The intrinsic slow growth of nitrifying bacteria and their high sensitivity to environmental perturbations often result in cell growth inhibition by toxicants. Nanoparticles are of great concern to the environment because of their small size and high catalytic properties. This work sought to determine size-dependent inhibition by Ag nanoparticles and evaluate the relationship between the inhibition and reactive oxygen species (ROS). Nanoparticles with an average size range of 9-21 nm were synthesized by varying the molar ratios of BH4-/Ag+ in the solution. The resulting ROS generation was quantified in the presence and absence of the bacteria while the degree of inhibition was inferred from specific oxygen uptake rate measurements, determined by extant respirometry. By examining the correlation between nanoparticle size distribution, photocatalytic ROS generation, intracellular ROS accumulation, and nitrification inhibition, we observed that inhibition to nitrifying organisms correlated with the fraction of Ag nanoparticles less than 5 nm in the suspension. It appeared that these size nanoparticles could be more toxic to bacteria than any other fractions of nanoparticles or their counterpart bulk species. Furthermore, inhibition by Ag nanoparticles as well as other forms of silver (AgCl colloid and Ag+ ion) correlated well with the intracellular ROS concentrations, but not with the photocatalytic ROS fractions. The ROS correlations were different for the different forms of silver, indicating that factors other than ROS are also important in determining nanosilver toxicity.

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Aims:  To investigate antibacterial activities of zinc oxide nanoparticles (ZnO NP) and their mode of action against an important foodborne pathogen, Escherichia coli O157:H7. Methods and Results:  ZnO NP with sizes of 70 nm and concentrations of 0, 3, 6 and 12 mmol l−1 and NP‐free solutions were used in antimicrobial tests against E. coli O157:H7. ZnO NP showed increasing inhibitory effects on the growth of E. coli O157:H7 as the concentrations of ZnO NP increased. A complete inhibition of microbial growth was achieved at the concentration level of 12 mmol l−1 or higher. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy were used to characterize the changes of morphology and cellular compositions of bacterial cells treated with ZnO NP and study the mode of action of ZnO NP against E. coli O157:H7. The intensity of lipid and protein bands in the Raman spectra of bacterial cells increased after exposure to ZnO NP, while no significant changes in nucleic acid bands were observed. Conclusions:  ZnO NP were found to have antibacterial activity against E. coli O157:H7. The inhibitory effects increase as the concentration of ZnO NP increased. Results indicate that ZnO NP may distort and damage bacterial cell membrane, resulting in a leakage of intracellular contents and eventually the death of bacterial cells. Significance and Impact of the Study:  These results suggest that ZnO NP could potentially be used as an effective antibacterial agent to protect agricultural and food safety.

Biochar as a carbon-rich coproduct of pyrolyzing biomass, its amendment has been advocated as a potential strategy to soil carbon (C) sequestration. Updated data derived from 50 papers with 395 paired observations were reviewed using meta-analysis procedures to examine responses of soil carbon dioxide (CO 2 ) fluxes, soil organic C (SOC), and soil microbial biomass C (MBC) contents to biochar amendment. When averaged across all studies, biochar amendment had no significant effect on soil CO 2 fluxes, but it significantly enhanced SOC content by 40% and MBC content by 18%. A positive response of soil CO 2 fluxes to biochar amendment was found in rice paddies, laboratory incubation studies, soils without vegetation, and unfertilized soils. Biochar amendment significantly increased soil MBC content in field studies, N-fertilized soils, and soils with vegetation. Enhancement of SOC content following biochar amendment was the greatest in rice paddies among different land-use types. Responses of soil CO 2 fluxes and MBC to biochar amendment varied with soil texture and pH. The use of biochar in combination with synthetic N fertilizer and waste compost fertilizer led to the greatest increases in soil CO 2 fluxes and MBC content, respectively. Both soil CO 2 fluxes and MBC responses to biochar amendment decreased with biochar application rate, pyrolysis temperature, or C/N ratio of biochar, while each increased SOC content enhancement. Among different biochar feedstock sources, positive responses of soil CO 2 fluxes and MBC were the highest for manure and crop residue feedstock sources, respectively. Soil CO 2 flux responses to biochar amendment decreased with pH of biochar, while biochars with pH of 8.1-9.0 had the greatest enhancement of SOC and MBC contents. Therefore, soil properties, land-use type, agricultural practice, and biochar characteristics should be taken into account to assess the practical potential of biochar for mitigating climate change.

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