THE NUTRIENT INPUT BY HARMATTAN DUST TO A FOREST ECOSYSTEM IN COTE-DIVOIRE, AFRICA

During December and January, dry northeasterly surface winds (the Harm attan) distribute dust over West Africa. Rate of deposition and some c hemical and physical characteristics of Hammattan dust were measured i n Tdi National Park in the southwest corner of Cote d'Ivoire during th e 1990-1991 dry season. The dust deposition was estimated by the class ical water-filled basin method and by using canopy drip to account for deposition on tree canopies. Contamination by local biotic debris in both, water-filled basin and canopy drip collectors, was corrected for by using Ti (which appears to be wholly of atmospheric origin) as a r eference element. Harmattan dust in Tai consisted mainly of kaolinitic silt finer than that collected in North Nigeria, closer to the source area in the Chad basin. The estimates of seasonal deposition rates we re 33 to 47 kg ha(-1) for the water-filled basin method and around 80 kg ha(-1) for the canopy drip method. The higher value in canopy drip was in agreement with expected higher deposition of fine dust on the c anopies than on a water surface, and was therefore considered more rel iable to estimate nutrient inputs by Harmattan dust deposition. The se asonal nutrient input by dust was thus estimated to be 0.11 kg ha(-1) for P, 2.5 kg ha(-1) for K, 3.5 kg ha(-1) for Ca and 0.4 kg ha(-1) for Mg.