Silicon isotope fractionation in bamboo and its significance to the biogeochemical cycle of silicon

A systematic investigation on silica contents and silicon isotope compositions of bamboos was undertaken. Seven bamboo plants and related soils were collected from seven locations in China. The roots, stem, branch and leaves for each plant were sampled and their silica contents and silicon isotope compositions were determined. The silica contents and silicon isotope compositions of bulk and water-soluble fraction of soils were also measured. The silica contents of studied bamboo organs vary from 0.30% to 9.95%. Within bamboo plant the silica contents show an increasing trend from stem, through branch, to leaves. In bamboo roots the silica is exclusively in the endodermis cells, but in stein, branch and leaves, the silica is accumulated mainly in epidermal cells. The silicon isotope compositions of bamboos exhibit significant variation, from -2.3 parts per thousand to 1.8 parts per thousand, and large and systematic silicon isotope fractionation was observed within each bamboo. The 63 Si values decrease from roots to stem, but then increase from stem, through branch, to leaves. The ranges of 63 Si values within each bamboo vary from 1.0 parts per thousand, to 3.3 parts per thousand. Considering the total range of silicon isotope composition in terrestrial samples is only 7 parts per thousand, the observed silicon isotope variation in single bamboo is significant and remarkable. This kind of silicon isotope variation might be caused by isotope fractionation in a Rayleigh process when SiO2 precipitated in stem, branches and leaves gradually from plant fluid. In this process the Si isotope fractionation factor between dissolved Si and precipitated Si in bamboo (alpha(pre-sol)) is estimated to be 0.9981. However, other factors should be considered to explain the decrease of 63 Si value from roots to stem, including larger ratio of dissolved H4SiO4 to precipitated SiO2 in roots than in stem. There is a positive correlation between the 63 Si values of water-soluble fractions in soils and those of bulk bamboos, indicating that the dissolved silicon in pore water and phytoliths in soil is the direct sources of silicon taken up by bamboo roots. A biochemical silicon isotope fractionation exists in process of silicon uptake by bamboo roots. Its silicon isotope fractionation factor (alpha(bam-wa)) is estimated to be 0.9988. Considering the distribution patterns of SiO2 contents and 63 Si values among different bamboo organs, evapotranspiration may be the driving force for an upward flow of a silicon-bearing fluid and silica precipitation. Passive silicon uptake and transportation may be important for bamboo, although the role of active uptake of silicic acid by roots may not be neglected. The samples with relatively high 63 Si values all grew in soils showing high content of organic materials. In contrast, the samples with relatively low 63 Si values all grew in soil showing low content of organic materials. The silicon isotope composition of bamboo may reflect the local soil type and growth conditions. Our study suggests that bamboos may play an important role in global silicon cycle. (c) 2008 Elsevier Ltd. All rights reserved.