Role of the silicophytoliths and amorphpus silica in the degradation of soils in the southeast of Pampean Plain, Argentina

OSTERRIETH M; ALVAREZ, M. FERNANDA; BENVENUTO, M.L.; BORRELLI, N.; FERNÁNDEZ HONAINE, MARIANA

Antwerp

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

The Molisolls of the southeast of Pampean plains in Argentina, are one of the most fertile soils of the world and they have been used for agricultural activities for more than 200 years. These important socioeconomic sectors of the Pampean region have modified the physical, chemical and biological properties of the soils and have a negative environmental impact. The rol of the silicophytoliths (amorphous silica biomineralization of plants) in the matrix conformation which in turn modify the type, morphology and consistency of aggregates, their stability and response to different types of use, is little known. Also, the importance of the contribution of the plant silica biomineralizations of the most representative environments and agroecosystems of the Pampean region is scarce studied. The aim of the study is to define and quantify the presence of silica biomineralizations in the natural plant communities, in the most common crops of the area and in the horizons of the typical Argiudolls affected by agricultural practices. Lastly, the matrix of the aggregates is analyzed at microscopic and submicroscopic levels. The silica biomineralizations of plants and the aggregates of the horizonts of typical Argiudolls in natural and agricultural plots were analyzed by loupe, optical and petrographic microscopes, SEM and EDAXs analyses. In the study area the plant communities that have been developed through all the Cenozoic are represented by grasslands. Mean values of silica content of the natural plant communities range between 2 and 7% dry weight. The presence of silicophytoliths in the soils ranges between 3% (parental material) and more than 50% (mollic epipedons). In these last horizons the contribution of amorphous silica is relevant. The content of silicophytoliths ranges between 60 to 105 tn/ha and Si ranges between 25 and 45 tn/ha. These values are higher in the cultivated plots due to burning activities and the presence of crop residues. The matrix of the aggregates of strong pedal, is made up in turn by microaggregates with a recurring elemental composition, mainly consisting of carbon and silicon. As the use of these soils is higher, the proportions of these elements and the substructures of the organominerals complexes are modified. A higher land use leads to lower carbon content and to an increment of amorphous silica, along with calcium, iron, aluminum subordinates. The weathering of silicophytoliths is an ongoing process in these soils and contribute to the formation of amorphous silica-rich matrix of the aggregates and thus to maintain stability. Despite the intensity of tillage in cultivated soils, the negative effect would be mitigated by the high contribution of the same silicophytoliths.