Soil organic carbon dynamics under Eucalyptus and Pinus planted on savannas in the Congo

In the Congo, Eucalyptus and Pinus have been planted commercially for 30 yr on savanna. Large differences in the abundance of carbon isotopes of the savanna Gramineae and the trees enabled a study to be made of the soil organic matter (SOM) turnover by means of its carbon isotope composition. The organic carbon content slightly increased with plantation age without any significant differences (90% confidence limit) between plantation types. The delta 13C of SOM varies with the plantation age from -14.4 per thousand, in the savanna reference plot to values close to -26.5 per thousand in the oldest tree plantations. The distribution of delta 13C as a function of plantation age was modelled by an exponential function and exhibited no significant variation (90% confidence level) between plantation type. As the relative contribution of organic matter inherited from the savanna and that derived from the tree species was proportional to the delta 13C value, the fraction of each organic compartment was evaluated for plantations grouped together without distinguishing between the different tree species. With time, the SOM inherited from the savanna tended to disappear. The half-life was estimated to be 16.5 yr, clearly illustrating the rapid SOM turnover which characterizes the savanna ecosystems in psammitic ferrallitic soils. The fraction of organic C of tree origin progressively introduced into the soils was linearly related to time. The annual increment of organic C was found to be equal to 0.3 mg C g-1 yr-1, but it was clear that the linear evolution of carbon with time could not be extrapolated over more than 30 yr. The various models predicted that after 30 yr there would remain in the soil, 1.8 and 0.8 mg C g-1 of savanna origin and 9.4 mg C g-1 of newly introduced organic C of tree origin respectively.