| Summary: | The nanoparticles of TiO<sub>2</sub> (TiO<sub>2</sub> NP) have been used as a plant-growth stimulant or catalyst in pesticide formulas. However, due to high resistance of TiO<sub>2</sub> NP to abiotic weathering, dissolved Ti is unlikely to act as an active compound in these preparations. Even if soil is acidic, TiO<sub>2</sub> NP do not dissolve easily and preferably remain as undissolved particles. The low dissolution rates of inorganic nanoparticles in the soil environment make Ti in TiO<sub>2</sub> NP largely unavailable for plants and soil microorganisms. To characterize the behavior of TiO<sub>2</sub> NP in soil under different pH conditions, we analyzed TiO<sub>2</sub> NP-size distribution in two soil materials, an alkaline and acidic one. We also cultivated <i>Aspergillus niger</i>, a fungus ubiquitously found in soils, in the growth medium spiked with TiO<sub>2</sub> NP to assess accumulation of the nanoparticles in fungus. In soil suspensions, the dissolved Ti was present in low concentrations (up to 0.010 mg L<sup>−1</sup>). Most of the TiO<sub>2</sub> NP remained in particulate form or appeared as aggregates sized 100–450 nm. In experiment on Ti accumulation by <i>A. niger</i>, TiO<sub>2</sub> NP either settled down to the bottom of the flask with growth medium or were actually accumulated by the fungus; about 7.5% of TiO<sub>2</sub> NP were accumulated in fungal mycelia. Most of the TiO<sub>2</sub> NP remain in particulate form in soil solutions, regardless of soil pH. Filamentous fungus <i>A. niger</i> has the ability to accumulate bioavailable TiO<sub>2</sub> NP, which hints at the possibility that some soil fungi can affect spatial distribution of this type of nanoparticles in soils.
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