Experimental in situ transformation of vermiculites to study the weathering impact of tree species on the soil

Weathering of soil minerals under forest seems to depend on the species pre sent. To study the influence of tree species we placed unweathered vermicul ites in the soil and assessed the impact in terms of saturation index of ea rth-alkaline cations and cation exchange capacity in 64 forest stands, domi nated by different species and growing side by side at 20 sites on acidic s oils. The vermiculites were of two types, one with a large charge and the o ther with a small charge. Minerals were maintained in the soil for 1 and 3 years. The minerals placed in the topsoil and in soils with low buffering capacity were more acidified and weathered than those deeper in the soil and in les s acid conditions. The vermiculites were transformed into hydroxylated inte rlayered vermiculites, and the formation of hydroxides in the interlayer sp ace decreased the vermiculites' cation exchange capacities. The high-charge vermiculite had a greater affinity for aluminium than the low-charge varie ty. The effect of tree species was significant but small compared with fact ors such as soil type, depth and duration of incubation. Nevertheless, we c an rank the acidifying and weathering caused by the trees in the following order: Picea abies, Abies alba >Pinus sylvestris, Pseudotsuga menziesii > Q uercus spp., Fugus sylvatica. This in situ experimental approach enabled us to study potential trends in pedogenesis in few years.