Fig. 4 Change in soil organic C as a function of time under conventional cropping for different crop sequences. Curves result from fitting a model proposed by Bartholomew and Kirkham (1960). Crop sequence description and model parameters can be seen in Tables 1 and 2, respectively. With N = received N fertilizer 3, 5, 7, 9, or 11 yr during the experiment (Table1); without N = never received N fertilizer