural source materials in Ontario.

Lead researchers

• Dr. Craig Drury, Agriculture and Agri-Food Canada, Harrow
• Dr. Gary Parkin, School of Environmental Sciences, University of Guelph

Objectives

This project will evaluate the current hydrologic soil group (HSG)-based Ontario N Index system for its ability to predict nitrate leaching losses from soils. Field and laboratory studies will be conducted to examine the fate of applied N in soils.

1. N losses through leaching and denitrification will be evaluated in 6 dominant soil types in Ontario that are representative of agricultural production areas. Two paired sites will compare a tiled to a non-tiled field.
2. Chloride tracers will be used as a tool to compare leaching potential with actual nitrate leaching.
3. Column studies with/without applied swine manure will be conducted to partition the fate of applied N as far as nitrate leaching and denitrification losses are concerned.
   

Expected benefits

The evaluation of the effects of HSG, soil characteristics (e.g. shrinking/swelling soils, soil organic matter as it affects N transformations), tile drainage, and N dynamics on N leaching behavior will be used to suggest enhancements to the N index for Ontario. This information will benefit farmers, consultants and provincial regulatory agencies as it will provide an improved risk classification for the agricultural regions of Ontario.

Results

The Nitrogen Index in Ontario has been proposed to determine the risk for contamination of surface and ground waters by nitrate leached out of the crop rooting zone. The proposed N Index is based on hydrologic soil group (HSG), with the most permeable soils, HSG-A (e.g. sands, sandy loams and loamy sands) generally having the greatest leaching risk, and the least permeable soils, HSG-D (e.g. clays and clay loam soils) generally having the lowest leaching risk.

However, the current N Index has not been evaluated for the potential impacts of tile drainage and nitrogen (N) transformations on leaching risk. The importance of tile drainage and N transformations were examined in this study by comparing nitrate loss from the soil profile over time to the corresponding losses of a non-reactive "tracer solute" (chloride). Nitrate is removed from the soil via plant uptake, leaching, denitrification and immobilization, whereas leaching is the predominant mechanism for chloride removal.

The study characterized: 1) nitrate and chloride removal (by leaching, uptake, transformations, etc.) from the soil profiles of 4 different hydrologic soil groups (1 HSG-A soil, 1 HSG-B soil, 2 HSG-C soils, and 1 HSG-D soil); 2) the influence of field tile drainage on nitrate and chloride removal rates (1 HSG-C soil and 1 HSG-D soil); and 3) the indirect effect of drainage tiles on soil hydraulic properties, and hence the risk of nitrate leaching out of the crop root zone.

Calcium nitrate fertilizer and potassium chloride were added at 3 rates (0, 100 and 200 kg N/ha) to all field sites in October, 2007 and the movement of inorganic N (ammonium, nitrate) and chloride was tracked over the following year. After 2-3 weeks, the majority of the applied nitrogen and chloride was still in the 0-10 cm depth of all soils, although there was some evidence of inorganic N leaching to the 10-20 cm depth, except for the not-tiled Brookston clay loam.

The 200 kg N ha^-1 application rate had significantly greater soil inorganic N contents than the 0 and 100 kg N ha^-1 application rates for both Perth and Brookston with/without field tiles. After 8 weeks, there was considerable movement of inorganic N down the profiles of the Fox, Guelph and Maryhill soils with maximum concentrations in the 20-30 cm depth. Inorganic N movement to the 40-60 cm depth was also evident in the Fox, Guelph, Maryhill, tiled Brookston and not-tiled Perth clay loam at 8 weeks.

Except for the not-tiled Brookston clay loam, very little of the applied N was present in the soils after 32 weeks (May 2008), and virtually no inorganic N remained in the soil one year after application. After 54 weeks, the nitrate concentrations at all depths with both the tiled and non-tiled fields were less than 10 mg N kg^-1.

Inorganic N and chloride can move rapidly in some soil profiles (within weeks), and soil information such as site-specific measurements of saturated hydraulic conductivity (Ksat) are required to accurately identify the potential leaching risk. The Fox and Maryhill sites had the greatest leaching potential of all soils examined, and this was consistent with the fact that the measured Ksat values for the Maryhill loam placed it in the HSG-A to HSG-B classes, rather than the HSG-C class and the Ksat values for the Fox soil was consistent with the HSG A class. Tile drainage of Brookston clay loam increased inorganic N and chloride leaching substantially, and it increased the Ksat-derived hydrologic soil group from HSG-D to HSG-C.

Parameters studied

N, subsurface flow

Recommendations relevant to nutrient management

It appears that site-specific application of the Ontario N Index should consider the measured Ksat of the crop rooting zone and the presence/absence of tile drainage.

Related information

Other projects funded through the Nutrient Management Joint Research Program:

• in 2006
• in 2007