Silicophytoliths: their role in the degradation and silicon biogeochemical cycle of Molissolls in the southesast of the Pampean Plains, Argentina.

OSTERRIETH, M.; DONNA, R.; FRAYSSINET, C.; BENVENUTO, M.L.; PAOLICCHI, M.; FERNANDEZ HONAINE, M.; BORRELLI, N.; ALTAMIRANO, S.M.; ALVAREZ, M.F.

Bengaluru

Congreso; 7th International Conference on Silicon in Agriculture; 2017

University of Agricultural Sciences-Bengaluru-India

The study area is located at the central-east part of the province of Buenos Aires, inside the so-called Chaco-Pampean Plain. The vegetation that covered the Pampas throughout the Late Cenozoic to have been mainly a grass steppe, which is one of the major producers of silicophytoliths. Silicophytoliths are amorphous silica biomineralizations deposited in plant tissues; following degradation of the tissue, the silicophytoliths are incorporated into the soil, where they can be preserved for thousands of years; but a high percentage of them are altered and disappear from the soil system. Thus, silicon is incorporated into the soil solution and the local hydrological system, forming part of the biogeochemical silicon cycle in humid temperate environments. The quantitative and qualitative analysis of the silicophytolith (status, type of alteration) in the most representative Molisolls under natural conditions and agroecosystem were performed. The quantity of silicophytoliths by the number per gram of soil and by ton per hectare in the different soil horizons was defined: In mollic epipedons (A horizon), we found 18 to 28,000,000 silicophytoliths per gram of soil, and 60 to 100 tons of silicophytoliths per hectare. In the B horizon, we found less than 3,000,000 silicophytoliths per gram of soil. In the parent materials, the C horizons gave 1,500,000 silicophytoliths per gram of soil. In these environments, there is a greater than 80% decrease in the average total content of silicophytoliths among the illuvial horizons with respect to the eluvial mollic horizons. The silica in the soil solution, moving from 453 to 1,243 mM/liter; in surface water oscillates between 100 and 1,000 /liter, according to whether the water comes from streams or ponds and the season of the year, rainfall and drought. And, in ground water with medium values of 840+ to -230 mM/liter. The agricultural (wheat, maize crops and pastures), of these areas are important providers of silicophytoliths. Studies show that the use and agricultural management of these soils generate a substantial loss of pelitic fractions. But, these fractions may have been reincorporated by these crops since more than 170 years ago. Also taphonomical, mineralo-chemical and chemical studies of the soil solution show that the soil matrix is enriched in amorphous silica from the chemical degradation of silicophytoliths. Therefore, silicophytoliths originally provided by the natural grasses and later reincorporated by the main crops into the system, compensate the losses of silt and very fine sand by wind and water erosion. The fluxes in the complex biogeochemical system of silicon in temperate humid grasslands, show that much of the silicon/amorphous silica re-circulate in the unsaturated zone, where they contribute to form an amorphous silica enriched matrix of aggregates, which increases and maintains the structural stability of soils. This work demonstrates the importance of silicophytoliths in the preservation of the physical and chemical properties of these productive soils, and the relevant role in the terrestrial silica biogeochemical cycle in the Pampean Plain of Argentine.