Influence d'une substitution d'espece sur le fonctionnement biogeochimique de l'ecosysteme forestier. L'exemple du cycle du soufre

The substitution of oak by spruce brings about changes in site factors, which determine nutrient element absorption. After 50 years a coniferous stand modifies the biogeochemical cycle. The element sulphur is often neglected, although it is important in nutrition ; the N/S equilibrium is involved in proteosynthesis. Sulphur is present in the external input to the ecosystem. The differential filter effect of conifers greatly modifies the S input in the ecosystem and as a consequence, the soil function, by acidification. A comparative study of the biogeochemical cycle is being carried out in the Ardennes (N.E. of France) in a traditional deciduous forest (oak) and in a coniferous stand (Picea abies). The spruce was planted 50 years ago after the clearfelling of a part of the deciduous forest. The soil is an acid brown earth (« sol brun acide .). Biomass and mineral content have been determined in the two stands, as well as the soil reserve. The concentration of elements and the flux have been evaluated using lysimeter techniques (plates without tension) in the different parts of the ecosystems (fig. 1). The flux of water in the soil was evaluated by calculating the drainage, using the TURC equation for evapotranspiration. The dry deposit of sulphur was evaluated by the Miller method (1984). The results are as follows : the mean annual concentration curves in the different parts of the ecosystems (fig. 3) show an increase in the S concentration from the wet input (precipitation above the canopy) to the (B) level in the soil. Seasonal curves show that aximum concentrations occur in winter. The species effect is clear. Under spruce, concentrations are twice the values under oak, with the greatest difference in the throughfall.Input of sulphur to the ecosystems by wet only deposit is high, about 32.2 kg - ha -1. y -1 which is characteristic of a polluted area ; 45 %, of this input occurs in winter. The value of dry deposit input in the deciduous ecosystem in about 6.1 kg - h -1. y -1. The filter effect of the Norway spruce canopy is very high : 23.5 kg - ha-1. y-1 of dry deposit S has been evaluated for this coniferous ecosystem. Figure 4 summarizes the current immobilization in the wood and the flux of S in the two ecosystems. Output by immobilization in the biomass is low, about 5 kg - ha -1. y -1, and it is similar in the two systems ; the fluxes by litterfall are similar too. The flux of soluble S in gravitational water shows large differences between the two ecosystems. The soil in the deciduous ecosystem is losing S sulphur, probably from the pyrite in the bedrock. Conversely, under the coniferous ecosystem, there appears to be a relative accumulation of S in the soil.The soil reserve of soluble sulphur (S) has increased by about l0 kg - ha -1. y-1 since the spruce plantation. The difference in flux between the two ecosystems seems to be the consequence of dry deposit in the coniferous stand. The mechanism of adsorption under spruce and desorption (and/or weathering) under deciduous trees could be related to the chemical properties of the Al complex in this range of soil pH.