Fig. 5 Arable layer soil organic C at equilibrium according to the model proposed by Bartholomew and Kirkham (1960) 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