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Estimation of nitrate concentrations in groundwater using a whole farm nitrogen budget

, : Estimation of nitrate concentrations in groundwater using a whole farm nitrogen budget. Journal of Environmental Quality 22(4): 767-775

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-), probably because more N 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.

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