Do Deep Tree Roots Provide Nutrients to the Tropical Rainforest?

The contribution of deep tree roots to the nutrition of a tropical rainforest were studied along an edaphic transect in French Guyana. Soil types were mapped in relation to the texture of the upper horizons and the depth of occurrence of the loamy saprolite. The position of mature individuals of four common species, differing by they rooting depth, was identified and tree leaves were analysed for major nutrients and strontium (Sr) isotopic ratios. On average, the range of leaf isotopic ratio (87Sr/86Sr = 0.714-0.716) was narrow compared to that of bulk soils (87Sr/86Sr = 0.72-0.77). Steep gradients of increasing 87Sr/86Sr in roots with soil depth were found in all investigated profiles, which indicated that the flux of Sr deposited in rain and leached from the litter layer was tightly retained in the upper soil layers. Over the whole of the site, as well as within each soil unit, tree 87Sr/86Sr ratios were very similar whatever the species, and close to litter and near-surface roots 87Sr/86Sr ratios, suggesting no or very little Sr contribution from deep tree roots. Variations of Ca and Sr concentrations in leaves were strongly correlated but not with leaf 87Sr/86Sr ratios. These results support the theory that Sr and Ca uptake and cycling are mostly superficial in tropical rainforests.