Silicophytoliths and silicon biogeochemical cycle in the Pampean Plain, Argentina

Abstract

The biogeochemistry cycle of silicon, the second most abundant element in the world, is scarcely known in continental environments and particularly in Argentina. We present a synthesis and revision of our work about the silica biogeochemical cycle in the southeast of the Pampean Plain, Argentina; with especial emphasis in the role of amorphous silica biomineralizations (silicophytoliths). Silicophytoliths are commonly produced in grassland species that have dominated the Pampean Plains during all the Cenozoic. The transfer of amorphous silica from these biotic systems to soils and surface-groundwaters in temperate and aquatic Pampean environments is considered as a condition for the silica biogeochemical processes. This chapter includes the study of plant communities, Mollisols and Fluvisols soils from dry environments, ponds and groundwaters. Soils were analyzed at different scale (mega, meso, micro and submicroscopic), with special emphasis in mineralochemical determinations (petrographic and scanning electron microscopies and EDAXS). The presence of silicophytoliths and their weathering degree were assessed through routine techniques in plants, soils, palaeosoils and sediments. The concentration of silicon was determined in the soil solution, in Los Padres wetland and its tributaries, and in groundwaters. The silicophytolith content represents about 1-18% dry wt. in monocotyledons and 1-4% dry wt. in dicotyledons. The silicophytolith content was higher and the content decreased from the surface (40%) to the subsurface levels (5%) of soils. The weathering of silicophytoliths is an ongoing process in these soils. The balances show that part of the silicon/amorphous silica, moves to the saturated zone and associated waters; but much of the silicon/biogenic amorphous silica re-circulates in the unsaturated zone, where it contributes to form amorphous silica enriched matrix of soil aggregates, which increases and maintains the structural stability of soils. Finally, we concluded that the silicophytoliths affect the terrestrial silica biogeochemical cycle in the Pampean Plain of Argentina.