|Liebman, M - IA STATE UNIVERSITY|
|Van Ouwerkerk, E - IA STATE UNIVERSITY|
Submitted to: Water Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 15, 2005
Publication Date: N/A
Interpretive Summary: Surplus nitrogen (N) from intensive agricultural systems has contributed to nitrate loads in streams that are a concern to land users and downstream water users. It is difficult to experimentally determine what parts of the agricultural system contribute most to surplus N. It was found that a Geographic Information System (GIS) N-budget model that integrates crop, livestock, and soil components of an agricultural system could be useful to estimate surplus or leachable N from complex agricultural landscapes. Application of the model in watersheds draining a corn-soybean and livestock system in western Iowa produced watershed-averaged surpluses of 25 to 40 kg N ha-1. Net losses of soil organic N were sufficiently small to suggest that the maize-soybean rotation is in near equilibrium with the soils of the region. The model will be useful to policymakers, watershed groups, and water resource planners because the general data requirements will make it applicable to a wide range of intensely managed watersheds.
Technical Abstract: Surplus nitrogen (N) in water is of concern in intensive agricultural regions such as the Midwest United States. Surplus N is available for leaching during long periods without crop cover in annual crop systems used in many temperate regions. Periods without crop cover frequently coincide with precipitation and snowmelt resulting in infiltration and ground water recharge. This paper presents a model to estimate the surplus N available for leaching to ground water beneath agricultural systems and demonstrates an application to watersheds in western Iowa where 71% of the land is intensively managed for maize and soybean production. The model was developed to use commonly available Geographic Information System data on soils, crops, and livestock so that it can be applied to watersheds in many regions. The model links stocks of N within agricultural systems, including soil, crops, and livestock, and external stocks representing fertilizer and atmospheric sources. Atmospheric exchanges include volatilization from fertilizer, soil, manure, and senescing crops, denitrification, atmospheric deposition, and symbiotic fixation. Nitrogen flow is centered on exchange between the soil inorganic and organic N stock. Differential N mineralization rates are defined for three soil organic matter pools, crop residue, and manure based on C:N ratios. Nitrogen exports from the system are accounted for by quantifying harvested crops and animals in addition to losses to the atmosphere. Application of the model to contiguous watersheds produced watershed-averaged surpluses of 6 to 9 kg N/ha, with net losses of soil organic N that were sufficiently small to suggest that the maize-soybean rotation is in near equilibrium with the soils of the region.