Bioturbation of Ag2S-NPs in soil columns by earthworms

Sewage sludge contains Ag 2 S-NPs causing NP exposure of soil fauna when sludge is applied as soil amendment. Earthworm bioturbation is an important process affecting many soil functions. Bioturbation may be affected by the presence of Ag 2 S-NPs, but the earthworm activity itself may also influence the displacement of these NPs that otherwise show little transport in the soil. The aim of this study was to determine effects of Ag 2 S-NPs on earthworm bioturbation and effect of this bioturbation on the vertical distribution of Ag 2 S-NPs. Columns (12 cm) of a sandy loamy soil with and without Lumbricus rubellus were prepared with and without 10 mg Ag kg −1 , applied as Ag 2 S-NPs in the top 2 cm of the soil, while artificial rainwater was applied at ∼1.2 mm day −1 . The soil columns were sampled at three depths weekly for 28 days and leachate collected from the bottom. Total Ag measurements showed more displacement of Ag to deeper soil layers in the columns with earthworms. The application of rain only did not significantly affect Ag transport in the soil. No Ag was detected in column leachates. X-ray tomography showed that changes in macro porosity and pore size distribution as a result of bioturbation were not different between columns with and without Ag 2 S-NPs. Earthworm activity was therefore not affected by Ag 2 S-NPs at the used exposure concentration. Ag concentrations along the columns and the earthworm density allowed the calculation of the bioturbation rate. The effect on the Ag transport in the soil shows that earthworm burrowing activity is a relevant process that must be taken into account when studying the fate of nanoparticles in soils. Earthworm bioturbation plays a more important role than rainfall in the vertical transport of Ag 2 S-NPs in soil.