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Contamination of groundwater under agricultural land by NO3 is influenced by the kind of farming system. One possible method of selecting farming systems that result in less NO3 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 N2 fixation by legume crops with different yields was estimated from regression equations. A net surplus in the N balance was converted to maximum mean NO3‐N concentration in groundwater by assuming a groundwater recharge rate of 160 mm yr−1, and no denitrification. Predicted NO3‐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 NO3‐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 NO3 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.

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