Empirical modelling of soil dynamics along a chronosequence of shifting cultivation systems in southern Cameroon

Models are simplified representations of a system, in this study the dynamic behaviour of soil properties in shifting cultivation systems in southern Cameroon. Soils were sampled synchronically from plots representing a chronosequence (from 0 to more than 30 years) of these land-use systems and also diachronically from the same plots over 7 years. The five soil properties (pH water, exchangeable calcium, available P, organic C and bulk density) that had been identified in a previous study as the most sensitive to these land-use systems were modelled. Linear/quadratic fractional rational functions were fitted to the synchronic series using nonlinear least squares. These functions were used to compute three metrics describing soil behaviour over time: maximum proportional deviation from the base state, time to reach this maximum, and relaxation time towards the original value. The curves of four variables showed an initial S-shaped rise from the value under virgin forest to a maximum during cropping, followed by an inverse-S-shaped decrease towards the original value during fallow or perennial plantations; the curves of organic C showed an inverse shape. The fitted function explained 50% to 80% of soil dynamics for the first four variables in the 0–20 cm layer on both Ferralsols and Acrisols but only 25% for organic C. These functions showed a very quick reaction to forest conversion for exchangeable calcium, available P and organic C which maxima are reached at the end of the first year. Soil reaction and bulk density showed significant changes somewhat later, 2.5 to 3.5 years. The relaxation times of soil chemical properties were much shorter than those for bulk density. The two sampling approaches showed some differences in absolute values but quite similar trends. The simpler synchronic approach can thus be used in studies of soil dynamics.