Nutrient cycling in a clonal stand of Eucalyptus and an adjacent savanna ecosystem in Congo

In this highly-weathered soil, atmospheric deposits and symbiotic N fixation by a legume species balanced the nutrient budgets in savanna, despite large losses during annual burnings. After afforestation, weeding in the Eucalyptus stands eliminated the leguminous species responsible for a N input by symbiotic fixation of about 20 kg ha−1 year−1. Whereas the budgets of P, K, Ca and Mg were roughly balanced, the current silviculture led to a deficit of about 140 kg N ha−1 in the soil, throughout a 7-year rotation. This deficit was large relative to the pool of total N in the upper soil layer (0-50 cm), which was about 2 t ha−1. Therefore, the sustainability of Congolese plantations will require an increase in N fertilizer inputs over successive rotations to balance the N budget. These results were consistent with field trials of fertilization. Practical consequences of these budgets were identified, in order to: (i) direct field trials of fertilization, (ii) select appropriate methods of soil preparation, weed control and harvest, (iii) highlight the importance of fire prevention in this area, and (iv) support the implementation of field trials aiming at introducing a biological nitrogen fixing understorey in Eucalyptus stands.