Nitrogen cycling in tree plantations grown on a poor sandy savanna soil in Congo

Fast growing #Eucalyptus$ trees are grown in Congo for industrial purposes, on a very poor sandy savanna soil. The sustainability of such land use is questionable, particularly regarding nutrient availability. Among the lacking nutrients, nitrogen (N) deserves special attention as N status is supposed to be improved by N-fixing trees. To assess N changes in tree plantations, N cycling was studied in #Eucalyptus$ and #Acacia$ stands and compared to the native savanna. Nitrogen accumulation in vegetation and soil, together with N flows in the ecosystem, were estimated. It was assumed that the difference between N amounts in #Acacia$ stands and in #Eucalyptus$ stands was an estimate of the amount of N fixed symbiotically in the former. The results showed that 800-1000 kg N/ha were accumulated during 7 years in #Acacia$ stands. Nitrogen fixation was higher in #Acacia auriculiformis$ than in #Acacia mangium$. Nitrogen cycling through litter fall was high in #Acacia$ stands, up to 170 kg/ha per year, and low in #Eucalyptus$ and #Pinus$ stands. However, in #Eucalyptus$ stands, slow litter decay and reduced N release from decaying litter resulted in a relative accumulation of organic N in the forest floor. Decrease in N content was observed in organomineral fractions under the trees. In top-soil (0-10 cm) N mineralization was higher in tree stands than in savanna and total N decreased significantly under 7-year-old #Eucalyptus$. Under older #Eucalyptus$, decreasing N content of organic matter was shown by the increase in C/N ratio. Under #Acacia$, soil N increase was significant in the older stands studied but not in the younger ones, and fixed N accumulated in trees and in the forest floor first. The implications of these observations are discussed. (Résumé d'auteur