TOTAL AND LIGHT-FRACTION CARBON DYNAMICS DURING 4 DECADES AFTER CROPPING CHANGES

The dynamics of soil organic C after a change in cropping practice are important in defining the potential impact of the new practice on soi l behavior and net CO2 exchange. Our objective was to determine the du ration and magnitude of a shift in soil organic C after a change in su mmer fallow frequency in spring wheat (Triticum aestivum L.) systems. Total organic and light-fraction C in continuous wheat (W), fallow-whe at-wheat (FWW), and fallow-wheat (FW) treatments were determined in so il samples collected periodically from 1954 to 1992 from a long-term s pring wheat study located in the northern Great Plains. Organic C conc entrations in the 0- to 15-cm depth were 11% lower in the FW and FWW t reatments than in the W treatment from 1967 to 1992. Organic C concent rations in the 15- to 30-cm depth were not affected by treatment, but varied among sampling years due to differences in effective sampling d epth. Light-fraction C concentrations were 38% lower in the FW and FWW treatments than in the W treatment from 1967 to 1992. Reduced light-f raction C accounted for about one-third of the decrease in total organ ic C due to inclusion of fallow in the crop rotation. The negative inf luence of fallow was primarily due to enhanced decomposition in the fa llow year. We conclude that most losses of soil organic C due to intro duction of a falloff system occur within two decades, and that further loss of organic C is negligible provided soil erosion is minimal.