Silica content in soil solution and its relation with phytolith weathering and silica biogeochemical cycle in Typical Argiudolls of the Pampean Plain, Argentina-a preliminary study

Purpose Little is known on the silica biogeochemical cycle in terrestrial environments. The aim of this work is to assess phytolith's role on the biogeochemical cycle of Si in Typical Argiudolls under different vegetation of the Pampean Plain, Argentina. Materials and methods The work has been developed in three plots with different vegetal cover: grasses and shelter-belt plantations of Acacia melanoxylon-Celtis tala and Eucalyptus globulus-C. tala. The heavy liquid separation in the soil samples was realized with sodium polytungstate. The silica concentration of the soil solution and groundwaters was determined by UV-VIS spectrophotometry. Results and discussion Acacia and eucalyptus do not produce phytoliths; instead, Dactylis glomerata (grass) is a silica-accumulating species, and their phytolith assemblage is composed basically by oblong and crenate, smooth elongate, rectangular and prickles within isolated phytoliths, and smooth long cells articulated. Phytolith content in soils decreases with depth. Total stock of phytoliths represents 5.9 to 12.9 wt.% and is higher in the arboreal plots. In the A horizons, phytolith fraction represents about 59.6 x 10(3) to 103.5 x 10(3) kg/ha. In these horizons, 90.7-94.4% of the phytolith content constitutes the labile pool and 9.3-5.6% the stable pool. In the arboreal plots, SiO2 content in soil solution is higher (406-1,106 mu mol/L) and decreases with depth, while in the grass plot, SiO2 content is lesser (421-777 mu mol/L) and increases with depth; probably because of differences in the nutritional requirements and root design between vegetal species, therefore, in the different depth uptake from the soil solution. In groundwaters, silica content is very high (932 mu mol/L). Conclusions Phytoliths are very representative in Typical Argiudolls and show a great degree of weathering so they could be into account in the biogeochemical studies since they could contribute with silica content in the soil solution, affecting the terrestrial silica biogeochemical cycle.