Carbon, nitrogen and phosphorus allocation in agro-ecosystems of a West African savanna. II. The soil component under semi-permanent cultivation

The assessment of carbon (C), nitrogen (N) and phosphorus (P) in agro-ecosystems of West African savannas (WAS) may be a useful tool to define sustainable intensification schemes needed to respond to the rapid increase in local populations as well as global change issues. Changes in soil properties, and particularly in the soil organic matter status, under semi-permanent cultivation were thus examined through a groundnut crop-fallow chronosequence in southern Senegal. The effect of fallowing was mainly restricted to the 0-20 cm soil layer and hardly affected soil physical properties. In this layer, steady improvements were recorded for Mg and Ca contents. Carbon and N amounts increased by 30%, and by 50% for available P (P(OD)) within the very first year of fallow and then remained steady (C and N) or dropped back to levels recorded for crops (P(OD)). The rapid initial change in organic status after crop abandonment was attributed to fast recovery of woody vegetation. The steady soil organic matter (SOM) content in oldest fallows compared to young fallows probably resulted from poor protection of soil organic matter from oxidation during biological activity. This hypothesis was confirmed by mesh-bag experiments, which indicated that >40-60% of decaying woody root biomass disappeared after 6 months of in situ incubation. In fallow systems in southern Senegal, soil fertility may in fact rely at least as much on fast organic matter cycling in soil food webs as on SOM build-up. Carbon storage in the soil-plant system of mature fallow ecosystems was only 27 t C ha(-1) higher than in crops and consisted mainly of pools with fast turnover. Consequently, the potential of semi-permanent cultivation for C sequestration in the WAS will be rather indirect, by a shift to more intensified practices, thus avoiding the conversion of dense forests to cropping in more humid areas.