Fall 1993 (v5n5)
	Modeling the fate of nitrogen in the root zone: Management and research applications. L Warden; B. W House, L.E. Jackson and K.J. Tanji Proceedings: 1992 California Plant and Soil Conference, Decision-making in an Uncertain Environment. California Chapter American Society of Agronomy. 1992 Intensive vegetable production systems, with high inputs of both water and nitrogen fertilizer, have been identified as a major source of nitrate pollution in California. This paper, presented at the 1992 California Plant and Soil Conference, describes a simulation model whereby best management practices (practices that minimize non-point source pollution, while remaining economically viable for the farmer) can be determined for a given set of climatic conditions and agronomic variables. Methods Lettuce was chosen as the case study crop. Specific objectives were to "demonstrate how physical-chemical-biological modeling approaches can assess environmental and economic consequences of nitrogen fertilizer and irrigation water management." The USDA Erosion/Productivity Impact Calculator (EPIC) was the model chosen for this study because it uses easily-obtained input data, and is able to account for irrigation and fertilization practices. EPIC first had to be calibrated to real field circumstances. This was accomplished using 1990 data for spring- and summer-planted lettuce grown on a 11-hectare field in the Saloons Valley, California. After calibration, the model was applied to hypothetical simulations of different rates of applied irrigation water and nitrogen fertilizer. The conditions for the case study simulation were as follows: maximum rate of N fertilization 168 kg nitrogen per hectare maximum rate of irrigation water 300 mm per crop residual nitrate from previous crop assumed to be 60 kg N03- nitrogen in the soil profile nitrogen fertilizer incorporated at a 50 mm depth on June 5 at 168 kg per hectare, and August 28 at 128 kg per hectare; 40 kg nitrogen per hectare was applied with irrigation water on October 5 each crop was furrow irrigated in equal amounts six times during the growing seasons at 10- to 14-day intervals Model simulations were run with decreasing irrigation and fertilizer rates (100% to 0% of maximum at 10% increments). Economic modeling involved calculating profit as the difference in revenue from lettuce yield and marginal costs of applied water and fertilizer. Results The authors present break point analyses for water and fertilizer inputs (figures 1 and 2). These data show that "leaching of nitrate was most effectively reduced up to 5O percent of the 'normal' quantity of applied irrigation water, and at 65 percent of applied fertilizer nitrogen." Beyond these break points, reductions in irrigation and fertilizer rates become less effective at reducing nitrate leaching. A combined analysis of the two variables showed that both fertilizer nitrogen and irrigation water could be reduced up to 50 percent of normal with no reduction in yield. The authors stress, however, that water and fertilizer management go hand-in-hand: If, for example, nitrogen fertilization is reduced without reducing irrigation water, crop uptake efficiency goes down because nitrate is flushed past the root system. The 50 percent reduction in fertilizer and irrigation water is also the point at which profit is maximized. Optimal rates were determined to be 150 mm of applied irrigation water and 84 kg nitrogen per ha per crop. Implementation of these management practices would reduce nitrate leaching by about 75 percent. The authors caution that this modeling work is still in the developmental stages, and that calibration of the model was conducted using data from a lettuce crop grown under near-optimal, disease-free conditions. Areas that require further study include: the effect of corky root disease on water and nitrogen uptake, crop quality considerations, and the low salinity tolerance of head lettuce. For more information write to: L. Jackson, USDA Ag Research Station, 1636 E. Alisal St., Saloons, CA 93905. Figure 1. Effect of irrigation reduction on nitrate leaching. Figure 2. Effect of fertilizer reduction on nitrate leaching. (DEC.315) Contributed by David Chaney [ Back | Search | Feedback ]