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Fluid Journal : Fluid Journal 2002-2004
Fall 2004 Fluid Journal 3 expression of yield potential was realized regardless of population density. This could also be an artifact of the general characteristics of this hybrid, which has very erect leaf structure. We are using this same hybrid in 2004. N rates. N application rates are adjusted as a function of projected yield potential, previous crop and spring residual soil NO3-N (Table 2). Application rates have remained more consistent for the corn/soybean rotation owing in part to the impact of soybeans on reducing residual soil NO3-N. Beginning in 2002, prior to plowdown in the fall, we began the practice of applying fluid N at the rate of 45 to 65 lbs/A to the residue of the corn M2 treatment. This was meant to facilitate decomposition and humification of the high amounts of residue we have experienced under this treatment with the intent of decreasing the competition of decomposers for N sources during the early growing season. The elevated NO3-N levels in the spring following these applications have resulted in a significant reduction in fertilizer rate (Table 2). NUE. The impact of these management changes on N--use efficiency (NUE) is given in Table 3. Average farm NUE in the U.S. is 1.03 bu/lb of N. Although average NUE for the continuous corn M2 treatment is below this level, we have experienced a steady increase in NUE over the course of the study. This indicates the potential that exists for increasing NUE in cornbased systems. We hypothesize that the increase in NUE we are observing is due to improvement in both soil quality from greater carbon inputs to the soil and concomitant sequestration of N with this carbon. Cumulative carbon/N. The cumulative effect of crop residue carbon (C) and N recycled in these systems is summarized in Figure 1. Over the four-year period, Table 3. Trend in NUE as influenced by crop rotation, population density, and fertility management. Treatments Average NUE Density* Fertilizer N rate Yield NUE '99 '00 '01 '02 '03 lbs/A bu/A bu/lbN bu/lbN Continuous Corn P1 MI 170 217 1.28 1.18 1.25 1.11 1.59 P2 P3 M2 268 247 0.94 0.71 0.94 0.94 1.18 Corn after soybeans P1 M1 116 236 2.04 1.89 1.83 1.98 2.06 2.31 P2 P3 M2 224 256 1.16 1.28 0.93 1.16 1.26 1.28 * M2 treatment with highest yielding plant density: P2 in 2000 and 2003; P3 in 1999, 2001, and 2002. 7.5 tons C/A were recycled in the recommended CS-P1-M1 treatment, which is the most widespread rotation in the Corn Belt. This amount increased to 9.1 tons C/A in the intensified corn/ soybean system (CS-P3-M2). Net C recycling in all of the continuous corn treatments was larger than in any of the corn/soybean treatments, reaching 9.3 tons C/A in continuous corn P1 M1 and a maximum of 11.7 tons C/A in continuous corn P3 M2. This increase represents a 48 percent gain in residue C inputs to soil over this four-year period. N recycled in crop residues was highest in the corn/soybean rotation with an average of 481 lbs/ A vs. 356 lbs/ A in the continuous corn treatment. Increased C inputs to soil can only build organic matter if there are not elevated losses of CO2-C from soil respiration. We have been monitoring soil CO2-C respiration since 1999 and have noted that fertility treatments have had a minor impact on CO2-C losses. Although CO2- C losses during the 2003 growing season were higher for continuous corn (owing to higher residue C input), fertility management did not result in CO2-C losses equivalent to C inputs. This suggests that increases in soil C and N should result. Drs. Arkebauer, Dobermann, Cassman, Drijber, Lindquist, Specht, Walters, and Yang are research scientists in the Department of Agronomy and Horticulture, University of Nebraska.
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