An Extended Version of a Conceptual Model for Evaluating Annual Nitrogen Leaching Losses from Croplands

Previously, a steady-state conceptual N model was applied to corn (Zea mays L.) field trials at two University of California Field Stations. That model examined only the inputs and outputs of N into a soil-plant-water system without considerations of carryover inorganic soil N and mineralization of organic soil N. It predicted N leaching losses reasonably well in the corn plots at the Kearney site where residual soil N was low, but not for the Davis site where considerable mineralization of soil organic N occurs. This steady state model was extended so that it would be applicable to the transient conditions prevailing at the Davis site. The revisions included provisions for the presence of residual inorganic soil N and the mineralization of organic soil N as well as inputs of organic N such as manures and sewage sludges and their mineralization rates. The original water submodel was not modified. The model is described in the appendix and required input parameters with examples are listed. The revised model was tested with 2 years of corn field trials at Davis which consisted of four (NH₄)₂SO₄ rates (0, 90, 180, and 360 kg N/ha) and three irrigation regimes (1/3, 3/3, and 5/3 of the corn evapotranspiration). Sensitivity analysis evaluated the importance of the harvested crop coefficient, the denitrification coefficient, and the mineralization rate constant for organic soil N, and aided in estimating the latter two experimentally unmeasured parameters. Computed results compared favorably with measured N content in the harvested (grain and stover) crop, the residual inorganic soil N after harvest, and the average annual N concentration in the soil solution beneath the root zone at the 3-m depth. There was no leaching of N in the 1/3 ET treatment; from 0.5 to 12 kg N/ha per year N leaching losses in the 3/3 ET treatment; and from 13 to 144 kg N/ha N leaching losses in the 5/3 ET treatments. Some differences were noted in the annual leaching losses between the 1975 and 1976 corn crops in the 5/3 and 3/3 ET treatments mainly due to differences in seepage out of the root zone. It is concluded that the applicability and utility of this expanded model have been significantly enhanced over that of the previous steady-state model.