Nutrient cycling in a clonal stand of Eucalyptus and an adjacent savanna ecosystem in Congo -: 1.: Chemical composition of rainfall, throughfall and stemflow solutions

The dynamics of nutrients was compared in a clonal Eucalyptus plantation and in a native savanna in Congo. This paper focuses on changes in the precipitation chemistry during the transfer of solutions aboveground in both ecosystems. Average rainfall during the rainy and the dry seasons was 151 and 7 mm per month, respectively. Chemical analyses performed monthly over a 3-year period showed that the concentration of all the elements in rainfall increased sharply during the dry season. Wet depositions were dominated by the cations Na+, N-NH4+, H+ and the anions Cl-, S-SO42-, N-NO3- and amounts of dry depositions of Ca2+, Cl- and H+ were highly variable depending on the characteristics of the collector. Throughfall and stemflow were enriched in most of the elements, but N foliar uptake was observed in both stands. Clear differences in behaviour between the savanna and the Eucalyptus stand were evidenced. Whereas the enrichment of solutions through the foliage mainly made up of dry depositions in the plantation, foliar leaching processes were much greater in the savanna, particularly for K+, Mg2+ and P-H2PO4-. Stemflow solutions followed the same pattern as throughfall solutions. They were greatly enriched for most elements but N uptake also occurred. The low availability of K+ in the soil might account for the particularly low levels of foliar leaching of this element in the Eucalyptus stand compared to values reported for most forest ecosystems. The time course of concentrations of elements in rainfall, throughfall and stemflow provided relevant data to distinguish external inputs from foliar leaching in the enrichment of solutions through both stands. This qualitative information will be helpful to establish input-output budgets of nutrients in these ecosystems. (C) 2002 Elsevier Science B.V. All rights reserved.