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Fluid Journal : Fluid Journal 2005-2007
Spring 2005 Unlocking The Secrets of Carbon/Nitrogen Cycling Drs. A Dobermann, D. Walters, F. Legoretta, T. Arkebauer, K. Cassman, R. Drijber, J. Lindquist, J. Specht, and H. Yang Summary: In intensive continuous corn (CC) systems, incorporation of large amounts of residue carbon (C) and nitrogen (N) has led to a significant buildup of soil organic matter over a few years, which most likely has contributed to the increased N-use efficiency (NUE) observed. Although corn yields and NUE were highest in the intensive corn/soybean (CS) system, this excellent performance was achieved at the cost of exploiting soil C and N reserves. Large grain N removal, less residue input, and rapid cycling of soybean residue through young organic matter fractions were observed in the CS rotation. The N- credit attributed to CS rotations appears to be due to "mining" of soil N reserves. In contrast, in systems that accumulate soil organic matter (SOM), credit should be given to the efficiency of added fertilizer N in augmenting soil N sequestration. Corn yields and N-use efficiency highest in intensive corn/soybean systems. Fluid Journal 1 The University of Nebraska research program on Ecological intensification of irrigated maize- based cropping systems was established in 1999 to 1) improve understanding of the yield potential of corn and soybeans and how it is affected by climate and management, 2) develop approaches for managing CC and CS rotations at 80 to 95 percent of yield potential, 3) conduct integrated assessment of productivity, profitability, input-use efficiency, energy balance, and environmental consequences of intensified cropping systems, and 4) develop a scientific basis and decision tools for extrapolation at other locations. The project has resulted in 1) a new physiologybased corn growth model and its user-friendly software, 2) new information on radiation-use efficiency in high-yield corn, 3) understanding of regional variation in corn yield potential, 4) new information on NUE, C and N cycling, greenhouse gas emissions in CC and CS systems, and 5) detailed data on soybean phenology and yield response to management and climate. New developments We have previously reported how differences in residue input and changes in SOM over time may affect yield performance and NUE. Since then, we have expanded our data collection and analysis, leading to somewhat different conclusions from those previously reported. Table 1 summarizes key results for four contrasting treatments that represent recommended and intensive crop management systems. In a period that included four years (2 complete CS cycles), the cumulative crop residue C input amounted to 17,410 lbs C/A (or 8.7 tons/A) in the CS-P1-M1 treatment, which serves as our reference because it represents a recommended best management practice. Compared to this, residue C input increased by 5 percent in CS-P3-M2, but by 11 percent and 35 percent in CC with recommended (CC-P1-M1) or intensive management (CC-P3-M2), respectively. In contrast, N recycled in crop residues was highest in the CS rotation (Table 1). Increased C inputs to soil can only build SOM if losses of CO2-C from soil respiration are not elevated and N is available for humification processes. Monitoring of soil CO2-C respiration has shown no significant differences between recommended and intensified management systems within each rotation, but generally has shown larger Table 1. Components of the C and N budget under recommended and intensive management practices in CC and CS rotation CS CC CS CC Corn yield --- bu/A 241 223 261 251 C recycled with residues1---lbs C/A 17,411 19,286 18,304 23,571 Annual soil CO2 flux, corn---lbs C/A 4,634 5,973 4,598 5,813 Change in soil C, 0 to 1 foot---lbs C/A 620 -980 -3,990 3,890 N recycled with residues1---lbs N/A 469 272 500 420 Fertilizer N input---lbs N/A 246 696 638 1,088 N removal with grain---lbs N/A 861 601 900 709 Fertlilzer N---grain N removal---lbs N/A -615 95 -263 381 Change in soil N, 0 to 1 foot---lbs N/A -134 -205 -500 196 1 Aboveground corn and soybean residue 4-Year C and N budget 2000 to 2003/4 Recommended Intensive P1-M1 P3-M2
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