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Fluid Journal : Fluid Journal 2002-2004
3 Fluid Journal Summer 2002 management (M2) averaged 7.9 tons/A. In continuous corn, annual stover yield averaged 6.5 tons/A for the M1-P1 treatment vs. 7.8 tons/A under very intensive management (M2-P3). Plant nutrient accumulation Higher plant density and intensive nutrient management resulted in greater plant accumulation of N and K per unit of grain yield, whereas no differences were observed for P, Ca, Mg, and S (Table 1). Average crop nitrogen accumulation in aboveground biomass (corn after soybeans) was 1.06 lbs N/bu yield in M1 treatments at normal plant density (P1), but 1.10 lbs N/bu under M2-P2/P3 management. Average crop potassium accumulation in aboveground biomass was 1.55 lbs K/ bu in M1, but increased to 1.84 lbs K/ bu under M2 management at high plant density. By contrast, nutrient removal with grain alone did not differ significantly among the nutrient management and plant density levels, except for a slight decrease in grain N removal with increasing cropping intensity (Table 1). As yields approach existing ceilings, internal plant nutrient requirements increase to sustain the physiological functions of a vastly increased amount of aboveground biomass. This is particularly true for nutrients such as potassium, which has both non-specific and specific plant functions and can be stored in large amounts in the vacuole. However, more work is needed to verify whether the increased K uptake represents a true increase in crop K requirements for achieving yield potential under non- stress conditions. Carbon Sequestration Other justifications for interest in optimal soil productivity are low commodity prices, and the need for mitigating local and global environmental effects of human activities. Corn production systems can contribute to solving environmental problems rather than being perceived to be the source of such problems. One such example is the potential of corn systems to fix atmospheric carbon dioxide (CO2) in crop biomass through the process of photosynthesis and to sequester a portion of this fixed carbon (C) in soil organic matter. Once in place, this C sequestration would contribute to reducing the rate of increase in greenhouse gases. In fact, international negotiations are under way that may create markets for such C storage. The annual C storage potential of corn production systems may range from 0.25 to 1.5 tons/A of C, depending on soil management and yield level. However, no studies have been conducted to quantify this at near- optimal yield levels. Soil samples collected after the first year indicated no significant differences in soil C and N stocks among treatments. As a baseline, average total soil C stored in the top foot was 26.1 tons/A, and average total soil N was 2 tons/A. Average concentrations were 14.8 g C kg-1 and 1.14 g N kg-1. However, depending on the nutrient management and plant density levels, total C input over two years (1999 to 2000) was 15 to 30 percent greater in continuous corn than in a corn/soybean rotation (Figure 2), mainly due to less vegetative biomass production in soybeans compared to corn. Total C input from recycled crop biomass was 1.5 tons/A greater in continuous corn than in a corn/soybean rotation. We expect the difference in C sequestration to be even greater because soybean residue decomposes much more rapidly than corn stover. It remains to be seen how the different levels of C input will affect soil stocks over the medium and long term, and whether potentially greater C sequestration can be achieved without increases in CO2 emissions and other greenhouse gases. Preliminary data indicate that intensive management schemes do not appear to cause increased soil surface CO2 flux that would offset increased soil carbon sequestration potential. However, efforts to increase sequestered carbon through high N applications may lead to other problems such as increased nitrous oxide (N2O) emissions, which must be mitigated through more detailed forms of N management. Dr. Dobermann is associate professor, Dr. Arkebauer is associate professor, Dr. Cassman is professor and head, Dr. Lindquist is assistant professor, Dr. Nelson is professor, Dr. Specht is professor, Dr. Walters is professor, and Dr. Yang is research assistant professor in Department of Agronomy and Horticulture at the University of Nebraska.
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