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Fluid Journal : Fluid Journal 2005-2007
Spring 2005 Fluid Journal 2 soil respiration losses in corn following corn than in corn following soybeans (Table l). The latter difference is primarily due to significantly greater residue amounts remaining after corn than after soybeans. However, within each rotation, increases in soil C and N should result from greater biomass production. Because of the large residue C input, the CCP3-M2 system is likely to have the greatest potential for building up SOM. Other important C budget components include C recycling from roots and CO2 respiration losses under soybeans and we will quantify those in the future. Unlike the carbon budget, a partial N budget analysis suggests that N removal in CS systems exceeded that of the CC systems by far (Table 1). Depending on yet to be quantified N contributions from N2 fixation and roots, as well as gaseous and leaching losses of N, changes in soil N may range from depletion to accumulation in these systems. Increase in NUE Figure 1 and Table 1 show the change in both soil C and N from June 2002 to June 2004. In the recommended CS system (P1-M1), corn yields were high (241 bu/A, Table 1) and soil C remained virtually unchanged, but a small loss of 134 lbs/A of N occurred over the four yearperiod. However, significant losses of both soil C (-3,990 lbs/ A) and soil N (-500 lbs/A) were measured in the intensive CS system (P3-M2), which also had the highest average yields (261 bu/A). In the recommended CC system (P1-M1), corn yields were lowest (223 bu/A, Table 1) and losses of both soil C and N were observed. In contrast, the intensive CC system (P3-M2) was the only one with a significant accumulation of both soil C (+3,890 lbs/A) and N (+196 lbs/A) over time. Stabilization of soil N in this system has probably resulted in increased indigenous soil N supply and better congruence of N supply with crop N demand during the growing season. This has probably been a major factor in the increase in NUE we have observed over the last five years in this treatment. Quantifying changes The results shown in Figure 1 differ from our previous reports. One more year of data was included in Figure 1, but the major reason for the revised conclusions was that we have used an improved method for quantifying changes in soil C and N over time. Soil C and N stocks were measured based on a fixed amount of dry soil sampled in each year, which better accounts for annual fluctuations in bulk density and moisture. This method provides more accurate estimates than methods that express C and N on a constant soil volume basis. In our example, volume- based estimates suggested an increase in soil C in CC-P3-M2 that was nearly twice as much as the actual increase calculated on constant soil mass basis. Similarly, the soil-mass calculation showed a large decline in soil C in CS- P3-M2 (-4,000 lbs/ A), but the volume- based estimate suggested a loss of only --800lbs/A. Qualitative differences More detailed studies were conducted to better understand qualitative differences in SOM in the different CS and CC systems. In particular, we were interested in finding more evidence and reasons for the decline in SOM under intensive CS management as opposed to the large buildup of SOM under intensive CC. C signatures were measured in archived soil samples (1999 to 2003) for both bulk soil and different organic matter fractions, including a mobile humic acid fraction (MHA). The MHA fraction is of particular interest for C and N cycling because it is a young pool (<10 years) of SOM with a faster turnover time. MHA is much involved in N mineralization, but probably also the precursor to more stable humus. It is less recalcitrant than other humic acids due to a lower degree of complexation with cations, has higher N(+or--5%)andlowerC(+or--50%) contents than more humified material. Drs. Dobermann, Walters, Leogretta, Arkebauer, Cassman, Drijber, Lindquist, Specht, and Yang are research scientists in the Department of Agronomy and Horticulture, University of Nebraska.
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