Biogeochemistry of an afrotropical montane rain forest on Mt. Kilimanjaro, Tanzania

Abstract

In contrast to their well-studied counterparts in the Neotropics and in Asia, East African montane rain forests are surrounded by semi-arid savanna plains. These plains have a high erosion potential for salt crusts accumulated at the soil surface. Hence it may be hypothesized that East African montane forest ecosystems experience strongly enhanced nutrient inputs via dry deposition, which alters their overall biogeochemistry. The aim of our study was to test this hypothesis by investigating K, Mg, Ca, Na and N-forms in rainfall, throughfall, fine litter, litter percolate and soil solution of a montane rain forest at Mt. Kilimanjaro. Four forest plots situated at elevations between 2250 and 2350 m asl on the south-western slopes of Mt. Kilimanjaro were studied for 2 y. In contradiction to our hypothesis, inputs of K, Mg, Ca and Na via rainfall (7.5, 0.9, 2.3 and 6.2kg ha⁻¹y⁻¹) and throughfall (35, 2.0, 3.5 and 11kg ha⁻¹⁻¹) were low on Mt. Kilimanjaro. Fluxes of NH₄-N and NO₃-N were within the range observed at other montane rain forests, with NO₃-N being the only nutrient partly absorbed in the forest canopies (2.9kg ha⁻¹y⁻¹ in rainfall, 0.9kg ha⁻¹y⁻¹ in throughfall). The highest overall nutrient concentrations in water samples occurred in litter percolate (1.4mg l⁻¹ K, 0.3mg l⁻¹ Mg, 0.8mg l⁻¹ Ca, 0.3mg l⁻¹ NH₄-N, 0.9mg l⁻¹ NO₃-N), with values still being low compared to other sites. Nutrient concentrations in seepage water strongly declined with increasing soil depth. Thus, both inputs and losses of base cations from the forest by water pathways are assumed to be low. N or P limitation of growth is not expected since high fluxes of N and P in fine litter (119 and 5.9kg ha⁻¹y⁻¹ for N and P respectively) indicate low within-stand efficiency.