Biomineralizations: An important source of elements to the soil solution

BORRELLI, N.; M. OSTERRIETH Y J. MARCOVECCHIO

Mar del Plata, Buenos Aires, Argentina

Congreso; 7th Internatinal Meeting on Phytolith Research – 4th South American Meeting on Phytolith Research; 2008

Society for Phytolith Research-Grupo de Estudios Fitolíticos Aplicados del Cono Sur- Instituto de Geología de Costas y del Cuaternario

Soil minerals are the main sources of elements at the geological time scale, however biomineralizations are important sources of elements to the soil solution at the ecological time scale. Phytoliths (SiO2.nH2O,hydrated amorphous silica) and calcium oxalate crystals (C2O4Ca, COC) are the most common biomineralizations in higher plants. Studying the weathering processes of these biomineralizations is important because silica and calcium are key elements in the formation of organo-mineral complexes necessary for the soil matrix conformation. The aim of this work is to determine the contribution of silica (SiO2) and calcium (Ca2+) to the solution, from plant biomineralizations and soil samples from Typic Argiudolls with different vegetation cover, through in vitro experimentation. We worked with samples of: 1) vegetation: pieces of leaves cleared with sodium hypochlorite (50%) of: a) Dactylis glomerata (Poaceae), phytolith producer; b) Acacia melanoxylon (Fabaceae: Mimosoidea) and Eucalyptus globulus (Myrtaceae), COC producers; c) Celtis tala (Rosaceae), phytolith and COC producer; 2) soil samples (Hz. A, Hz. B, Hz. C) of Typic Argiudolls with different vegetal cover: a) grasses (P1), b) acacia and tala (P2), c) eucalyptus and tala (P3); and 3) volcanic ashes (20000 ± 7000 years BP). On the soil samples, heavy liquid separation was realized with sodium polytungstate (ä=2.3 g/cm3) to separate the amorphous silica fraction (phytoliths + volcanic ashes) from the heavy fraction of the soil. Three subsamples were exposed to different treatments for 6 months: 1) control (distilled water), 2) acid solution (pH2, glycine buffer), 3) solution similar to the natural soil conditions (pH6, phosphate buffer), 4) basic solution (pH12, phosphate buffer). The biomineralizations weathering state was analyzed with an optical microscope and scanning electron microscope, and the SiO2 and Ca2+ solution content was determinated by UV-VIS spectrophotometry. Regarding the phytoliths, the basic treatment produced the weathering of D. glomerata biomineralizations. However, this was not reflected in the solution, since this treatment showed the lowest silica content, which could be attributed to methodological errors. In general, amorphous silica fraction produced higher silica contents (14.05-2558.38mg/L) than the rest of the soil minerals (11.06-1176.65mg/L) and, within the amorphous silica fraction, phytoliths contribution was higher than the volcanic ashes (16.59-313.36mg/L). Regarding the COC, acid treatment produced the dissolution of the biomineralizations and the largest calcium release into the solution (18.73mg/L). Celtis tala (12.38-65.86mg/L) and Eucalyptus globulus (14.65-53.98mg/L) leaves were the ones that made the greatest calcium contribution, with values significantly higher than the soil minerals (0.29-20.72mg/L). The results show that phytoliths and COC produce larger contributions of silica and calcium to the soil solution than the soil minerals; therefore, they represent important sources of these elements and have an important role in the soil biogeochemistry.