NUTRIENT DISTRIBUTION IN AN ACID BROWN SO IL DEVELOPED FROM A VOLCANIC TUFF BEDROCK (BEAUJOLAIS, FRANCE) - CONSEQUENCES FOR THE LONG-TERM FERTILITY

The study of soil fertility, often measured only by its short-term com ponent (ie, exchangeable cation reserves), appears insufficient and ca n mask different situations. This work shows that the middle and long- term fertility characterized by the nutrient content of minerals of th ree acid brown soils was very different although their exchangeable ca tion reserves were comparable. The middle and long-term fertility of t hese soils corresponds to reserves of nutrient elements in the fresh t uff. in the weathered tuff and in the coarse soil fractions originatin g from the desquamation of the weathered tuff. Differences in the fert ility of these three soils were due to the colluviation and to the int ense hydrothermalism that affected the very top layer of tuff and prov oked a major calcium depletion in the bedrock. Calcium appeared to be the first limiting factor for plant nutrition in this ecosystem. Only the flux of calcium originating from the dissolution of calcic primary minerals in the soil fractions (> 2 mu m) was able to maintain the te mporary reserve of this element on the adsorbing complex and to meet t he nutritional demands of the Douglas-fir stand. The stability of this ecosystem therefore depends mainly on the rate of calcium flux origin ating from minerals located in the coarse soil fractions, assuming tha t the organic matter compartment is stable.