Submitted to: Soil and Water Conservation Society Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: July 28, 2004
Publication Date: N/A
Surplus agricultural nitrogen (N) and erosion are indicators of the potential for nitrate and sediment to reach water resources. This paper quantifies the surplus N available for leaching, changes in soil organic N (SON), and erosion under existing land use patterns in watersheds in western Iowa and then evaluates these same indicators under two alternative land use scenarios. The alternative land use scenarios integrate animals with several cropping systems including a two-year maize-soybean rotation; a six-year rotation of maize, soybean, oat, and forage; and permanent pasture. In order to compare existing land use patterns with the alternatives, this paper presents a model to estimate surplus N and applies the Water Erosion Prediction Program (WEPP) to estimate erosion. Because the N model developed here utilizes commonly available georeferenced data on soils, crops, and livestock, these methods are widely applicable. Model results indicate watershed-averaged annual surpluses of up to 43 kg N ha-1 under current conditions, but surpluses less than 14 kg N ha-1 under alternative land use. Under current conditions, there were net losses of SON of as much as 23 kg ha-1, whereas SON increased by more than 18 kg ha-1 under alternative land use. WEPP indicated watershed maximum annual erosion of 22 Mg ha-1 under current conditions, double the regional maximum of 11.2 Mg ha-1 at which soil is maintained as a medium for plant growth (the "T" value). Under alternative land use, erosion was between 1.1 Mg ha-1 and 5.5 Mg ha-1, well below T. These results indicate that stream loads of N and sediment could be reduced when better accounting of N inputs is combined with changes in the distribution and species composition of crop and pasture systems.