The nutrient budgets of a watershed and its forest ecosystem in the Tai National Park in Cote d'Ivoire

Atmospheric inputs and stream water outputs of P, K, Ca and Mg were estimated for an undisturbed forested watershed and the forest ecosystem within it in Tdi National Park, Cote d'Ivoire in 1990/91. The study included measurements of wet and dry deposition, and suspended sediments, organic debris and solutes in the water flows. Base flow as well as quick flow were sampled. The nutrient budgets of the entire watershed and the forest ecosystem (comprising vegetation and rooted soil layers) were distinguished on the basis of two assumptions (i) solutes in the base flow are derived from the soil layers below the rooted zone only, and hence not from the forest ecosystem; (ii) the total soil mass in the rooted zone remains constant, i.e. the export of topsoil material by erosion is compensated for by deepening the root zone. The first assumption was supported by the resemblance of the molar ratios of solutes in the base flow and those calculated for the weathering of the migmatite found in the soil layers below the rooted zone. It is concluded that the stocks of P, K, Ca and Mg in the watershed are decreasing with 1.4, 12.7, 15.3, and 8.1 kg ha(-1) yr(-1) respectively. Losses are mainly a result of nutrient exports by erosion and solutes in the base flow. Nutrient stocks for the forest ecosystem are also apparently decreasing, but to a much lesser extent, indicating the importance of distinguishing between the watershed and the forest ecosystem within.