N biogeochemistry and cycling in two well-drained savannas: a comparison between the Orinoco Basin (Llanos,Venezuela) and Ivory Coast (western Africa)

When a savanna burns, a decline in the input of organic matter and nutrients to the soil occurs. However, the existence of recurrent fires is a natural condition and N depletion by fire is not incompatible with the existence of savannas per se. Consequently, savanna vegetations have evolved under fires, implying a near to steady-state N budget. In some Australian and African ecosystems, N fixation appears to be insufficient to replace losses inducing soil-N depletion, whereas in neotropical Llanos and western African savannas, the N-fixation and precipitation seem to be enough to maintain production despite fires. This review presents information about well-drained savannas, namely Trachypogon savannas in Orinoco Llanos and Andropogonae savannas in Ivory Coast. The sites present similarities in climate and fire regime differing in soil parent material. A report on N budgets is presented. The budget was positive, since losses seem to be balanced by inputs in rainfall and biological fixation. Uncertainties in flux measures indicate that more work on those aspects are required. Results indicate that the estimated amounts of dinitrogen fixation and deposition seem to be enough to maintain plant production. Data support the hypothesis that the vegetation in the natural savanna has evolved under fire constraint.