The increasing use of engineered nanoparticles (NPs) in industrial and household applications will very likely lead to the increased release of such materials into the public sewer systems. During the wastewater treatment process, some fraction of NPs would always be concentrated in the biosolids. When biosolids is applied on the agricultural land, NPs are introduced into the soil matrix. In the current study we investigate the influence of five different metal nanoparticles present in biosolids on soil microbial community as a function of time. Results indicate that ZnO and Zero Valent Cu NPs were not toxic to soil bacterial community. Biosolids mixed with Ag NPs and TiO2 (both anatase and rutile phase) in contrast changed the bacterial richness and composition in wavering pattern as a function of time. Based on the observations made in the study, we suggest caution when interpreting the toxicity of NPs based on single time point study. (C) 2014 Elsevier B.V. All rights reserved.