Citation: Burkart, M.R., James, D.E., Liebman, M.Z., Herndl, C. 2005. Impacts of integrated crop-livestock systems on nitrogen dynamics and soil erosion in western Iowa watersheds. Journal of Geophysical Research-Biogeosciences. 110:G1. G01009,10.1029/2004JG000008. Interpretive Summary: Suspended sediment and nitrate loads frequently contribute to the impairment of streams in watersheds dominated by row-crop agriculture. Soil erosion and nitrogen (N) leaching are variables which can be estimated to compare a variety of existing and alternative systems that integrate crop and livestock production. It was found that erosion rates and nitrogen leaching losses could be substantially reduced if crop rotations included more perennial vegetation under an alternative crop pattern which was theoretically placed on watersheds in western Iowa. Annual erosion under current conditions was as much as 22 Mg/ha, a level more than double the maximum soil loss tolerance factor (T) of 11.2 Mg/ha. Erosion was reduced to as little as 1.1 Mg/ha under the alternative crop pattern. Annual leachable nitrogen, as determined by a nitrogen-budget model defined in this paper, was reduced from 44 kg N/ha per year under current conditions to less than 25 kg N/ha under the alternative crop pattern with optimum livestock production. Also, net changes in soil organic nitrogen shifted from losses of as much as 23 kg N/ha under current conditions to gains of more than 21 kg N/ha under the alternative crop and livestock pattern. Reduction in erosion and leachable N under the alternative crop pattern could substantially reduce stream sediment and nitrate-nitrogen loads, although the lag time for such a change may be substantial. These results illustrate that ultimate improvements in water quality could occur when changes in the distribution of crop and pasture systems are linked with expanded use of manure-N credits. Such information will be useful for policy makers at the watershed scale as well as the national level in considering support for alternate agricultural systems that maintain crop and livestock production while providing long-term improvements in water quality.
Technical Abstract: This paper quantifies the surplus N available for leaching, changes in soil organic N and the erosion under existing land use patterns and compares that to the surplus N available for leaching, changes in soil organic N and the erosion produced by an alternative land use. The alternative land use integrates animals with crops including a two-year and six-year rotation and permanent pasture using slope thresholds to distribute crops. In order to compare existing land use patterns with the alternative, this paper presents a model to estimate the surplus N available for leaching to ground water beneath agricultural systems and applies the Water Erosion Prediction Program (WEPP) to estimate erosion under both current conditions and under the alternative pattern of land use. The N model developed here utilizes georeferenced data commonly available in many regions of intensive agriculture. The model links stocks of N within agricultural systems, including soil, crops, and livestock, and external stocks representing fertilizer and atmospheric sources to estimate the surplus N available for leaching. Nitrogen flow is centered on exchange between the soil inorganic and organic N stocks. Application of the N model to current conditions produced annual surpluses of up to 44 kg N/ha, whereas the alternative land use yielded surpluses of less than 25 kg N ha-1 when optimum swine production is included. When adjusted for perennial vegetation, leachable N under current conditions was 43 kg N/ha less than 14 kg N/ha under the alternative land use. The annual nitrate-N load in regional streams was 23 kg N/ha, more than 25% less than the median leachable N (32 kg N/ha) under current conditions. Net changes in soil organic nitrogen (SON) were losses of as much as 23 kg N/ha under current conditions and gains of more than 21 kg N/ha under alternative land use. The erosion model was applied using data from the National Resources Inventory and rainfall generated by NEXRAD radar as input to WEPP. Annual erosion was reduced from a watershed maximum of 22 Mg/ha under current conditions, a level well above the regional maximum T of 11.2 Mg/ha. Under the alternative land use scenario, erosion was reduced to between 1.1 Mg/ha and 5.5 Mg/ha.