THE EFFECT OF SPRUCE (PICEA-ABIES KARST) ON SOIL DEVELOPMENT - AN ANALYTICAL AND EXPERIMENTAL APPROACH

Changes in soil and biogeochemical processes following the replacement , 60 years ago, of part of a deciduous forest by a coniferous stand we re evaluated by (i) a balance-sheet approach using soil and biomass el ement content analyses and the mineral flux, and (ii) the inclusion of test-mineral and resin bags in an integrative experimental approach. The comparison of soils under different stands shows that the change t o spruce is inducing physical, chemical and biological modifications i n soil properties. Changes in the humus layers lead to a reduction in mineralization and the mobilization of active organic acids in the sys tem. Under spruce, the soil structural stability is reduced as fine cl ay is dispersed. Soil acidification is increased, linked to a decrease in soil pH and desaturation of the soil exchange complex. Soil minera logy also indicates this tendency, particularly with an increase in th e stability of Al intergrades. The biogeochemical cycle is modified es pecially for N, S and Al, with large changes in the internal fluxes be tween the ecosystem components, but the modification of the input-outp ut balance due to spruce introduction is large only for S. The acidifi cation is partly due to a change in litter quality, which inhibits bio logical activity, and partly because, during the dormant season, everg reen species intercept 'dry and occult' depositions from the atmospher e containing acidic or potentially acidic components. Use of test-mine ral bags improves understanding and interpretation of the current soil mechanisms. The test mineral, a vermiculite, introduced into the soil or placed in the lysimeter flux very quickly undergoes measurable tra nsformation characterized by desaturation and fixation of non-exchange able Al in the interlayer zone, easily identified by Al speciation. Th e acidifying effect of spruce is clearly demonstrated by the experimen tal approach used, including the characterization of soil solutions by resin bags. All the techniques are complementary and could be used to gether or individually, depending on the aim of the study.