ROTATION AND NITROGEN-FERTILIZATION EFFECTS ON CHANGES IN SOIL CARBONAND NITROGEN

Long-term effects of cropping systems on soil properties, such as orga nic soil C and N levels is necessary so more accurate projections can be made regarding the sequester and emission of CO2 by agricultural so ils. This information can then be used to predict the effects of cropp ing systems on both soil degradation, maintenance, or improvement and global climate changes. My objective was to evaluate the effects of cr op rotation and N fertilizer management on changes in total soil C and N concentrations that have occurred during an 8-yr period in a long-t erm study, in the Western Corn Belt. Seven cropping systems (three mon oculture, two 2-yr, and two 4-yr rotations) with three rates of N fert ilizer were compared. Monocultures included continuous corn (Zea mays L.), soybean [Glycine - (L.) Merr.], and grain sorghum [Sorghum bicolo r (L.) Moench]. The 2-yr rotations were corn-soybean and grain sorghum -soybean, and the two 4-yr rotations were corn-oat (Avena sativa L.) dover (80% Melilotus officinalis Lam. and 20% Trifolium pratense). gr ain sorghum-soybean and corn-soybean-grain sorghum-oat + clover. Soil samples were taken in the spring both in 1984 and 1992 to a depth of 3 0 cm in 0- to 7.5-cm, 7.5- to 15-cm, and 15-to 30-cm increments. No di fferences were obtained in 1984, but both rotation and N rate signific antly affected total soil C and N concentrations in 1992. The results indicate that C could be sequestered at 10 to 20 g m-2 yr-1 in some cr opping systems with sufficient levels of N fertilizer. Greater storage of C in soils suggests CO2 emissions from agricultural soils could be decreased with improved management practices and may in the long term have a significant effect on CO2 in the atmosphere under current clim ate conditions.