This study, in the Journal of Environmental Monitoring, focused on the effect of titanium dioxide and zinc oxide nanoparticles on wheat growth and soil enzyme activities under field conditions.  According to the authors, the increased use of nanoparticles has raised concerns about their possible harmful effects within the environment, but most studies have been in aqueous systems.  This study found that both of the nanoparticles reduced the biomass of wheat.  The TiO2 nanoparticles stayed in the soil for long periods and primarily adhered to the cell walls of wheat.  The ZnO nanoparticles dissolved in the soil, enhancing the uptake of the nanoparticles by the wheat.  Significant changes were also induced in soil enzyme activities.  These enzymes are bioindicators of soil quality and health, and soil protease, catalase, and peroxidase activities were all inhibited in the presence of nanoparticles, although urease activity was unaffected.  The authors conclude: “The nanoparticles themselves or their dissolved ions were clearly toxic for the soil ecosystem.”

http://pubs.rsc.org/en/Content/ArticleLanding/2011/EM/c0em00611d