LOCALIZATION OF MINERAL UPTAKE BY ROOTS USING SR ISOTOPES

To assess the contribution of deep soil horizons to the mineral supply of trees, we investigated the natural variation in the Sr-87/Sr-86 is otopic ratio of plant-available strontium with soil depth. In three si tes of North-western Spain, this ratio increased with soil depth. The comparison of isotopic ratios of tree leaves and roots at different de pths showed that most of the Sr accumulation in Eucalyptus globulus an d Pinus pinaster growing on shallow and poor soils in this rainy clima te originated from the upper soil layers. As Ca and Sr behave similarl y in the soil-plant system, this conclusion can be applied to Ca. This superficial uptake is attributed to the low availability of Sr and Ca in the soil as well as to the shortness of the drought period as comp ared to the length of the growth period. This technique appears to off er a promising way of studying relative root distributions in soils an d plant competition for nutrients.