Most studies on the safety of engineered nanoparticles (ENP) have neglected the long term effect of ENPs as they enter the environment. These Duke studies should lead to more similar studies:
http://www.merid.org/Content/News_Services/Nanotechnology_and_Development_News/Articles/2010/08/11/Silver_Beware_Antimicrobial_Nanoparticles_in_Soil_May_Harm_Plant_Life.aspx
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0057189
https://scholars.duke.edu/display/pub747389https://scholars.duke.edu/display/pub747389
We should also take care to ensure that once a silver ENP has chemically changed, that its effects on root growth and micro-organism health has truly been mitigated. I don't find any studies tracking long term effects, that is, over several seasons.