Estimation of Nitrate Concentrations in Groundwater Using a Whole Farm Nitrogen Budget

Contamination of groundwater under agricultural land by NO 3 is influenced by the kind of farming system. One possible method of selecting farming systems that result in less NO 3 leaching is to calculate whole farm N budgets, that are simplified by assuming soil‐N remains constant from one cycle of a rotation to the next. This method was applied to two model crop rotations using average crop yield data for two regions of Ontario, and to a cash‐crop farm and a dairy farm using information on purchases, sales, and crop yields, for these farms. The model rotations were corn ( Zea mays L.)‐soybean [ Glycine max (L.) Merr.]‐wheat ( Triticum aestivum L.) and corn‐soybean‐wheat‐hay (mixture of timothy, Phleum pratense L. and alfalfa, Medicago sativa L.)‐hay‐hay. Atmospheric deposition (18.4 kg N ha −1 yr) was obtained by literature review. Symbiotic N 2 fixation by legume crops with different yields was estimated from regression equations. A net surplus in the N balance was converted to maximum mean NO 3 ‐N concentration in groundwater by assuming a groundwater recharge rate of 160 mm yr −1 , and no denitrification. Predicted NO 3 ‐N concentrations in leachate for the model corn‐soybean‐wheat rotation were greater for southwestern Ontario (22.4 mg L −1 ) than western Ontario (8.5 mg L −1 ), probably because more fertilizer was recommended in the southwest. Including hay in the model rotation increased the amount of N leached by a factor of two in western Ontario, but only by 9% in the southwest. Predicted NO 3 ‐N concentration in groundwater for the cash crop farm was 6.7 mg L −1 , compared with an average measured value of 9.5 mg L −1 in the tile drainage water. For the dairy farm the predicted value was 58 mg L −1 and a measured value was not available. The simplified N balance method provided useful estimates of potential NO 3 leaching losses even though it relied on some major assumptions. A major uncertainty was atmospheric deposition of ammonia volatilized from on‐farm sources. Denitrification could be as much as 62 kg N ha −1 yr −1 under continuous production of grain corn, based on differences between N present after harvest and amount of N leached.