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Fluid Journal : Fluid Journal 2005-2007
Winter 2005 Fluid Journal 3 from native grass in the fall of 1995. During the one year prior to April 1996 it was planted to winter wheat and grazed. N applied. The amount of N applied to the whole field (N1) at Texline was 315, 321, 253, and 200 lbs/A of N in 1999, 2000, 2001, and 2002, respectively. The amount of N applied to the corn residue after harvest on the eastern half of the field (N2) was 88, 132, and 100 lbs/A of N in 1998, 1999, and 2000, respectively. No fall N was applied to the corn residue after the 2001 corn harvest in the (N2) treatment area because of a large amount of residual soil NO3-N. Grain yield. From 1999 to 2002, combine grain yields ranged from 178 to 244 bu/A and averaged 213 bu/A at the (N1) fertility level. At the (N2) fertility level, they ranged from 169 to 245 bu/A and averaged 210 bu/A. Residue. Estimated corn residue amount returned to the soil ranged from 10,780 to 14,653 lbs/A and averaged 12,126 lbs/ A at the (N1) fertility level. At the (N2) level, it ranged from 10,404 to 14,437 lbs/ A and averaged 11,643 lbs/A. Residue C concentration averaged 44.7 percent and N concentration averaged 0.93 percent. Estimated amount of residue C and N returned to the soil surface has aver- aged 5,365 lbs C/A and 109 lbs/A of N (N1) and 5,260 lbs C/A and 103 lbs/A of N (N2). Yearly differences in grain yields and residue production at both sites reflect differences in growing season climatic conditions and corn hybrids grown. A large quantity of residue C is being incorporated into the soil each year in these irrigated, reduced-till continuous corn production systems. Soil carbon/nitrogen Soil organic C in reduced-till continu- ous corn production systems in Texas has increased with each additional crop year since 1999 in the 0 to 6-inch soil depth (Figure 1). Increases were also observed in the 0 to 12- and 0 to 24-inch soil depths. Because the C inputs to the soil have been similar for both N fertility treatments, difference in SOC accumula- tion between (N1) and (N2) fertility levels is not significant. TSN has also increased with each additional crop from 1999 through 2002 in the 0 to 6-inch soil depth at both sites (Figure 2). This supports the observation of increasing SOC with time. The SOC level in the cropped area exceeded the level present in the native sod in 1999 at Dalhart (7.76 tons C/A) and at Texline (11.7 tons C/A) in the 0 to 6-inch soil depth. The increase in SOC level within the irrigated continuous corn system indicates that SOC is accumulating in the fine sandy loam soil at Dalhart and in the clay loam soil at Texline when averaged over both N fertility treatments. Since corn residue levels are high for both N fertility management levels, differences in SOC between(N1) and (N2) treatments are small. Several more years of data collection are needed to determine if differences can be detected in SOC changes between the (N1) and (N2) fertility management treatments. Soil NO3-N levels At both Texas sites, soil NO3-N levels under native sod were very low com- pared to the residual soil NO3-N in the cropped areas (Figure 3). Residual soil NO3-N following corn harvest has been greater for the (N2) treatment than for the (N1) treatment since 1999. This indicates that the addition of extra liquid N to the corn residue after harvest with the (N2) treatment is contributing to a higher residual soil NO3-N level than with the normal (N1) fertilizer program. For this reason, Mr. Poole reduced the after-harvest N application to the residue at Dalhart in 2001 to 50 lbs/A of N for the 2002 corn crop and applied no N to the residue at Texline in the fall of 2001. Because the Texas fields have been continuously cropped to corn for seven years since conversion from native sod to cropland, it will be interesting to observe the change in SOC with time and to assess the effects of maximum soil productivity on SOC sequestration and NO3-N leaching potential. The increase in residual soil NO3-N levels at both sites with the (N1) treatment 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 and available for leaching below the root zone. Mr. Poole increased the 2003 planting rate to 42,000 seeds/A, trying to increase yield potential toward 300 bu/A. Dr. Halvorson is soil scientist, Dr. Mosier is research chemist, and Mr. Reule is soil scientist, USDA/ARS, Ft. Collins, CO.
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