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

Long-term effects of cropping systems on soil properties, such as organic soil C and N levels is necessary so more accurate projections can be made regarding the sequester and emission of CO2 by agricultural soils. This information can then be used to predict the effects of cropping systems on both soil degradation, maintenance, or improvement and global climate changes. My objective was to evaluate the effects of crop 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-term study, in the Western Corn Belt. Seven cropping systems (three monoculture, two 2-yr, and two 4-yr rotations) with three rates of N fertilizer were compared. Monocultures included continuous corn (Zea mays L.), soybean [Glycine - (L.) Merr.], and grain sorghum [Sorghum bicolor (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). grain 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 30 cm in 0- to 7.5-cm, 7.5- to 15-cm, and 15-to 30-cm increments. No differences were obtained in 1984, but both rotation and N rate significantly 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 cropping 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 climate conditions.