Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: December 17, 2004
Publication Date: December 17, 2004
Citation: Burkart, M.R., James, D.E., Liebman, M., Van Ouwerkerk, E. 2004. A nitrogen budget model as an indicator of leachable nitrogen under agricultural catchments [CD-ROM]. Washington, D.C. American Geophysical Union.
Surplus nitrogen (N) in ground and surface water is of concern to land users and downstream water users in intensive agricultural regions. Surplus N is available for leaching during lengthy periods without crop cover in annual crop systems used in many temperate regions. This paper presents a model to estimate surplus N available for leaching to ground water beneath agricultural systems and demonstrates an application to catchments in western Iowa where the landscape is intensively managed for maize and soybean production. Biochemical processes of N transfer and transformation within agricultural systems are simulated. Stocks of N in soil, crops, and livestock, and external stocks representing fertilizer and the atmosphere are linked using commonly available georeferenced data on soils, crops, livestock, and inorganic fertilizer, making it applicable to catchments and watersheds in many regions. 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 carbon: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 annual surpluses of 7 to 26 kg-N ha-1. Annual nitrate loads in local streams are 17 to 30 kg-N ha-1. Net losses of soil organic N are sufficiently small to suggest that the maize-soybean rotation is in near equilibrium with the soils of the region.