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 59.6×10 3 to 103.5×10 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, SiO 2 content in soil solution is higher (406-1,106 μmol/L) and decreases with depth, while in the grass plot, SiO 2 content is lesser (421-777 μmol/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 μmol/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.

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Amorphous silica accumulation in grasses is widely described in numerous species; however, work done in relation to the factors affecting this accumulation in the tissues and different type of cells, is still incipient. Here, we analysed a perennial tussock grass, Cortaderia selloana (Schult. & Schult.f.) Asch. & Graebn. (Pampas grass), a native and widely spread species from South America, which is also considered an invasive plant in many regions of the world. We analysed the relation between silicification process, soil type and environment, and anatomical features. Silicophytolith content and distribution in plants was analysed through calcination and staining techniques and SEM-EDAX analyses. Silicophytolith content was higher in leaves (2.48–5.54% DW) than in culms (0.29–0.43% DW) and values were similar to other tussock grasses. A relationship between the habit of a grass species and the potential maximum content of amorphous silica is suggested. Plants grown in soils from modified environments with high silica content (Anthrosol and Tecnosol) produced a higher content of amorphous silica. In leaves, the distribution of silicified cells was conditioned by leaf xeromorphic features. Soil Si content and environmental conditions (radiation, disturbance) are more relevant in silicification process than phenological factors. Within leaves, the accumulation of amorphous silica is conditioned by anatomy and transpiration.

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