Fig. 3 Arable layer soil organic C after 11 yr of conventional tillage as a function of mean annual residue C returned by different crop sequences arranged in two groups: G1, none or only one summer crop every 3 yr; G2, two summer crops every 3 yr. With N = received N fertilizer 3, 5, 7, 9, or 11 yr during the experiment (Table1); without N = never received N fertilizer