Silica biogeochemical cycle in temperate ecosystems of the Pampean Plain, Argentina

OSTERRIETH, MARGARITA; BORRELLI, NATALIA; ALVAREZ, FERNANDA; FERNÁNDEZ HONAINE, MARIANA

Antwerp

Congreso; 6th International Congress of Chemistry and Environment; 2013

The knowledge and understanding of silica biogeochemistry is important for the global biogeochemical cycles, since it is linked to the global carbon cycle by marine organisms. Although there are several studies focused on the marine silica cycle, the interest in the terrestrial silica cycle has recently begun to increase, since much of the reactive Si reaching the oceans has undergone prior biological cycling on the continents. In Argentine, the soils of the Pampean Plain, Typical Argiudolls, are one of the most fertile soils of the world and they have been used for agricultural activities for more than 150 years. The pristine vegetation predominant in the study area across the Quaternary was grasslands. The grasses, along with Cyperaceae and Arecaeae, are one of the major producers of silicophytoliths. Silicophytoliths are particles of hydrated amorphous silica (SiO2.nH2O) formed in the cells of living plants, who deposit solid silica in an intracellular or extracellular location after absorbing silica in a soluble state from the soil solution and groundwater. The transfer of amorphous silica from these biotic systems to soils in these temperate Pampean environments is considered as a condition for the silica biogeochemical processes. The aim of the study is to define and quantify the presence of amorphous silica biomineralizations in soils with pristine and arboreal vegetation, silica content in soil solution, surface and groundwaters. We worked with Hz. A, Hz. B and Hz. C of Typical Argiudolls with different vegetal cover: grasses, Acacia melanoxylon plantation, Eucalyptus globulus plantation, developed in Los Padres Basin, Argentine. Soils were analyzed at different scale (mega, meso, micro and submicroscopic), with special emphasis in mineralochemical determinations. The percentage of phytoliths with respect the total mineralogical components of the soil and the phytolith content (Kg/Ha) was calculated. The concentration of silica was determined in the soil solution, surface waters (Los Padres Pond and Los Padres Stream, the only inflow stream) and in groundwaters. In the grasses plot, phytoliths represent about 40% (Hz.A), 10% (Hz. B), 5% (Hz. C) of the total mineralogical components of the soil. These values represent about 6-12% of phytolith (%wt) and 2,5-5% of Si from phytolith (%wt) in the soils. In the epipedons (Hz. A) the stock of phytolith is about 59-72x103 Kg/ha, and the stock of Si from phytolith is 25-30x103 Kg/ha. In the arboreal plots, phytolith content is very representative too. In these plots, phytoliths represent about 40-60% (Hz.A), 20% (Hz. B) of the total mineralogical components of the soil; and these values represent about 9-13% of phytolith (%wt) and 3.5-5.5% of Si from phytolith (%wt) in the soils. In the epipedons (Hz. A) the stock of phytolith is about 83-105x103 Kg/ha, and the stock of Si from phytolith is 34-43x103 Kg/ha. The concentration of SiO2 in soil solution and its distribution along the profile is variable and relates to the present and past plant communities, the 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 µmol/L), whereas in plots with grass cover silica concentration increases with depth (from 421 to 777 µmol/L). Grasses are silica accumulators that have a shallow root system so the absorption from the soil solution is higher at the surface, and this could explain the lower silica content in epipedons with grass vegetation. In these soils, inorganic soil minerals show no features or weak features of weathering in Ca?Na feldspars and K feldspars, and there is no evidence of argilization processes. Within the amorpous silica fraction, silicophytoliths show different weathering degreeswhich is evident in the important percentage of unidentified silicophytoliths (10-40%) observed in the soil assemblages; whereas volcanic shards show a hyaline surface with no features of weathering and in some portions, there are evidences of coatings that could prevent their dissolution. So, phytoliths could play an important role in the silica cycle being an important sink and source of Si in these soils. 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). These balances show that much of the silicon/amorphous silica re-circulates in the unsaturated zone, where it contributes to form amorphous silica enriched soil solutions. Part of the silicon/amorphous silica moves to the saturated zone and associated surface waters, contributing to the biological silica cycle.