The Effect Of Cadmium Levels On Nitrogen Transformation In The Soil-plant System And The Microbial Mechanism Exploration

Cadmium(Cd)is a toxic heavy metal which harms plant by disturbing the nitrogen metabolisms and inhibiting the nutrient uptak.Apart from this,the mechanism of Cd affecting the nitrogen transport and transformation in soil-plant nitrogen system is still poorly understood.Two soil culture experiments were conducted with methods of isotopic trace,real-time quantitative(q-PCR)and Illumina Miseq high to explore the effects of soil cadmium levels on plant nitrogen accumulation and utilization,input nitrogen residual,soil nitrification and denitrification enzyme activity,and the abundance and community diversity of functional genes,aim at uncovering whether Cd contamination can destroy the soil nitrogen cycle and cause further environmental nitrogen pollution in the plant-soil system and revealing the microbial mechanism.Two Chinese cabbage [Brassica campestris L.ssp.Chinensis(L.)] cultivars(L: Jinxiashi and H: Aijiaohuang)and vegetable soil were chosen as the experimental materials.Overall,the main obtained results are as follows:(1)The effect of different levels of Cd on plant growth and nitrogen utilizationThe growth of the Chinese cabbage was inhibited by relatively high concentration of soil Cd contamination.The biomass of the cabbages slightly stimulated by 0~1.0mg/kg soil Cd and stayed relatively stable,while decreased significantly to some extent by 10 mg/kg Cd.The critical value of soil Cd concentration affecting the dry matter was 4.22 and 1.00 mg/kg for cultivar L and H respectively.The N accumulation and utilization of both cultivars were slightly stimulated by 0~1.0 mg/kg soil Cd concentrations,while notable decreased by about 10 mg/kg Cd.The critical value of Cd concentration affecting the nitrogen utilization was 6.64 and 5.02 mg/kg for cultivar L and cultivar H respectively,which suggested that cultivar L endure higher Cd contamination.(2)The effect of different levels of Cd on nitrogen from soil(Nfs)and fertilizer(Nff),total nitrogen concentration and the residual fertilizer nitrogenThe nitrogen content that absorbed from soil and fertilizer of cultivar L were higher than that of cultivar H;both of them were all significantly decreased by 10.0 mg/kg Cd.The critical value of soil Cd concentration affecting the Nfs was 5.07 and 5.27 mg/kg for cultivar L and cultivar H respectively.The critical value of Cd concentration affecting the Nff was 5.83 and 6.03 mg/kg for cultivar L and cultivar H respectively.In addition,the soil residual nitrogen of fertilizer was increased with Cd contamination,and the residual in bulk soil was higher than that of rhizospheric soil.(3)The effect of different levels of Cd on soil properties and nitrogen componentsNo significant effect of Cd on soil p H,total nitrogen and organic matter.However,the soil microbial biomass nitrogen was remarkably reduced by 10.0 mg/kg Cd.In addition,the soil NO3-concentration increased along with the soil NH4+ concentration decrease under Cd treatments.Soil mineral nitrogen concentration increased and then kept stable along with the increasing Cd.They reached the maximum at 7.11 and 7.13 mg/kg Cd treatment respectively for cultivar L and cultivar H.(4)The effect of Cd on soil nitrogen transformationThe soil nitrifiers(AOB and AOA)abundance and soil potential nitrification rate was accelerated by Cd as well,which suggested that Cd contamination had positive effect on soil nitrification.However,the denitrifiers(nirS,nar G and nosZ)and soil potential denitrification was inhibited by Cd.Those results made us speculate that the increased nitrification,decreased denitrification and the decreased nitrogen absorption by plants were the main reason resulting the input nitrogen residual to some extent,especially significant for cultivar H.Therefore,it was estimated that Cd contamination attributed to the residual of input nitrogen by interrupting the nitrogen transportation in plant-soil system.(5)The effect of Cd on the soil AOB microbial community structureCd and root had no significant effect on the main species of AOB(Proteobacteria(71.9%),Environmental_sample_k_norank(16.6%)and others(11.5%))at Phlum level,but the lower soil Cd concentration made some difference on the abundance of different communities: promoted the relative abundance of the Environmental_samples_k__norank and Unclassified_k__norank_d__Bacteria,while inhibited the Proteobacteria abundance.The effect of root was more significant than Cd which could be seen from the results that there was significant difference of AOB diversity between rhizosphere soils and bulk soils with or without Cd most of the time.Thus,soil Cd pollution may affect the nitrification by converting the community structure of AOB gene.(6)The effect on the soil nirS and nosZ microbial community structureThe nirS gene included three main communities at phylum level: 62.6% Unclassified_k_norank_d_Bacteria,25.3% Environmental_sample_k_norank and 12.0% Proteobacteria.Soil Cd contamination had no significant effect on the nirS community structure.There were significant difference between rhizosphere soil and bulk soil,especially at higher Cd lever,which suggested that there was interaction effect between Cd and root on nirS community structure.The nosZ gene included two main communities at phylum level: proteobacteria(55.9%)and Unclassified_k_norank_d_Bacteria(37.0%).Soil Cd contamination had no effect on the dominant species at the phylums level,but made some difference on their relative abundance: promoted the abundance of Unclassified_k__norank_d__Bacteria,while inhibited the abundance of proteobacteria.The soil of cultivar H was influenced more greatly by Cd.In addition,there was significant difference between rhizosphere and bulk soil.Thus,soil Cd pollution may affect the denitrification by converting the community structure of nosZ gene.