Soil fertility enhancement by planted tree-fallow species in the humid lowlands of Cameroon

Four shrub/tree species, Alchornea cordifolia, Pennisetum purpureum, Chromo laena odorata, and Calliandra calothyrsus were evaluated for their potentia l contribution to soil fertility restoration after two years fallow. Standi ng biomass, root distribution, nutrient content in the biomass, decompositi on and nutrient release patterns, and association with mycorrhizae were the evaluation parameters. Alchornea and Pennisetum produced thehighest above- ground biomass, 66 t and 54 t/ha respectively. Pennisetum had more than 19 t/ha of root, 92% of which was in the 20 cm top soil. Alchornea had 74% of it roots in this soil layer, mostly as coarse roots while Calliandra had a deeper root system. Alchornea fallow accumulated more N and Ca, and Pennise tum fallow, more K than others, and mycorrhizae were mostly associated with Alchornea roots. The ranking of the different species for the decompositio n rate was: Chromolaena > Pennisetum > Calliandra = Alchornea. Also release of nutrients during decomposition followed the order K > N > Ca. Alchornea and Pennisetum could be recommended as green manure species especially whe n high quantities of material are needed for weed or erosion control. Calli andra and Chromolaena, because of the flush of nutrient during early mass, loss can be used as mulch when the crop demand of nutrient is high. Alchorn ea decomposed slowly and therefore could be used to improve Chromolaena mul ch, thus contributing to the build up of soil organic N and providing both short- and long-term nutrient release.