Fig. 1 Arable layer soil organic C after 11 yr of conventional tillage under different crop sequences (a) without N and (b) with N applied at 3, 5, 7, 9, or 11 yr during the experiment. Crop sequences are described in Table 1. C = corn, Sb = soybean, Sf = sunflower, W = wheat. The same letters within columns indicate that the sequences do not differ according to Duncan (P < 0.05)

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