Aboveground and belowground biomass, productivity and nutrient accumulation in tree improved fallows in the dry tropics of Cameroon

The accumulation of biomass and nutrients by three tree fallows planted in a previously cultivated ferruginous soil in the Sudanian zone of Cameroon were compared with that of a natural grazed herbaceous fallow and a natural ungrazed herbaceous fallow. The planted tree fallows, that were protected against fire and grazing, included: Acacia polyacantha, Senna siamea, and Eucalyptus camaldulensis. After 5 years of fallow, mean aerial woody biomass production rates ranged from 5.30 t ha-1 per year for Acacia and 5.73 t ha-1 per year for Eucalyptus to 3.81 t ha-1 per year for Senna; furthermore, the total dry matter (DM) accumulation (root, shoot and litter) of tree fallows represented 2.5-4 times that of the herbaceous fallows. Regarding root mass, these proportions are increased to 5-7 times for the three tree species studied. Acacia had higher N and Mg contents in total DM than the other treatments while Eucalyptus had the greatest accumulation of Ca. Acacia and Eucalyptus had similar accumulation of total DM, P and K. Senna was not so productive as the other tree species but it accumulated a large quantity of K. Over a 7-year period, Acacia, a N-fixing species, accumulated in its total DM, 340 and 412 kg N ha-1 more than Eucalyptus and Senna (non-N-fixing species) did, respectively. In the context of intensive harvest of aerial biomass, the accumulation of nutrients in the belowground components is a process of high importance. The N accumulation in the Acacia root mass (342 kg ha-1) represented roughly the total N accumulation in Eucalyptus or Senna stands. In Eucalyptus the root system accounted for only 18-27% of the accumulation of total DM and nutrients, while much greater proportions were found under Senna (39-52%) and Acacia (32-47%). Thus, the harvesting methods of the aerial biomass will play an important part in the conservation of the nutrients of the system, especially in the case of Eucalyptus. Nutrient removal by harvesting under different logging scenarios was evaluated. For Eucalyptus, the technique of de-barking on-site reduced the removal of Ca and Mg by 70 and 43%, respectively. Furthermore, the results show that in the long term, in comparison to removal of timber at the end of the rotation, annual harvesting of grass for fodder in tree fallows in the Sudanian zone of Africa, would be an equally important or even a more serious risk to site fertility. (Résumé d'auteur