The Development of Environmental Indicators for Nitrogen: Residual Soil Nitrogen and the Indicator of Risk of Water Contamination by Nitrate.

Nitrate leached from agricultural soils can be both an environmental concern when it enters rivers and lakes, and a human health concern when it enters aquifers used for potable water. Residual soil nitrogen (RSN) is the amount of inorganic nitrogen that remains in the soil at the end of the growing season after crops have been harvested. RSN can be estimated as the difference between all N inputs (fertilizer, manure-N, biological fixation, and atmospheric deposition) and all N outputs (N removed in crop harvest, N lost from ammonia volatilization and N lost from denitrification). RSN was calculated on a regional scale as well as on a provincial and national level from 1981 to 2006. The Canadian average RSN values increased steadily from 9.3 kg N/ha in 1981 to 15.5 kg N/ha (67 % increase) in 1996 and then to 25.0 kg N/ha (61% increase) by 2001. The RSN values decreased to 17.7 kg N/ha in 2006. The spike in RSN values in 2001 was due to a combination of factors including low crop yields as a result of drought conditions in many parts of Canada in the summer of 2001 as well as an increase in legume crop acreages which contributed to large N inputs (fixation of atmospheric N) without a corresponding decrease in either fertilizer or manure inputs. In Ontario, the RSN estimate was greater than all other provinces in 1981 at 42.2 kg N/ha which was over 4 times the Canadian average (9.3 kg N/ha). These levels were high as a result of the increased amount of N added to Ontario soils through fertilizer, manure and nitrogen fixation in the highly productive regions in Eastern Canada. RSN levels remained fairly steady over the 6 census years except for 2001 when the RSN level reached 59.9 kg N/ha due to low N uptake under drought conditions and an increase in legume acreage. The amount and concentration of N lost through leaching (IROWC-N) after harvest is dependent upon the RSN levels and the water budget (antecedent soil moisture content, precipitation, evaporation and surface runoff). In Ontario, the amount of N lost remained fairly constant with a range of 14.4 kg N/ha in 1981 to 17.6 kg N/ha in 1991, while N concentration in the drainage water ranged from 8.4 mg N/L in 1981 to 9.6 mg N/L in 2001.