The biogeochemical cycle of aluminium : the effect of replacing oak forest (Quercus sessiliflora) by Norway spruce (Picea abies) The substitution of oak forest by Norway spruce induces changes in the physico-chemical properties of soil and in the dynamics of the biogeochemical cycle of elements. Aluminium is a good indicator of pedogenesis. Do conifers induce podzolisation in an acid brown earth soil (« sol brun acide ») ? '? In the French Ardennes, on a brown soil, after the substitution of

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... e species, the mull humus under the oak forest became a moder after 50 years of spruce cultivation. The consequence of the change on the soil chemical properties is an increase in exchangeable acidity (fig. 2) in the litter and the upper organic layer of the soil (A,). In relation to the increase of organic matter, the C.E.C. and the Al concentration are greater under spruce ( fig. 3) in the A, level ; but the acidity ratio Ai/C.E.C. remains the same. In figure 4 , the weathering complex shows higher levels of translocation of free Al under spruce, with adsorption in the (B) horizon. That is typical under the deciduous forest of a « sol brun acide », and under the spruce forest of a « sol brun ocreux ». The evolution of the soil towards the podzolic group due to conifer plantation has been verified by a study of flux and in particular, the Al flux in the ecosystems. Aluminium flux has been evaluated in the two ecosystems using lysimeter techniques. A comparison of Al distribution in the two ecosystems shows that no difference exists in the upper part of ecosystems (similar biological cycles) : current immobilization in biomass and flux by litterfall or throughfall are the same. The input of Al by wet or dry deposits are negligible. The differences in the processes are greater in the soil :