Total and Light‐Fraction Carbon Dynamics during Four 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 soil behavior and net CO 2 exchange. Our objective was to determine the duration and magnitude of a shift in soil organic C after a change in summer fallow frequency in spring wheat ( Triticum aestivum L.) systems. Total organic and light‐fraction C in continuous wheat (W), fallow‐wheat‐wheat (FWW), and fallow‐wheat (FW) treatments were determined in soil samples collected periodically from 1954 to 1992 from a long‐term spring wheat study located in the northern Great Plains. Organic C concentrations in the 0‐ to 15‐cm depth were 11% lower in the FW and FWW treatments than in the W treatment from 1967 to 1992. Organic C concentrations in the 15‐ to 30‐cm depth were not affected by treatment, but varied among sampling years due to differences in effective sampling depth. Light‐fraction C concentrations were 38% lower in the FW and FWW treatments than in the W treatment from 1967 to 1992. Reduced light‐fraction C accounted for about one‐third of the decrease in total organic C due to inclusion of fallow in the crop rotation. The negative influence of fallow was primarily due to enhanced decomposition in the fallow year. We conclude that most losses of soil organic C due to introduction of a fallow system occur within two decades, and that further loss of organic C is negligible provided soil erosion is minimal.