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ISU AGRON 515 - Impacts of integrated crop-livestock systems

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Impacts of integrated crop-livestock systems on nitrogen dynamicsand soil erosion in western Iowa watershedsM. Burkart and D. JamesNational Soil Tilth Laboratory, Ames, Iowa, USAM. Liebman and C. HerndlIowa State University, Ames, Iowa, USAReceived 23 December 2004; revised 18 April 2005; accepted 13 May 2005; published 24 September 2005.[1] Agricultural land uses impact leachable nitrogen (N) and erosion, indicators of thepotential for nitrate and sediment contamination of water resources. This paper evaluatesthe potential impact of alternative land uses on leachable N, soil organic nitrogen (SON)and erosion in western Iowa watersheds using a combination of widely availablemodels and georeferenced data. The alternative land uses increase land area underperennial cover, integrate livestock with cropping systems, and reduce inorganic fertilizeruse. We used the Water Erosion Prediction Program (WEPP) to estimate erosion and aN-budget model to estimate leachable N and changes in SON. The N model describedhere is widely applicable because it utilizes commonly available georeferenced dataon soils, crops, and livestock. Maximum annual erosion rates were estimated to be22 Mg ha1under current conditions, double the regional maximum at which soil ismaintained as a medium for plant growth (T). Under alternative land uses, erosion wasbetween 1.1 Mg ha1and 5.5 Mg ha1, well below T. Annual leachable N was as much as43 kg ha1for current conditions, but consistently less than15 kg ha1under alternativeland uses. Maximum SON losses were 23 kg N ha1under current conditions whileSON increased by as much as 18 kg N ha1under alternative land uses. These resultsindicate that erosion may be minimized, leachable N could be decreased and SON may beincreased by better accounting of N inputs and altering the distribution and speciescomposition of crop and pasture systems.Citation: Burkart, M., D. James, M. Liebman, and C. Herndl (2005), Impacts of integrated crop-livestock systems on nitrogendynamics and soil erosion in western Iowa watersheds, J. Geophys. Res., 110, G01009, doi:10.1029/2004JG000008.1. Introduction[2] Nitrate leached from agricultural systems has beenshown to contaminate groundwater [Nolan et al., 1997;Burkart and Stoner, 2002] and ultimately streams [Ferm,1998; Schilling and Libra, 2000; Howarth et al., 2002 ],and to contribute to seasonal hypoxic zones in coastalwaters [Turner and Rabalais, 1991]. Leachable inorganicnitrogen in agricultural systems serves as an indicator ofthe potential for nitrate contamination of groundwater thatdischarges to streams. Increased use of nitrogen (N)fertilizer in the United States, particularly during the1960s and 70s, has been cited as the prime contributorto the hypoxia observ ed in the Gulf of Mexico [Turnerand Rabalais, 1994] and coastal eutrophication [NationalResource Council, 2000]. This was also a period duringwhich annual crops substantially replaced perennial cropsand other perennial vegetation, such as pasture. Sinceleaching of inorganic N is facilitated during periods ofthe year with no canopy cover and this can coincide withperiods of substantial infiltration potential [Randall andGoss, 2001], this shift from perennial to annual cropsaccelerated the flux of N to groundwater. Nitrate leachingbeneath annual crops has been shown to be an order ofmagnitude larger than under perennial vegetation [Randallet al., 1997]. Conversely, relative to annual crops, peren-nial vegetation can maintain or increase soil organicnitrogen (SON) stocks, thereby reducing nitrate leachingpotential and improving long-term soil productivity[Drinkwater et al., 1998; Weil and Magdoff, 2004].[3] Sediment in surface water is of concern throughoutthe midwest U.S., a region where annual crops dominatethe landscape. Estimates of sheet and rill erosion provideindicators of sediment contamination of the region’s waterresources. Erosion is a concern for landowners throughoutthe midwest, but particularly on the steep slopes andhighly erodible soils of the deep loess region in westernIowa (Figure 1). Soil erosion rates are among the highestin the U.S. and can reach more than 100 Mg ha1yr1inthis region. Erosion from rainfall and flooding removessoil from the hills, and deposits it on floodplains where itis ultimately delivered to streams. Water users’ concernsabout stream sediment are reflected in the high frequencyJOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, G01009, doi:10.1029/2004JG000008, 2005Copyright 2005 by the American Geophysical Union.0148-0227/05/2004JG000008$09.00G01009 1of16of documented sediment or turbidity impairment (IowaDNR, 2004: http://www.iowadnr.com/water/tmdlwqa/wqa/303d/2002/2002FinalList_EPA%20approved.pdf).[4] The objectives of this paper are to (1) describeprocesses by which the magnitude and direction ofchanges in soil inorganic and organic N stocks anderosion rates can be estimated using commonly availableand georeferenced data; and (2) compare the magnitudeand direction of change in these indicators betweencurrent conditions and alternative land uses that increaseland area under perennial cover, integrate crop andlivestock production, and decrease reliance on syntheticN fertilizer. We focus our analyses on watersheds inwestern Iowa (Figure 1), a region that is among the mostintensively cropped in the United States. The indicatorswe have chose n (leachable inorganic N, SON, anderosion) allow comparisons of agroecosystem functioningat the watershed scale and can facilitate the developmentof solutions to contamination of local and regionalaquatic systems by agricultural sources of N and sediment[Goolsby et al., 1999; Burkart and James, 1999].[5] The N-budget model used here (see Appendix A) isintended to complement existing deterministic models.Models such as SWAT [Neitsch et al., 2002] use gener-alized descriptions of watershed space to link N dynamicswith hydrologic processes and to accumulate contami-nants in the watershed. Other models such as EPIC[Williams et al., 1984], APEX [Williams et al., 2000],and LEACHM [Hutson, 1992] have the ability to simu-late complex leaching dynamics for individual points. TheN-budget model developed and applied in the presentstudy links the attributes of one-dimensional models togeoreferenced data using geographic information systemstechnology. The necessary data are readily available at avariety of resolutions in intensive agricultural areas in theU.S. By linking stocks and flows with


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