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The increasing production and use of engineered silver nanoparticles (AgNP) in industry and private households are leading to increased concentrations of AgNP in the environment.An ecological risk assessment of AgNP is needed,but it requires understanding the long term effects of environmentally relevant concentrations of AgNP on the soil microbiome.Hence,the aim of this study was to reveal the long-term effects of AgNP on soil microorganisms.The study was conducted as a laboratory incubation experiment over a period of one year using a loamy soil and AgNP concentrations ranging from 0.01 to 1 mg AgNP/kg soil.The short term effects of AgNP were,in general,limited.However,after one year of exposure to 0.01 mg AgNP/kg,there were significant negative effects on soil microbial biomass (quantified by extractable DNA;p =0.000) and bacterial ammonia oxidizers (quantified by amoA gene copy numbers;p =0.009).Furthermore,the tested AgNP concentrations significantly decreased the soil microbial biomass,the leucine aminopeptidase activity (quantified by substrate tover;p =0.014),and the abundance of nitrogen fixing microorganisms (quantified by nifH gene copy numbers;p =0.001).The results of the positive control with AgNO3 revealed predominantly stronger effects due to Ag+ ion release.Thus,the increasing toxicity of AgNP during the test period may reflect the long-term release of Ag+ ions.Nevertheless,even very low concentrations of AgNP caused disadvantages for the microbial soil community,especially for nitrogen cycling,and our results confirmed the risks of releasing AgNP into the environment.