Mineral evolution at geosphere-biosphere interface: investigation on the endemic shrub Helichrysum microphyllum Cambess. subsp. tyrrhenicum Bacch., Brullo & Giusso growing in an abandoned mining area

Beyond the fundamental interaction between water and rock, the active role of living organisms in the formation of new minerals stable at the Earth's surface must be considered (Velde & Barré, 2009). Transformation of primary minerals caused by biological degradation is up to several orders of magnitude faster than transformation of minerals due to water-rock interaction. In the rhizosphere, root exudates and associated soil microorganisms promote mineral alteration, precipitation of new phases, and the transfer of macro- and micro-nutrients from minerals to plants (Cabala et al., 2004). The reported processes allow plant utilization in remediation actions through phytostabilization strategies. In our study we selected an endemic shrub, Helichrysum microphyllum Cambess. subsp. tyrrhenicum Bacch., Brullo & Giusso (hereafter H. tyrrhenicum), growing on a Zn-Pb ore lotation tailings pond (Campo Pisano mine, Sardinia, Italy). Quartz and dolomite were detected in the bulk soil and in the rhizosphere, whereas phyllosilicates and pyrite were found only in the rhizosphere. The bulk soil is characterized by the presence of goethite, gypsum and jarosite. Pyrite probably precipitates at the soil-root interface because of the synergistic action of roots and associated microorganisms thus controlling rhizosphere mineralogy. In H. tyrrhenicum roots we detected the same minerals of the rhizosphere, suggesting that minerals are embedded in plant tissues. Soil-to-plant transfer of Zn by H. tyrrhenicum was characterized by Zn accumulation in plant roots (Znroots 2900-4000 mg/kg). The Zn coordination environment changes from the rhizosphere (mainly hydrozincite, Zn sulfate and Zn acetate) to the plant roots (mainly Zn apatite) pointing out an active role of the physiological mechanisms of the plants in incorporation of Zn into the biological tissues and biomineralization (Medas et al., ?017). These inding strongly motivate future researches aimed to investigate the reaction mechanisms governing mineral evolution in the rhizosphere and the reaction kinetics in the presence or absence of mycorrhiza and soil microbes.