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(-1) 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(-1) 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.