Fig. 3. (A) Relationship between simulated and observed changes in soil organic C (SOC) (kg C ha^-1 yr^-1). Change in SOC is the SOC at each sampling date minus SOC at 1939. Simulated SOC at each date was calculated with Eq. [10] using parameter values reported in Table 7 for the Cum model. (B) Mass of SOC was simulated using adjusted values of k to distinguish wheat–fallow (WF) from wheat–oat–barley–hay–hay (WOBHH) rotations and compared with observed values for the Breton plots over 51 years. Adjustment entailed multiplying each k by 1.3 for the WF rotations and dividing them by 1.3 for the WOBHH rotations