ORGANIC-MATTER QUANTIFICATION AND CHARACTERIZATION OF ALLUVIAL SOILS DURING VEGETATION EVOLUTION

In order to determine if the physical and chemical criterias of organic matter are in phase with the evolutive state expressed both by the soil morphology and by the present vegetation, forms of organic matter are identified and then quantified in the humus of 2 different series of alluvial soils (Fluviosols). One series is situated on mainly calcareous alluvia and the other on alluvia principally constituted of orthogneiss (figs I and 2; table I). A first fractionation, using the method of water sieving, is undertaken to separate and to estimate the particulate plant material from the best humified plant material. The latter is then selectively chemically extracted (fig 3; table III). Results show that the amount (stock in carbon and nitrogen) of organic matter is a better indicator (table II) than its quality, which virtually does not change in a calcareous environment and only slowly in acid conditions (tables IV and V). As compared with the vegetation dynamic, the slow humus evolution in these soils seems to reveal in less than 100 years a shift in the soil-vegetation relationship.