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Fluid Journal : Fluid Journal 2005-2007
SPRING 2007 Fluid Journal 19 average carbon concentration of 44 and 45 percent and N concentration of 0.90 and 0.87 percent for the N1 and N2 sites, respectively. Estimated amount of carbon and N returned to the soil surface has averaged 4,774 and 4,935 lbs/A of carbon and 95 and 91 lbs/A of N, respectively, for the N1 and N2 treatments (Figures 1 and 2). Soil carbon and N The reduced-till, continuous corn production system from 1999 through 2003 has increased soil organic carbon each additional crop year in the 0- to 6-inch soil depth. Increases were also observed in the 0 to 12-inch and 0 to 24-inch soil depths. Because the soil or carbon inputs have been similar for the N1 and N2 sites at both locations (Figure 3), the difference in soil organic carbon accumulation rates between the N1 and N2 treatments (liquid N added to residue) was not significant. Total soil N has also increased linearly with each additional crop year from 1999 through 2005 in the 0 to 6-inch soil depth at both sites, which supports the observation of increasing soil organic carbon with time and building the N fertility of the soil. The soil organic carbon data from 2005 indicated a decrease in soil organic carbon from 2003 and 2004. The samples were rerun to verify the decrease. This decrease may be an indication that the heavy removal of corn residue by cattle grazing the corn stalks in 2003 and 2004 at Texline and the effects of drought stress at Dalhart, plus inserting wheat in the rotation in 2003, are slowing the rate of soil organic carbon accumulation at both locations. Another year of data will be needed to verify this trend. The increase in soil organic carbon level within the reduced-till, irrigated continuous corn/wheat system at Dalhart indicates that soil organic carbon is building in this fine sandy loam soil at a rate of about 1,000 lbs/A per year when averaged over the N1 and N2 sites. The change in soil profile soil organic carbon levels at the clay loam Texline site shows the same trends as the Dalhart site of increasing soil organic carbon (about 1,000 lbs/A per year), at least through 2003, and total soil N in the cropped area. NO3-N levels Dalhart. Addition of liquid N to the corn residue at the N2 site increased residual soil NO3-N levels significantly compared with the N1 site. Residual soil NO3-N levels at the N2 site declined when this practice was terminated in 2002 and winter wheat was added to the crop rotation. The lower rates of N applied to winter wheat resulted in a Cumulative Residue C Year 1999 2000 2001 20022 003 2004 2005 Residue C (lb C/A) 0 5000 10000 15000 20000 25000 30000 35000 40000 N2-Dalhart N1-Texline N2-Texline W W W W W = winter wheat grown Corn all other years N1-Dalhart reduction in soil NO3-N levels at both N1 and N2 sites, with 2004 and 2005 having the lowest level of residual soil NO3-N since 1999. Texline. Residual soil NO3-N levels had generally increased at both the N1 and N2 sites with each additional crop through 2004, even though N application to the corn residue was discontinued in 2001 at the N2 site. This probably reflects the result of fertilizing for a greater than 250 bu/A corn crop but not achieving this yield potential, which leaves residual N fertilizer in the soil available for leaching below the root zone. Application of N to the corn residue resulted in higher residual soil NO3-N levels at the N2 site than at the N1 site. Soil NO3-N had declined from 2004 levels at both sites in 2005, but were still higher than in 1999. Dr. Halvorson is soil scientist and Mr. Reule is soil scientist, USDA/ARS, Ft. Collins, CO. Figure 3. Cumulative increase in residue carbon returned to soil at Dalhart and Texline at N1 and N2 sampling sites.
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