Toxicological Effects Of Silver Nanoparticles With Different Concentrations And Particle Sizes On The Functional Traits Of Wheat(Triticum Aestivum L.)and Soil N-fixing Bacterial Communities In The Rhizosphere As Well As The Driving Mechanism

Currently,the ever-increasing environmental pollution has been became one of the most serious global environmental problems.The large amount and extensive use of nanomaterials has led to an increasing trend towards environmental pollution.Among the various nanoparticles,one of the most widely used one is silver nanoparticles?AgNPs?because of its diverse antimicrobial activities and other unique physiochemical properties.However,AgNPs may be released into the environments during the process of production,use,and disposal of AgNPs-containing products.Accordingly,AgNPs may trigger multiple effects on the structure and functions of ecosystems,especially the physiological ecology of plant species and the structure of soil microbial communities in the soil sub-system.Therefore,it has become one of the most key scientific issues in the field of environmental science that exploring and elucidating the environmental ecotoxicological effects and the driving mechanisms of AgNPs on the physiological ecology of plant species and the structure of soil microbial communities.In addition,nitrogen?N?is one of the most important nutrients needed for plant's growth,and the N nutrient required by plant species in nature ecosystems is mainly derived from the biological N-fixation mediated by soil N-fixing bacterial communities.Soil N-fixing bacterial communities can obviously improve the available N levels in the rhizosphere of Triticum aestivum L.which is one of the world's major crops.Therefore,the present study aims to insights into the toxicological effects of AgNPs with different concentrations and particle sizes on the functional traits of wheat?i.e.,plant height,leaf length,leaf width,leaf shape index,leaf chlorophyll concentration,leaf N concentration,single leaf wet weight,single leaf dry weight,specific leaf area,aboveground wet weight,and aboveground dry weight?and soil N-fixing bacterial communities in the rhizosphere as well as the driving plant physiological ecological mechanism and soil micro-ecological mechanism.The AgNPs solutions?with a gradient of particle sizes of 30 nm,50 nm,and 70 nm?and AgNO₃ solutions were thoroughly mixed with the soil substratum separately and the final concentrations?all based on the amounts of Ag ions?were set as a gradient levels,namely,low level:100 mg kg^–1?soil?,moderate level:200mg kg^–1?soil?,and high level:400 mg kg^–1?soil?],using distilled water as a control.In addition,a metagenomics approach based on high-throughput sequencing technology was used to comprehensively analyze the alpha and beta diversity of soil N-fixing bacterial communities.The main results of the present study are as following:1.Effects of AgNPs with different concentrations and particle sizes on functional traits of wheat:?1?AgNPs significantly decreased the leaf length,leaf width,and leaf chlorophyll and N concentrations of wheat,especially the plant height,which can notably reduce the competitive ability for resource acquisition?especially sunlight?of wheat.?2?The leaf length,single leaf wet weight,and aboveground wet weight of wheat were significantly lower under moderate concentration of AgNPs with 70 nm than those under moderate concentration of AgNO₃.Hence,the environmental ecotoxicity of AgNPs on wheat's functional traits may be more likely to be toxic from itself but not Ag ions.Ag ions can be complexed by the bound ligands?e.g.,chloride,phosphate,and sulfide?and can be absorbed by colloids and/or humic acid in the soil sub-system.?3?The leaf chlorophyll and N concentrations of wheat significantly decreased with increasing concentration of AgNPs with 30 nm.The major reason may be that AgNPs with higher concentration can release more Ag ions and then induce more production for the reactive oxygen species.?4?AgNPs with smaller particle size significantly exert greater environmental ecotoxicity on the leaf chlorophyll and N concentrations of wheat than those with larger particle size at the same concentration.The possible reason for this is that AgNPs with smaller particle size can increase the effective surface areas as well as the concomitant environmental ecotoxicity.Another major reason may be due to the fact that AgNPs with smaller particle size can release more Ag ions than those with larger particle size and thus show more environmental ecotoxicity.?5?AgNPs with larger particle size significantly exhibit stronger environmental ecotoxicity on the aboveground wet weight of wheat than those with smaller particle size at the same concentration.One possible reason was that AgNPs with larger particle size can induce the formation of new larger sizes of cell wall pore size and then absorbed by plant roots with more volume.Another major reason may be ascribed to that AgNPs with larger particle size accumulated longer inside the body than those with smaller particle size due its less aggregation and more stable.2.Effects of AgNPs with different concentrations and particle sizes on soil N-fixing bacterial communities in the wheat rhizosphere:?1?The soil pH was significantly and negatively correlated with Shannon's diversity index of soil N-fixing bacterial communities.The most probable mechanism of soil pH effects on soil N-fixing bacterial diversity may include mediation of nutrient availability in the soil sub-system.?2?All treatments both of AgNPs and Ag ions significantly increased soil Ag concentration compared with the blank control treatment.But soil Ag concentration was similar between the treatments of AgNPs and Ag ions.Thus,the significantly reduced Shannon's diversity index of soil N-fixing bacterial communities mediated by AgNPs may be presumably due to the released Ag ions.?3?Ag ions significantly decreased the Shannon's diversity index,Chao1 index,ACE index,and OTU richness of soil N-fixing bacterial communities compared to AgNPs.The reason may be due to that AgNPs can be converted to Ag ions and then reduced to AgNPs quickly by polysaccharide and other chemical substances in soil sub-system,thereby reducing the environmental ecotoxicity of AgNPs on soil N-fixing bacterial communities.?4?The ACE index,Chao1 index,and OTU richness of soil N-fixing bacterial communities significantly decreased with decreasing the particle size of AgNPs.The possible reason for this is that smaller particle size of AgNPs can increase its effective surface areas and dispersity,thereby enhancing the related concomitant ecotoxicity of AgNPs,especially the easier to enter the interior of cells,thereby enhancing its environmental ecotoxicological effects.Another and maybe equally important reason may be that AgNPs with smaller particle size can lead to a significant increase in the production of reactive oxygen species on the surface,which ultimately generate the corresponding ecotoxicity,especially can induce the membrane damage and oxidative stress.?5?AgNPs with different concentrations did not significantly alter soil N-fixing bacterial community structure.Thus,the particle size,rather than the concentration,was the most important determinant of the ecotoxicological effects of AgNPs on the community structure of soil N-fixing bacterial communities.