Research was conducted in soils of the Chesapeake Bay area to determine the effects of tillage practice and nitrogen (N) fertilizer application rates on N leaching from corn fields. Three well known computer models (NTRM, CERES-Maize, and VT-MAIZE) were tested to determine their ability to predict the distribution of N in (a) soil and crop, (b) the components of the N cycle, and (c) corn yields.

To accomplish the above objectives, two field sites were selected on agronomically important soils for either a corn (Zea mays L.)-wheat (Triticum aestivum L.)-soybean (Glycine max [L.] Merr.) or a continuouscorn rotation. The corn-wheat-soybean rotation was located on a Suffolk sandy loam soil (coarse loamy, siliceous, thermic Typic Hapludult) in the immediate Chesapeake Bay drainage basin. The continuous-corn rotation was located on a Groseclose silt loam soil (clayey, mixed, mesic Typic Hapludult) typical of finer-textured soils located in the upper reaches of tributaries that drain into the Chesapeake Bay. Management practices evaluated included tillage system and rate, source, and time of N application. Specifically, we looked at conventional vs. no-till; inorganic N vs. sewage sludge; preplant vs. split application of N; and a variety of N application rates. The N treatments for corn were 0, 75, 150, and 225 kg N ha-1 applied preplant; 150 kg N ha-1 applied 4 weeks after emergence; and 150 kg of mineralizable N ha-1 from anaerobically digested and either lime- or polymer-conditioned sewage sludge. The N treatments for wheat were 20 kg N ha-1 applied in the fall and 30, 60, or 90 kg N ha-1 applied in the spring; 60 kg N ha-1 split application; and 80 kg of mineralizable N ha-1 applied in the fall from either lime- or polymer-conditioned sewage sludge.

In the Groseclose soil, there was an increase in total yield and N uptake when sewage sludge was applied compared to the split and preplant application of inorganic N. There was no difference between polymer-or lime-conditioned sewage sludge application. Also, there were no differences between preplant and split application of N. Where no-till was used, there was an increase in both yield and N uptake compared with conventional till. In the Suffolk soil, tillage management did not influence yield or N uptake where time and source of N application were studied. This lack of response on the Suffolk soil is attributed to severe moisture deficits that were present during the growing season on this coarse-textured soil.

Nitrogen losses from the soil profile were directly related to the quantity of N remaining in the upper 1 m of the soil profile after the crop was harvested. Larger quantities of N were lost from the Groseclose soil where conventional till was employed during the first year of the study. This was attributed to enhanced mineralization where no-till was converted to conventional till and to lower yields and lower N recovery with conventional till. Losses of N tended to be higher from the conventional till plots because of the larger quantities remaining at the end of the growing season. It should be noted that the years during which these xvii studies were conducted were extremely dry, and the in the Groseclose soil where no-till management was employed resulted in increased yield and increased N recovery. This study also emphasizes the need for better methods for making N recommendations for crop production.

The model performances varied from year to year and from one tillage practice to another. Because they were written for average soil and climatic conditions, they did not make satisfactory predictions in many instances. Such models require adjustment to reflect the moisture stress conditions that often prevail in this region for corn production.