The Potential Mechanisms and Environmental Controls of TiO2 Nanoparticle Effects On Soil Bacterial Communities.

It has been reported that engineered nanoparticles (ENPs) alter soil bacterial communities, and reduce microbial biomass and enzyme activities, but the underlying mechanisms and environmental controls of such effects remain unknown. In the real soil matrix, besides direct toxicity, ENPs may indirectly affect soil bacteria by changing soil water and nutrient availabilities. Alternatively, environmental factors, e.g. soil water, may mediate ENP effects on soil bacterial communities by changing the interactions between ENPs and soil bacteria. To test these hypotheses, we incubated nano-TiO₂-amended soils across a range of water potentials through an isopiestic equilibration approach. Following a 9-month equilibration, the soil water characteristic, organic carbon (C), total C, total nitrogen (N), C/N ratio, and microbial activity were measured; bacterial community shifts were characterized through terminal restriction fragment length polymorphism (T-RFLP). We found that nano-TiO₂ altered the bacterial community composition, and reduced TRF diversity, but did not change the endpoint soil water characteristic and nutrient status. Furthermore, nano-TiO₂-induced community dissimilarities increased, but tended to approach a climax, when soils became drier, indicating that soil water mediates nano-TiO₂ effects on soil bacterial communities. Taken together, nano-TiO₂ effects on soil bacteria are likely due to direct toxicity which is affected by soil environmental factors. This study contributes to our understanding of the mechanisms of ENPs effects on soil bacterial communities, and highlights the complexity of such effects under varying environmental conditions.