Fig. 5. Modeled trajectory of change (line) in the phase space of mineral nitrogen (Nmin) and soil organic matter. Initial condition is a field after years of maize cultivation, based on Sonneveld et al. (2002), and afterward receiving 125 kg fertilizer N/ha. Mineral nitrogen determines the yield of the field and in 200 years the farmer will have almost the same yield with half of the external fertilizer input. The trajectory is strongly nonlinear, the fast variable of mineral nitrogen decreases steeply, and recovery of this variable is slow with the increase of soil organic matter.

Fig. 5