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Fluid Journal : Fluid Journal 1996-1998
2 Fluid Journal Summer 1998 Figure 2. Spatial variability of corn grain yields of two N treatments, Stratton, CO, 1996. Figure 3. Spatial variability of soil profile (4-ft depth) NO3-N, Sterling, CO, 1996. the resulting yield variability of dryland corn in a dryland winter wheat- corn-fallow cropping system. The specific objectives were to: • quantify spatial variability of corn yields, soil N and "in-situ" net N mineralization over "typical" landscapes of eastern Colorado • determine spatial relationships among corn yields and soil properties. Spatial variability Grain yield. At the Sterling site, grain yield at all N rates showed a high spatial dependence. This was measured using the Moran's I technique1, which varied from 0.15 to 0.35 for 30 and 120 lbs/A of N, respectively. Range for spatial dependence2 of grain yield varied between 138 and 402 feet for 30 and 120 lbs/A of N, respectively, with most soil property values falling between 261 and 402 feet. Kriged3 corn yields varied 720 percent, from 15 to 108 bu/A, depending on slope position and N rate. Highest yields occurred at the toeslope and summit positions while lowest-yielding areas occurred on the eroded sideslope (Figure 1). Highest nitrogen-use efficiency values (61 percent) occurred on the toeslope, although no definite pattern was observed over the landscape. At the Stratton I site, corn yields had similar spatial dependence as shown by index values (Moran's I) of 0.10 to 0.18 for 0 and 120 lbs/A of N, respectively. Range of spatial dependence was 74 and 122 feet for the unfertilized strip (0 lbs/A) and 120 lbs/ A, respectively. Kriged corn yields varied 252 percent from 44 to 111 bu/A over the landscape, depending on position and N rate (Figure 2). Highest yields occurred at the toeslope. Yields were not normally distributed. Coefficients of variation were low (18 percent) and similar for both N treatments. Nitrogen-use efficiency was less variable over the Stratton I landscape than at the Sterling site, yet similar values occurred at both landscapes. Higher nitrogen-use efficiency values (37 percent) occurred at the toeslope, with all other positions showing similar values. Soil available N. Total available N (NO3-N + NH4-N) measured to the 4-foot soil depth varied from 28 to 71 lbs/A of
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