Effects of Silver Nanoparticles on Soil Ammonia-Oxidizing Microorganisms Under Temperatures of 25 and 5℃

The excellent bactericidal performance of silver nanoparticles(Ag NPs) has led to their wide applications, resulting in increasing concerns about their potential environmental impacts. This study evaluated the influences of different concentrations of Ag NPs(0,1, 10, and 100 μg g^(-1) dry soil) on the ammonia-oxidizing microorganisms in soil at cultivation temperatures of 25 and 5℃ for 37 d. The results showed that 1 μg g^(-1) dry soil of Ag NPs had no acute effects on the ammonia-oxidizing microorganisms. However,10 and 100 μg g^(-1) dry soil of Ag NPs levels were found to significantly inhibit the activities of soil nitrification, with a decrease of69.89% and 94.55%, respectively, at 25℃ and 61.65% and 83.79%, respectively, at 5℃ compared to the control(0 μg g^(-1) dry soil of Ag NPs). These levels of Ag NPs also obviously decreased soil urease activity from about 380.47 ± 0.07(at 5℃) and 529.76± 13.44(at 25℃) mg N g^(-1) dry soil d^(-1) to 61.70 ± 2.97 and 68.29 ± 8.22 mg N g^(-1) dry soil d^(-1), respectively, after 37 d of cultivation. Quantitative polymerase chain reaction showed the abundance of ammonia-oxidizing archaea and bacteria. For the same exposure time, the effects of Ag NPs on the activities of ammonia-oxidizing microorganisms and urease decreased with decreasing temperature. The threshold concentration of Ag NPs that induced negative effects on ammonia-oxidizing microorganisms was higher at 5℃ than at 25℃. Therefore, the temperature has a major impact on the toxicity of Ag NPs to ammonia-oxidizing microorganisms and on the urease activity, with toxicity being reduced with decreasing temperature.

The excellent bactericidal performance of silver nanoparticles （Ag NPs） has led to their wide applications, resulting in increasing concerns about their potential environmental impacts. This study evaluated the influences of different concentrations of Ag NPs （0, 1, 10, and 100 μg g^-1 dry soil） on the ammonia-oxidizing microorganisms in soil at cultivation temperatures of 25 and 5 ℃ for 37 d. The results showed that 1μg g^-1 dry soil of Ag NPs had no acute effects on the ammonia-oxidizing microorganisms. However, 10 and 100μg g^-1 dry soil of Ag NPs levels were found to significantly inhibit the activities of soil nitrification, with a decrease of 69.89% and 94.55%, respectively, at 25 ℃ and 61.65% and 83.79%, respectively, at 5℃ compared to the control （0 μg g^-1 dry soil of Ag NPs）. These levels of Ag NPs also obviously decreased soil urease activity from about 380.47 ± 0.07 （at 5℃） and 529.76 ± 13.44 （at 25℃） mg N g^-1 dry soil d^-1 to 61.70 ± 2.97 and 68.29 ± 8.22 mg N g^-1 dry soil d^-1, respectively, after 37 d of cultivation. Quantitative polymerase chain reaction showed the abundance of ammonia-oxidizing archaea and bacteria. For the same exposure time, the effects of Ag NPs on the activities of ammonia-oxidizing microorganisms and urease decreased with decreasing temperature. The threshold concentration of Ag NPs that induced negative effects on ammonia-oxidizing microorganisms was higher at 5 -C than at 25 -C. Therefore, the temperature has a major impact on the toxicity of Ag NPs to ammonia-oxidizing microorganisms and on the urease activity, with toxicity being reduced with decreasing temperature.