Evaluation of a simple model to describe carbon accumulation in a Brown Chernozem under varying fallow frequency

Soil organic C (SOC) is readily influenced by crop management practices, su ch as summerfallowing. On the Canadian prairies, the area summerfallowed ha s decreased significantly in recent years. Our objectives were to determine the influence of fallow frequency on the rate of change in SOC in an Orthi c Brown Chernozem, and to test the effectiveness of an empirical equation d eveloped in an earlier study for estimating SOC changes in these rotations over 33-yr period. The rotations, which were initiated in 1967, all receive d adequate N and P fertilizers. They were (i) fallow-spring wheat (Triticum aestivum L.) (F-W), F-W-W, F-W-W-W-W-W and W-lentil (Lens culinaris L.) (W -Lent). Soil organic C was measured in the 0- to 15-cm. and 15- to 30-cm de pths in 1976, 1981, 1984, 1990, 1993, 1996 and 1999. No measurements of SOC were made in 1967; we estimated SOC starting values to be 30.5 Mg ha(-1) i n the 0- to 15-cm depth. In the period 1967 to 1990, when growing season pr ecipitation was near normal for this semiarid region, SOC in the four rotat ions approached a steady state. However, a decade of much more favourable g rowing season precipitation in the 1990s increased C inputs, which resulted in a marked increase in SOC in the treatments. The empirical equation sugg ests, and the F-W and W-Lent rotations appear to confirm, that these rotati ons are approaching a new steady state at a higher level of SOC, reflecting the decade of favourable precipitation. Measured SOC levels were quite var iable, emphasizing the difficulty of relying on measurements made over shor t time frames (e.g., 5-6yr) when quantifying SOC changes. The equation effe ctively simulated the trends in SOC changes in all rotations, but consisten tly underestimated SOC levels in the W-Lent rotation by about 2 Mg ha(-1). Estimates of difference in SOC between treatments were generally similar wh ether expressed on a mass/fixed depth or a mass/equivalent depth basis. Bas ed on the estimates derived by the empirical equation, we estimated rates o f SOC sequestration during the 1967-1990 period to be 0.03 Mg ha(-1) yr(-1) for F-W, 0.10 Mg ha(-1) yr(-1) for F-W-W, and 0.15 Mg ha(-1) yr(-1) for W- Lent. If we include the decade of more favourable precipitation (1967-1999) , the rates were between 0.05 Mg ha(-1) yr(-1) for F-W and 0.20 Mg ha(-1) y r(-1) for W-Lent. These values are much higher than those estimated by othe rs using the CENTURY model. We concluded that (i) simple models, such as th at used in this study, are very useful for estimating management effects on SOC changes, and (ii) we must be cautious in extrapolating C sequestration estimates based on data from short-term experiments because future weather conditions are not easily predicted and weather can have an important impa ct on C sequestration.