Speciation Transformation And Bioavailability Of Selenium Nanoparticles In Soil-Wheat System

Selenium（Se）is an essential micronutrient element in human body,and its biogeochemical cycle in the environment affects human health.Food crops,especially wheat,are the main source of dietary Se intake in human body.Se deficiency caused by uneven distribution of Se level in soil leads to low Se concentration in wheat grains.Therefore,Se biofortification in wheat is of great significance to meet the safe and reasonable Se supplement requirements of human body and the development and utilization of Se-enrich products.In this study,Se nanoparticles（SeNPs）were selected as Se supplement materials,and the distribution of SeNPs with different sizes in wheat rhizosphere and in different parts of plants were studied by soil application,which revealed that size was the key factor mediating the transformation of SeNPs and plant absorption;low molecular weight organic acids（LMWOAs）were used to simulate root exudes,and microbial community characteristics related to SeNPs transformation were analyzed to clarify plant regulation mechanism of rhizosphere SeNPs transformation;based on dilution extinction method,four microbial community diversity gradients were constructed,and the model of SeNPs transformation driven by microbial diversity was analyzed;finally,the bioavailability and residual effects of SeNPs in soil were evaluated by wheat-maize rotation experiment.The main results of this paper are as follows:（1）Soil application(2-100 mg kg^-1)of SeNPs decreased the biomass of leaf,but exhibited no obvious negative influence on other parts.SeNPs significantly increased grain Se concentration by 0.56-37.03 folds higher than control,where large-sized（200 nm）SeNPs were better than small-sized（50 nm and 100 nm）,especially 200 nm SeNPs spiked with 2 mg kg^-1had reached wheat Se-enriched standard.Furthermore,the concentrations of SOL-Se,EXC-Se,FMO-Se,AVA-Se and SOL-Se⁴⁺in soil treated with large-sized SeNPs were significantly higher than small-sized.SeNPs size or most of their existing forms in soil were significantly correlated with grain Se content,indicating SeNPs transformation in soil regulated by size ultimately influenced the effect of wheat Se biofortification.（2）LMWOAs affected the distribution of Se fractions in krasnozem,loess soil and black soil,the differences between sterilized and non-sterilized soils were found.Further,LMWOAs significantly increased EXC-Se concentration by 19.0-180.5%and decreased FA-Se concentration by 9.6-61.8%in non-sterilized soils.Meanwhile,LMWOAs affected soil enzyme activities,bacterial communities and co-occurrence network patterns,especially,Burkholderia-Caballeronia-Paraburkholderia,Azotobacter and Sphingomonas were enriched in three soils,respectively.In short,LMWOAs mediated SeNPs transformation indirectly via their influence on microbial community,invertase,FA-Se and EXC-Se.（3）Microbial diversity affected available Se concentration in krasnozem,loess soil and black soil,and there was a correlation between different SeNPs size and soil type.Further analysis of loess soil showed that diversity dilution increased SOL-Se,EXC-Se and TR by2.43-18.09%,15.29-27.77%and 2.71-20.7%,respectively.The dilution treatment also significantly reduced the microbial diversity and complexity of co-occurrence network.The loss of microbial diversity caused by serial dilution weakened the functional redundancy of the recombinant microbial community,resulting in high enrichment of microorganisms（e.g.,Bacillus）that might be involved in SeNPs transformation.In addition,there was a significant correlation between microbial diversity and EXC-Se,suggesting that the biotransformation products of SeNPs in soil were mainly EXC-Se.（4）Different inorganic Se with 2 mg kg^-1（selenite,selenate,SeNPs）had no significant effect on crop yield,and could effectively increase grain Se concentration of wheat and maize.The grain Se concentration of two crops treated with SeNPs(0.078-0.369 mg kg^-1)was significant lower than those treated with selenite(1.620-2.923 mg kg^-1)and selenate(48-129mg kg^-1).Also,the grain Se distribution and utilization efficiency of SeNPs were significantly lower than those of selenite and selenite.As planting years increasing,the available Se treated with SeNPs increased gradually,but the levels of selenite and selenite decreased significantly.In addition,the residual rate of SeNPs in soil was much higher than that of selenite and selenite.Therefore,compared with ordinary inorganic Se,SeNPs immobilized in soil in large quantities,and can provide long-term effective Se source for crops through slow release,so that it has the potential to be used as a slow-release fertilizer for a long time.In conclusion,soil application of SeNPs can effectively improve the Se nutrition level of wheat grain,and meet the Se enrichment standard of wheat.In rhizosphere microenvironment,SeNPs transformation can be absorbed and utilized by plants mainly through the regulation of biotic and abiotic factors.Meanwhile,SeNPs has the characteristics of high residual amount and slow release rate,which also reduces the economic cost and environmental risk of Se supplement strategy.Therefore,the application prospect of SeNPs in the agricultural production of Se biofortification is very promising.