Fig. 4. Distribution of the resulting proportion of new maize soil organic carbon (SOC) (%) as a function of depth in the profile and the duration of maize cropping following wheat: (a) 0 to 15 cm, (b) 15 to 30 cm, (c) 30 to 45 cm, (d) 45 to 60 cm, (e) 60 to 75 cm, (f) 75 to 90 cm, and (g) 90 to 105 cm. All 224 data points represented. ***, significant at P 0.001; NS, not significant; S, slope.