Calcium biominerals in typical Argiudolls from the Pampean Plain, Argentina: An approach to the understanding of their role within the calcium biogeochemical cycle

Abstract

The aims of this work are: (a) to describe the presence of calcium oxalate crystals from fungus and vegetal origin; (b) to determine the contribution from leaves; and (c) to assess the role they could play within the calcium biogeochemical cycle, in typical Argiudolls with different vegetal cover in the Pampean Plain, Argentina. Two plots with different vegetation were investigated: Acacia melanoxylon– Celtis tala and Eucalyptus globulus– Celtis tala. Leaves of the vegetation species were diaphanized and cleared with sodium hypochlorite (50%). Leaves and mycelial mats of the organic horizons were analyzed by optical, petrographic and scanning electronic microscopy. Soil samples were taken to determine particle size distribution and mineralogy, and to test calcium concentration by UV–vis spectrophotometry both in the soils’ saturated paste extract and in leached soil. Different morphologies of weddellite (i.e., primary nuclei, rosettes and druses) in the fungus hyphae were observed. Calcium oxalate crystal production was highest in A. melanoxylon and C. tala. Ca–Na feldspars are the only minerals that could release calcium to the system, but show no features of weathering. Calcium concentration in soil solution was higher in the Eucalyptus plot than in the Acacia, and it decreased from the organic to the mineral soil horizons. Fungus and vegetal calcium biomineral production, in addition to their higher susceptibility to weathering than minerals of inorganic origin, represents an important contribution to the calcium biogeochemical cycle.