Variation des rapports 87Sr/86Sr dans les sols et les plantes d’un inselberg tropical

Abstract

Abstract• 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 87 Sr/ 86 Sr ratios of 1.3 and 0.71, respectively. The 87 Sr/ 86 Sr ratio of cyanobacteria (0.72) suggests granite weathering by cyanobacteria crusts. The 87 Sr/ 86 Sr 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. • Du sommet vers la base d'un inselberg granitique (Nouragues, Guyane Française), les sols et la végétation évoluent depuis des savanes sur des ilots sableux entre les affleurements rocheux couverts de cyanobactéries, vers une forêt basse sur sols peu épais, riches en minéraux altérables puis une forêt haute sur sols très profonds et altérés. Mots• Les variations isotopiques du strontium des sols et des plantes ont été mesurées pour rechercher les sources de nutriments des différentes communautés végétales.• Les rapports 87 Sr/ 86 Sr du granite et des dépôts atmosphériques sont respectivement de 1,3 et 0,71. Le rapport 87 Sr/ 86 Sr des cyanobacteries (0,72) suggère une libération de Sr par altération du granite. Le rapport 87 Sr/ 86 Sr de la savanne est de 0,73 et varie entre 0,75 et 0,76 dans les litières de forêt basse et haute, quelle que soit la profondeur, et la richesse en minéraux altérables des sols.• La faiblesse et l'homogénéité surprenante de ces rapports suggèrent une alimentation en Sr des forêts essentiellement à partir de dépôts atmosphériques et des sols de la partie haute de l'inselberg, via des écoulements de surface, des redistributions latérales de litière et de particules minerales lors de crises érosives. Cependant, en raison de l'extrême pauvreté de la roche et des sols en calcium, le Ca des communautés végétales provient de la pluie.

Impact of agricultural practice on the Sr isotopic composition of food products: Application to discriminate the geographic origin of olives and olive oil

“…In fact, in contrast to lighter stable elements, Sr isotopic ratios are theoretically not affected by kinetic and equilibrium fractionations during low temperatures and biologic processes, due to their high atomic mass and small mass difference Faure and Mensing 2005). Only a few data outlined very discrete Sr isotope fractionation in soil environments (Fietzke and Eisenhauer, 2006;Halicz et al, 2008;Shalev et al, 2013) and during uptake by plants (De Souza et al, 2010 Compilation of literature data focuses on two main sources for Sr supply, one derived from bedrock weathering and the second from exogenous atmospheric input (Graustein and Armstrong, 1983;Graustein, 1989;Miller et al, 1993;Stewart et al, 1998;Vitousek et al, 1999;Probst et al, 2000;Whipkey et al, 2000;Poszwa et al, 2004Poszwa et al, , 2009Reynolds et al, 2012;Hartman and Richard, 2014;Clauer and Semhi, 2016). Recently, an additional potential source has been suggested in the context of forest ecosystems:…”

“…The atmospheric source of Sr, that is to say Sr that is brought by precipitation or wind to a vegetable, is considered as major for some authors (Graustein and Armstrong, 1983;Graustein, 1989;Fichter et al, 1998;Chadwick et al, 1999;Probst et al, 2000;Poszwa et al, 2004Poszwa et al, , 2009Maurer et al, 2012;Hartman and Richard, 2014). Atmospheric source material can be of natural continental or marine origin (precipitation, dust…) or of anthropogenic origin (sprayed fertilizer, pollutant particles, …), and can be used by a plant either through its roots or its leaves (Clauer and Semhi, 2016).…”

“…Even, when reaching the soil, these deposits can constitute an important nutrient source for plants. Shallow rooting plants (annuals and perennials) that utilize water and nutrients from upper soil layer are thus more likely to be affected by atmospheric deposition of bioavailable strontium than deep rooting plants (Poszwa, et al, 2004(Poszwa, et al, , 2009Hartman and Richards, 2014). It is thus assumed that atmospheric contribution gradually decreases with soil depth Vitousek et al, 1999;Hartman and Richards, 2014).…”

Arbuscular mycorrhizal community structure on co-existing tropical legume trees in French Guiana

“…The sampling was restricted to the Inselberg camp area (4°05'N; 52°41'W) in minimally disturbed tropical forest where two large sampling plots ('Grand Plateau' and 'Petit Plateau') have been delimited. These two plots have differing edaphic conditions: the Grand Plateau is based on metamorphic geology with more fertile clay-rich soils and the Petit Plateau is based on granitic geology with more sandy soils that are less fertile, although both are Ultisols (Poszwa et al 2009). The annual rainfall is around 2900 mm with a drier season from late August to early November.…”

Constraining soil mineral weathering 87Sr/86Sr for calcium apportionment studies of a deciduous forest growing on soils developed from granitoid igneous rocks

“…It is now recognized that the atmosphere, either from wet or dry deposition, is an important source of Ca in many settings. In fact, some forests seem to satisfy their Ca requirements almost entirely from atmospheric inputs (Kennedy et al, 1998;Drouet et al, 2005;Perakis et al, 2006;Poszwa et al, 2009;Bélanger and Holmden, 2010), whereas, in others, Ca nutrition is mostly supported by soil mineral weathering (Bailey et al, 1996;Vitousek et al, 1999;Blum et al, 2002;Bern et al, 2005). Some other forests depend on a mixture of these two primary Ca sources (Miller et al, 1993;Blum et al, 2002;Poszwa et al, 2004;Bélanger and Holmden, 2010).…”