Variations of plant and Soil 87Sr/86Sr along the slope of a tropical inselberg

From the summit downslope a granitic inselberg in French Guiana, soils and vegetation evolve from bare granite covered by cyanobacteria, to a savannah-type vegetation on thin patchy sandy accumulations, then to a low forest on shallow young soils and to a high forest on deep highly weathered ultisols. We have used element budgets and Sr isotopic variations in soils and plants to investigate the mineral nutrient supply sources of the different plant communities. Granite and atmospheric deposition have Sr-87/Sr-86 ratios of 1.3 and 0.71, respectively. The Sr-87/Sr-86 ratio of cyanobacteria (0.72) suggests granite weathering by cyanobacteria crusts. The Sr-87/Sr-86 ratio of the savannah-type vegetation is 0.73 and varies between 0.75 and 0.76 in the low and high forest leaf litter regardless of soil depth, age and degree of impoverishment. These almost constant ratio suggest that forest Sr comes from rainwater and from the summit of this inselberg, where it is released and redistributed along the slope, by surface flow, lateral redistribution of litter, and mineral particles. However, because of its very low content in the rock and soils, Ca is supplied to plants by atmospheric deposition.