, Volume 60, Issue 1, pp 97-118

Do deep tree roots provide nutrients to the tropical rainforest?

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The contribution of deep tree roots to the nutrition of a tropicalrainforest were studied along an edaphic transect in French Guyana. Soil typeswere mapped in relation to the texture of the upper horizons and the depth ofoccurrence of the loamy saprolite. The position of mature individuals of fourcommon species, differing by they rooting depth, was identified and tree leaveswere 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 ofincreasing 87Sr/86Sr in roots with soil depth were foundin all investigated profiles, which indicated that the flux of Sr deposited inrain and leached from the litter layer was tightly retained in the upper soillayers. Over the whole of the site, as well as within each soil unit, tree87Sr/86Sr ratios were very similar whatever the species,and close to litter and near-surface roots 87Sr/86Srratios, suggesting no or very little Sr contribution from deep tree roots.Variations of Ca and Sr concentrations in leaves were strongly correlated butnot with leaf 87Sr/86Sr ratios. These results support thetheory that Sr and Ca uptake and cycling are mostly superficial in tropicalrainforests.