Pedosphere 31(6): 934--943, 2021
ISSN 1002-0160/CN 32-1315/P
©2021 Soil Science Society of China
Published by Elsevier B.V. and Science Press
Impacts of silver nanoparticles on enzymatic activities, nitrifying bacteria, and nitrogen transformation in soil amended with ammonium and nitrate
Xiaohong LIU, Juan WANG, Lingli WU, Li ZHANG, Youbin SI
Anhui Province Key Laboratory of Farmland Ecological Conservation and Pollution Prevention, School of Resources and Environment, Anhui Agricultural University, Hefei (China)
ABSTRACT
      Silver nanoparticles (AgNPs) are effective antimicrobial compounds that are used in a myriad of applications. Soil microorganisms play crucial roles in nitrogen cycling, but there is a lack of comprehensive understanding of the effects of AgNPs on enzymatic activity in the nitrogen cycle, nitrifying bacteria, and nitrogen transformation in soil. Herein, enzyme activities were determined following the addition of different forms of nitrogen, ammonium nitrogen ((NH4)2SO4), nitrate nitrogen (KNO3), and amide nitrogen (urea, CO(NH2)2) at 200 mg N kg-1, into the soil amended with AgNPs at 0, 10, 50, and 100 mg kg-1. After 7 d of incubation with 10 mg kg-1 AgNPs, the activities of urease, nitrite reductase (NiR), nitrate reductase (NaR), and hydroxylamine reductase (HyR) were reduced by 12.5%, 25.0%, 12.2%, and 24.2%, respectively. Of particular note, more than 53.5%, 61.7%, and 34.7% of NaR, NiR, and HyR activities, respectively, were inhibited at 100 mg kg-1AgNPs. The abundance (most probable number) of ammonia- and nitrite-oxidizing bacteria (AOB and NOB, respectively) was measured using real-time quantitative polymerase chain reaction (qPCR) and the Cochran method. The abundance of AOB and NOB decreased when AgNPs were present in the soil. The NH4NO3 amendment increased copy numbers of bacterial and archaeal amoA nitrification functional genes, by 38.3% and 12.4%, respectively, but AgNPs at 50 mg kg-1 decreased these values by 70% and 56.4%, respectively. The results of 15N enrichment (atom% excess) of NH4+ and NO3- experiments illustrated the influence of AgNPs on soil nitrogen transformation. According to the 15N atom% excess detected, the conversion of 15N-labeled NH4+ to NO3- was significantly inhibited by the different levels of AgNPs in soil. The reduced gross nitrification rate further confirmed this finding. Overall, this study revealed considerable evidence that AgNPs inhibited nitrogen cycle enzyme activity, the number of nitrifying bacteria, the abundance of the amoA gene, and the gross nitrification rate. Silver nanoparticles inhibited nitrogen transformation, and the rate of nitrification was also negatively correlated with AgNP levels.
Key Words:  amoA,gross N transformation,nitrification gene,nitrogen cycling,nitrogen isotope fractionation,soil enzyme activities
Citation: Liu X H, Wang J, Wu L L, Zhang L, Si Y B. 2021. Impacts of silver nanoparticles on enzymatic activities, nitrifying bacteria, and nitrogen transformation in soil amended with ammonium and nitrate. Pedosphere. 31(6): 934–943.
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