Amorphous Silica Biomineralizations in Plants and Soils, and their Role in the Biogeochemistry of Silicon in the Southeast of the Pampean Plain, Argentina.

OSTERRIETH, MARGARITA; BORRELLI, NATALIA; ALVAREZ, M FERNANDA; FERNÁNDEZ HONAINE, MARIANA; DEL RÍO, LUIS; GOMES COE, HELOISA; MADELLA, MARCO

Colorado

Encuentro; 8th International meeting on phytolith research; 2011

PaleoResearch Institute (PRI), Golden, Colorado

Amorphous silica biomineralizations (silicophytoliths) are commonly produced in the plant communities that have predominated in the Pampean Plains during Cenozoic. The transference from these biotic systems to soils in these temperate- wet Pampean environments is considered as a conditioning of the silica biogeochemical processes. The silicon biogechemistry, the second most abundant element in the world, are scarcely known in continental environments, and particularly in Pampean Plain from Argentina. The study included Molisols, Fluvisols of continental environments, ponds and groundwaters. Soils were analyzed to different scales (mega, meso, micro and submicroscopic), with especial emphasis in mineralochemical determinations. The presence of silicophytoliths and their weathering degree in plants, soils, palaeosoils and sediments were quantified through routine techniques. The concentration of silica was determined in soil solution, temporary ponds, in Los Padres wetland and its tributaries (Los Padres stream, inflow stream; and La Tapera stream, outflow stream), and in groundwaters. The results indicated that the values of silica content ranged from 2 and 18% (dry weight) for the most abundant monocotyledons of the region, while the values for dicotyledons ranged between 1 and 4%. The content of silicophytoliths in epipedons of Molisols is high, with values of 59.6 x 103– 103.5 x 103 Kg/ha. The concentration of SiO2 in soil solution and its distribution along profile is variable depending on the present and past plant communities, in relation to root development and the nutritional requirements of the species. In plots planted with Acacia sp. and Eucalyptus sp., the concentration of SiO2 in the soil solution of the Mollisols decreases with depth: from 1106 to 406, whereas in plots with grass cover silica concentration increases with depth: 421 to 777 µmol/L. The grasses are silica accumulator that have a shallow root design so the absorption from the soil solution is higher at the surface, which could explain the lower silica content in epipedons containing them. The average concentration of silica in surface waters is 60 µmol/L. In Los Padres wetland, the silica content is 19.1 to 917 µmol/L, being higher in autumn-winter and decreasing in spring because of  the diatom bloom. In the inflow stream, values increase (660-917,5 umol/L) in relation to silica concentration in the soil solution and groundwater, where the average value is 840 umol/L).   The contribution of sílicophytoliths by grasses communities has been ongoing during the Cenozoic in the pampean plain, and has increased with agricultural and livestock handling of the last 150 years. These balances show that much of the silicon/amorphous silica recirculated in the unsaturated zone, where it contributes to form matrix of aggregates enriched of amorphous silica, which increases and maintains the structural stability of soils; and part goes to the saturated zone and associated waters.