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Water Contamination by Nitrogen

Drury, C.F., Yang, J.Y. and De Jong, R. 2016 Water Contamination by Nitrogen. Pages 121-130 in Clearwater, R. L., Martin, T. and Hoppe, T. (eds.) 2016. Environmental sustainability of Canadian agriculture: Agri-environmental indicator report series – Report #4. Ottawa, ON: Agriculture and Agri-Food Canada.


The “Indicator of the Risk of Water Contamination by Nitrogen” (IROWC-N) provides an estimate of the amount and concentration of nitrate which is lost from the soil through leaching. This indicator uses model outputs from the residual nitrogen indicator (an indicator which estimates the amount of inorganic nitrogen remaining in the soil after harvest) as well as climatic data to estimate the nitrate concentrations and losses from agricultural soils. IROWC-N is divided into 5 risk classes from very low to very high which consider both the nitrate concentrations and total losses due to leaching. Data from the last Census of Agriculture in 2011, indicate that the majority (74%) of farmland in Canada was in a very low risk class for loss of soil nitrate, although some pockets of land in higher risk classes were observed in Central and Atlantic Canada. From 1981 to 2011, the proportion of farmland in the very low risk class decreased gradually from 88% to 74%, while the proportion in the low risk class increased from 2% to 16%. On a national scale, 14% of farmland moved from the very low risk category to the low risk category between 1981 and 2011. In general, the annual nitrogen (N) leaching loss in 1981 was very low in the Prairie Provinces (0 to 0.5 kg Nitrogen/hectare), intermediate in the coastal provinces (British Columbia and Newfoundland, at 6.2 to 7.6 kg N/ha, respectively) and considerably higher in Central Canada and the rest of the Atlantic Provinces (11.5 to 22.2 kg N/ha). In British Columbia, Alberta, Saskatchewan and Ontario, N leaching losses from soils remained fairly constant across Census years from 1981 to 2011; however, substantial increases were observed in all other provinces. In 1981, the IROWC-N model estimated nitrate concentrations in drainage water at less than 5.0 mg nitrogen/litre for all provinces except British Columbia, Ontario and Quebec, which had concentrations ranging from 5.6 to 8.6 mg nitrogen/litre. The nitrate concentration in drainage water increased over the 30-year period from 1981 to 2011 in all provinces except Ontario. In 2011, all provinces except Saskatchewan had nitrate concentrations higher than 5 mg nitrogen/litre. Concentrations in drainage water exceeding 10 mg nitrogen/litre were observed in Manitoba (13.6 mg nitrogen/litre) and Quebec (10.5 mg nitrogen/litre). The IROWC-N risk indicator was used to determine the changes in nitrate leaching from soils over the past 30 years. IROWC-N was also used to identify which areas in Canada are at higher risk and policy makers may consider strategies which encourage the adoption of best management practices to improve nitrogen management in these higher risk areas.

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