HR: 0800h
AN: V51A-0520    [Abstracts]
TI: Silicon Isotope Fractionation by Banana Under Continuous Nutrient and Silica Flux
AU: * Opfergelt, S
EM: opfergelt@sols.ucl.ac.be
AF: Universit\'{e} Catholique de Louvain, Croix du Sud, 2/10, Louvain-la-Neuve, 1348 Belgium
AU: Cardinal, D
EM: damien.cardinal@africamuseum.be
AF: Mus\'{e}e Royal de l'Afrique Centrale, Chauss\'{e}e de Louvain, 13, Tervuren, 3080 Belgium
AU: Henriet, C
EM: henriet@sols.ucl.ac.be
AF: Universit\'{e} Catholique de Louvain, Croix du Sud, 2/10, Louvain-la-Neuve, 1348 Belgium
AU: Delvaux, B
EM: delvaux@sols.ucl.ac.be
AF: Universit\'{e} Catholique de Louvain, Croix du Sud, 2/10, Louvain-la-Neuve, 1348 Belgium
AU: Andr\'{e}, L
EM: luc.andre@africamuseum.be
AF: Mus\'{e}e Royal de l'Afrique Centrale, Chauss\'{e}e de Louvain, 13, Tervuren, 3080 Belgium
AB: Silicon is absorbed by plants as aqueous H$_{4}$SiO$_{4}$ with other essential nutrients, and precipitates in aerial parts of the plant as phytolith, a biogenic opal. Phytoliths are restored to the soil by decomposition of organic debris from plant material. The role of higher plants in the biogeochemical cycle of silicon is therefore major although it is still poorly studied. Biomineralization processes are known to fractionate the three stable silicon isotopes with a preferential uptake of light isotopes. Therefore, following some preliminary results from Douthitt (1982), and studies presented in recent conferences (Ziegler et al., 2002; Ding et al., 2003), we suspect that phytolith production by plants could also fractionate the silicon isotopes. Inversely, intensity of phytolith-related isotopic fractionations might contribute to a better understanding of the soil-plant silicon cycle. Our study focused on banana, a silicon accumulating plant ($>$1% Si, dry weight).{\it Musa acuminata} cv Grande Naine has been grown in hydroponics under controlled conditions (light, temperature, humidity, nutrients) during six weeks. The nutrient supply was kept constant: three batches of five plants were grown with a continuous nutrient solution flow of 5, 50 and 100 ppm SiO$_{2}$ respectively. Si isotopic compositions were measured in the source solution, and in silica extracted from the various parts of banana (roots, pseudostems, midribs and petioles, leaves), using a Nu Plasma multicollector mass spectrometer (MC-ICP-MS) operating in dry plasma mode. The results are expressed as $\delta^{29}$Si relatively to the NBS28 standard, with an average precision of $\pm$ 0.03$\permil$. Silicon contents and morphological studies of phytoliths were also achieved. Banana $\delta^{29}$Si varied between -0.18 and -0.76$\permil$ with a source solution at -0.02$\permil$. Values of $\delta^{29}$Si were less fractionated, relatively to the nutrient solution, in roots, where no phytoliths have been observed until now, than in upper parts of banana where phytoliths were clearly abundant as long chain of typical cone shaped morphotypes truncated saddle-like. The bulk isotopic composition of the leaves in the five plants grown at 100 ppm SiO$_{2}$ displayed a homogeneous negative signature (-0.44 $\pm$ 0.08$\permil$) indicating a small inter-specimen variability. The difference between $\delta^{29}$Si in roots and in upper parts of the plant was much larger with a silica offer of 100 ppm SiO$_{2}$ (0.58$\permil$) than with 50 ppm SiO$_{2}$ (0.08$\permil$). However, silicon isotope fractionation in leaves was not affected by a change in Si supply. Our preliminary results show that biomineralization of silica in bananas fractionates silicon isotopes in a similar extent as marine diatoms.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1615 Biogeochemical processes (4805)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting