Effects Of ZnO Nanoparticles On Mineralization Of Exogenous Organic Matter In Typical Farmland Soil And Its Contribution To Soil Respiration

Returning crops to farmland is an important way to increase the soil organic matter and nutrients in farmland soil and an important measure in the utilization of agricultural organic waste resources.Soil microorganisms play an important role in promoting the matter circulation and maintaining the normal structure and function of soil.With the development of nanotechnology,nanomaterials are widely used in various fields,which will inevitably be released into the soil environment through irrigation discharge,sewage sludge treatment,and the use of nano-fertilizers and nano-pesticides.Nanomaterials may affect the soil microbial community and the degradation of exogenous organic matter through toxic effects on soil microorganisms.Furthermore,it affect the soil ecosystem health.In this study,the effects of different concentrations of ZnO nanoparticles?ZnONPs?on soil respiration,soil enzymes and microbial community were studied,and the influence of high concentration ZnONPs on the decomposition of wheat straw,carbon distribution of each component,functional microorganisms which are associated with carbon conversion were studied by isotope tracer technique through laboratory culture experiments.The research results are of great significance to deepen the understanding of the toxicological effects of ZnONPs on the decomposition of exogenous organic matters,and to explore the toxic effects of Zn ONPs on the ecological processes of soil ecosystem.Meanwhile,the research conclusions can also provide an important theoretical basis for the toxicity evaluation of ZnONPs in agricultural fertilization or soil remediation.The main research results are as follows:1.The different concentrations of ZnONPs have obvious effects on biological processes and microbial activities.Heterotrophic respiration was significantly inhibited by a high concentration of ZnONPs under the concentration of 500 mg kg^-1,but it was stimulated by the addition of ZnONPs at a low concentration at 100 mg kg^-1.The change of soil dissolved organic carbon was similar to that of heterotrophic respiration.With the increase of ZnONPs concentration,the release amount of Zn²⁺increased in soil,and the absorption of unstable Zn²⁺in microorganisms increased significantly.2.The change of soil microorganism is closely related to the change of heterotrophic respiration.In the early stage of the experiment?0-7 days?,polyphenol oxidase,?-glucosidase,cellulose,and cellobiohydrolase were promoted by low concentration of ZnONPs(100 mg kg^-1),while at the end of the experiment?84 days?,the activities of?-glucosidase,cellulose,and cellobiohydrolase were inhibited by high level exposure of ZnONPs(500 mg kg^-1).Illumina Miseq sequencing results showed that the operational taxonomic unit numbers of bacteria and fungi ranged in each sample between 1425 to 4071 and 263 to 602,respectively.Proteobacteria,Actinobacteria and Acidobacteria were the dominant phyla.Most bacterial communities were inhibited except for Acidobacteria and Bacteroidetes after ZnONPs addition in all treatments.The proportion of Ascomycota was the highest in fungi,and was inhibited by ZnONPs,followed by Basidiomycota,Zygomycota and Chytridiomycota.With the increase of the concentration of ZnONPs,the diversity of the bacterial community decreased,while the diversity of the fungal community first decreased and then increased,indicating that the bacterial community was more sensitive of to ZnONPs than that of the fungal community.3.High concentration of ZnONPs(1000 mg kg^-1)has obvious effect on the decomposition of wheat straw.The CO₂ released from the decomposition of wheat straw in the soil was 40%-83%after the addition of wheat straw.After 56 days,compared with treatment of add only wheat straw,the CO₂ released from the decomposition of wheat straw was lowed by 43.9%er after the addition of Zn ONPs.The?¹³C-CO₂ values in the ZnONPs treatment group was lower than that of the wheat straw treatment group,indicating that ZnONPs inhibited the mineralization of wheat straw.The addition of ZnONPs of 1000 mg kg^-1)increased the contents of soil organic tarbon?SOC?and ¹³C-SOC,and decreased the contents of microbial biomass carbon?MBC?,¹³C-MBC,dissolved organic carbon?DOC?and¹³C-DOC.The?¹³C value shows that straw organic carbon is more easily distributed into MBC?4321.86‰-5083.93‰?,followed by DOC?1168.86‰-1440.13‰?and SOC?381.07‰-397.76‰?.The values of?¹³C-SOC,?¹³C-MBC and?¹³C-DOC in the ZnONPs treatment group were all lower than those in the wheat straw treatment group,indicating that the contribution of ZnONPs to the soil organic carbon pool after the addition of wheat straw was relatively lower.4.The functional microbial changes associated with carbon conversion are closely related to the decomposition of wheat straw.The activities of polyphenol oxidase,peroxidase,?-glucosidase and cellulose were promoted by the addition of wheat straw,while were inhibited by ZnONPs.The sensitivity of soil enzymes to ZnONPs was peroxidase>polyphenol oxidase>?-glucosidase>cellulose.ZnONPs of 1000 mg kg^-1 inhibited the abundance of laccase-like multicopper oxidase genes,family 48 glycosyl hydrolases genes and cellobiohydrolase I genes.The soil functional gene abundance of wheat straw treatment had the highest similarity with that of control treatment,and was separated from that of ZnONPs treatment group,indicating that the addition of ZnONPs changed the functional gene abundance of cellulose and lignin degradation in soil.The results of community level physiological profile analysis showed that the amount of carbon source used by microorganisms gradually increased and finally stabilized with the culture time.The addition of high concentration ZnONPs(1000 mg kg^-1)reduced Average Well Color Development,the index of Shannon-Wiener diversity,richness and community Mclntosh,indicating that ZnONPs inhibited soil microbial species,distribution and carbon source utilization.