Promoting Effect And Mechanism Of Nano Biochar On The Formation And Transformation Of Iron Plaque On Rice Root Surface

The ubiquitous presence of biochar（BC）in the environment endows its critical role in many biogeochemical processes.After entering the water and soil environment,BC is prone to fragmentation due to biological or chemical aging,and may even form nano-sized biochar particles,namely nano biochar（nano-BC）.Compared with bulk BC,nano-BC has stronger migration ability,can diffuse to the rhizosphere environment and attach to the root surfaces,forming an important root-BC-soil interface to directly affect the growth and development of roots.However,the mechanism of nano-BC on plant root activities are still unclear.Additionally,root exudates are ubiquitous in vegetated soil and water environment and frequently participate in various biogeochemical reactions such as contaminant degradation,and mineral transformation.However,the impact of root exudates on the activity and reactivity of nano-BC in the rhizosphere is largely unknown.Therefore,the impact of root exudates,especially the low molecular weight organic acids（LMWOAs）on the physicochemical properties（including specific surface area,minerals and surface groups,etc.）of nano-BC were firstly studied.Secondly,the regulation mechanism of nano-BC on the formation,composition,structure and content of iron plague on rice root surface was analyzed.Finally,silver ions commonly existing in water environment were selected as representative toxic metal ions,and the passivation mechanism of nano-BC on silver ions in the rice rhizosphere was revealed.The main contents and results of this thesis are as follows:（1）The release of nano-BC was enhanced by the introduction of root exudates,where low molecular weight organic acids played a dominating role in the dissociation of nano-BC from carbon skeleton.The surface properties of nano-BC were greatly modified by root exudates including lowered surface area（18.13%）and mineral contents（43.90%-69.57%）,increased O-containing groups（11.46%）and graphitization（18.65%）.Meanwhile,the presence of root exudates not only reduced the colloidal stability of nano-BC（30.43%for Ca Cl₂）in aqueous solutions but also lowered the intensity of free radicals（19.44%-22.22%）in nano-BC.Consequently,the oxidative stress of nano-BC（root exudates-treatment）to rice seedlings was significantly（p<0.05）alleviated,which was supported by the lower antioxidative enzyme activities and soluble protein content in rice leaves and roots.The enzyme activities positively correlated to the concentrations of hydroxyl radicals in nano-BC.These results indicate that the surface properties of nano-BC could be remarkably modified by its surroundings（e.g.,root exudates）in the rhizosphere,where the relevant reaction mechanisms merit further investigations to fully understand the environmental behaviors of nano-BC.（2）The data identify that show that the formation of Fe（III）-（oxyhydr）oxide coating（used to stand for iron plaque）on rice root surfaces is to a great extent regulated by the concentration of nano-BC attached on roots,where low concentration of nano-BC enhanced the formation of iron plaque due to electron transfer mechanism while excess nano-BC not only impeded the formation of iron plaque but also reduced plant growth,nutrient uptake,and downregulated the gene expression（Os FRDL1）in rice plants.The critical concentration of nano-BC is highly dependent on its redox potential stemmed stemming from electroactive components in nano-BC.Results show that Fe（II）direct oxidation by redox-active moieties accounted for～70%to the formation of iron plaque for low-temperature（400℃）nano-BC,while electron transfer from Fe（II）to oxidants in root exudates via graphitic structures accounted for at least 55%in the case of high-temperature（700℃）nano-BC.Nano-BC induces the formation of new iron plaque,which can significantly reduce the uptake and accumulation of toxic metals（p<0.05）.This directly verified that the iron plaque can effectively block toxic metals(Cu²⁺or Cu NPs).Our findings suggest that the impact of BC on the formation of iron plaque might be a crucial indicator for accurate evaluating and guiding the application of BC materials in agricultural production.（3）Multiple lines of existing evidence indicate that natural organic matter（NOM）could protect poorly crystalline Fe（III）（oxyhydr）oxides from Fe（II）-catalyzed mineral transformations.Conversely,we find that nano-BC,a pyrogenic form of NOM,promotes phase transformation of ferrihydrite（Fh）in nano-BC/Fh heteroaggregates in the presence of aqueous Fe（II）and rice root exudates.By contrast,the crystal form of nano-BC/goethite（Goe）heteroaggregates are more stable to phase transformation and difficult to dissociate or transform even exposed to root exudates.The nano-BC/Fh heteroaggregates are composed of a core-shell like structure,where the inner-layered nano-BC is more compacted and plays the dominant role in accelerating the phase transformation of Fh relative to that in the outer sphere.The extent of phase transformation is more regulated by the reversible redox reactions between quinone and hydroquinone in nano-BC than the electron transfer via its condensed aromatic structures.Additionally,the presence of root exduates（mainly reductive substances）synergistically contributes to the dissolution of Fe and transformations of nano-BC/Fh associations by donating electrons to nano-BC.Our results suggest that heteroaggregates between nano-BC and Fe minerals are subjected to partly dissociation during their co-transport and the attached nano-BC is favorable to phase transformations of poorly crystalline Fe minerals（e.g.,Fh）,which might have profound implications on biogeochemical cycles of carbon and Fe in the prevailing redox environments.（4）Ag NPs can be formed from aqueous silver ions in the presence of nano-BC in rice rhizosphere environment.It is found that nano-BC has high oxygen-containing functional groups and aromatic carbon,which makes it contain high free radicals and promotes the secretion of oxygen by rice roots to form·O₂^-.Quenching and trapping experiments were carried out to verify that the formation process of Ag NPs was closely related to·O₂^-.The reduction of silver ions by nano-BC follows the quasi first-order kinetic equation under the condition of rice rhizosphere environment.However,the formation of Ag NPs decreased with the increase of the concentration of nano-BC,because excessive nano-BC effectively inhibited the formation of oxygen secreted on the surface of rice roots and further reduced the formation of·O₂^-.The experimental results are beneficial to further deepen the understanding of the migration and transformation law of toxic variable-valence metal ions including silver ions in rice rhizosphere environment.