ESTIMATION OF NITRATE CONCENTRATIONS IN GROUNDWATER USING A WHOLE FARM NITROGEN BUDGET

Contamination of groundwater under agricultural land by NO3 is influen ced by the kind of farming system. One possible method of selecting fa rming systems that result in less NO3 leaching is to calculate whole f arm 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 informatio n on purchases, sales, and crop yields, for these farms. The model rot ations 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. Atmosphe ric deposition (18.4 kg N ha-1 yr) was obtained by literature review. Symbiotic N. fixation by legume crops with different yields was estima ted from regression equations. A net surplus in the N balance was conv erted to maximum mean NO3-N concentration in groundwater by assuming a groundwater recharge rate of 160 mm yr-1, and no denitrification. Pre dicted NO3-N concentrations in leachate for the model corn-soybean-whe at rotation were greater for southwestern Ontario (22.4 mg L-1) than w estern Ontario (8.5 mg L-1), probably because more N fertilizer was re commended in the southwest. Including hay in the model rotation increa sed 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 groundw ater for the cash crop farm was 6.7 mg L-1, compared with an average m easured 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 av ailable. The simplified N balance method provided useful estimates of potential NO3 leaching losses even though it relied on some major assu mptions. A major uncertainty was atmospheric deposition of ammonia vol atilized 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 dif ferences between N present after harvest and amount of N leached.