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Nanomaterials 2020, 10, 1763 12 of 24 several authors have concluded that the toxic effects on microbial communities are highly dependent on their concentration in the soil [194–198]. However, most studies have evaluated NPs at higher levels than actually occur in nature [199–201]. Dinesh et al. stated that NPs might have an impact on the soil in different ways. Firstly, a direct effect could be attributed to their properties, such as their antimicrobial activity, which might reduce soil microbiota mostly by generating ROS. In the second place, NPs could induce changes in toxins and nutrients bioavailability. Finally, the indirect effects might result from their interaction with natural organic toxic compounds, thus enhancing their toxicity [191]. In addition to that, different studies have proven that AgNPs affect microorganisms that promote plant growth and nutrient cycling in soils, such as rhizobacteria, Pseudomonas fluorescens, Pseudomonas putida, and others. A major concern is related to the inhibition of denitrifying bacteria, where studies have reported that at concentrations of 100 mg of AgNPs/kg of soil, can be observed a complete reduction of these colonies without recovery signals and with the consequence of the reduced conversion of nitrates to nitrogen [191,202]. NPs can affect other soil organisms, such as earthworms. Unrine et al. determined AuNPs bioavailability from soil to Eisenia fetida. They reported that NPs sizes ranging from 20–55 nm did not influence distribution among tissues in contrast to the ones smaller than 20 nm, which were more available. In addition to that, AuNPs could cause adverse effects on earthworm reproduction [203]. Bourdineaud et al. also used E. fetida to evaluate the transfer of AuNPs and AgNPs from soil to the earthworm tissues. In this study, the invertebrate was exposed to soil containing 2, 10, and 50 mg of NPs/kg of soil for 10 days. Both NPs showed similar transfer coefficients and induced the onset of oxidative stress that caused DNA modifications even at the lowest evaluated concentration [204]. Another study by Schlich et al. evaluated the effects of AgNPs on Eisenia Andrei earthworm reproduction. Ag uptake from the NPs was slightly higher compared to AgNO3 , where both substances showed similar toxicities in the reproduction test. However, Ag uptake was not reported to inhibit reproduction [205]. In contrast, Ploeg et al. described the reduction in the reproduction of Lumbricus rubellus earthworms influenced by AgNPs, which also caused mortality upon long-term exposure [206]. In addition to NPs concentration in soil, ecotoxicological studies should take into consideration dissolution rate, size, surface area, electric charge, and their surface chemistry since these control NPs’ stability and their transport, giving relevant and more accurate outcomes about their toxicity. Furthermore, dispersibility is also considered as a key factor; agglomerates of NPs have shown less toxicity compared to well-dispersed forms. A way of promoting that is by using biodegradable polymers as stabilizing agents obtained through green methods, such as chitosan, wood cellulose, gelatin, among others [207]. Moreover, the behavior of NPs within soil systems is influenced by the presence of surface charged components, such as clay, which alters their association with the solid phase. This interaction between NPs and solid surfaces controls their transport along with the soil, which is influenced by environmental conditions and the NPs’ physicochemical properties. Particle retention in pore soils is especially relevant to the bigger NPs, whereas smaller particles tend to move more freely and can penetrate to reach groundwater [208]. Meier et al. presented the concern that anthropogenic activities could disrupt soil ecosystems, resulting in the reduction of its microbial health. In order to evaluate the previous, they exposed freshly collected sandy loam soil to solutions ranging from 0–2000 mg/kg of AgNPs. After that, they expanded traditional soil microbial analysis with genomics-based tests through the measure of alterations in community taxonomic structure and function using 16S-rDNA profiling and metatranscriptomics. The research group found that AgNPs affected bacterial taxonomic structure, as well as genes involved in heavy metal resistance. Besides, their presence induced some toxicity response pathways to become highly upregulated [209]. In Canada, AgNPs are employed for direct application in soils to reduce the utilization and degradation of conventional pesticides, provide micronutrients, increase crop yields, and control plant pathogens. Regarding that, Asadishad et al. evaluated the effects of AgNPs on the soil microbiota and enzyme activity of agricultural soils at different concentrations (1, 10, or 100 mg of AgNPs/kg of soil)PDF Image | Green Synthesis of Gold and Silver Nanoparticles from Plant Extracts
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