Study On How Molybdenum Regulate Soil Nitrogen Transformation And Its Microbial Mechanism In Yellow-brown Soil

Molybdenum ?Mo? is an essential micronutrient for the growth of plants,animals and microbes.Previously studies indicated that Mo could promote the N uptake and assimilation of higher plants,enhance the N fixation of soil.However,researches on the regulatory mechanisms of Mo impact soil N transformation,especially on the key processes such as nitrification,denitrification and N₂O emission are rare.Therefore,in the present study,long-term field experiment,rhizobox culture experiment,soil cultivated experiment,high-throughput sequencing,and qPCR techniques were applied to explore the mechanism of Mo regulate the key processes of soil N transformation,and to research the effects of plant roots,soil pH,and seasonal variation on these regulatory processes.The main results are as follows:1.In the long-term field experiment,After 7 years experiment,continuous Mo application significantly increased soil available Mo content.Compared to the Mo deficient treatment,continuous Mo application significantly increased the crop cumulative yield,plant N uptake,and N fertilizer use efficiency by 11.56%,18.41%,and,8.18%,respectively.The concentration of soil NO₃^--N,NH₄⁺-N,microbial biomass N,and total N were decreased by 13.53%,29.48%,40.37%,and 11.88%,respectively.The data suggest that a deficiency in soil available Mo may induce the risk of soil N accumulation and environmental N emission in vegetable soil,whereas continuous Mo application could mitigate this risk by increasing crop yield and N uptake and,by decreasing soil N residues,soil nitrification and denitrification.2.The soil in the long-term experimental field has abundant specie,14 bacterial phyla were detected,among them Proteobacteria,Acidobacteria,Actinobacteria,and Chloroflexi were predominant.7 fungal phyla were detected,among them Ascomycota and Basidiomycota were predominant.Continuous Mo fertilization increased the gene copy number of bacterial 16 S rRNA;induced the abundance of nirK gene in early spring?January,2016?;inhibited the abundance of nosZ gene in early summer?May,2016?;decreased bacterial and fungal unique OTUs,reduced the specificity of soil microorganisms.At the bacterial level,Mo deficient induced the presence of Verrucomicrobia,Mo fertilization significantly increased the relative abundance of OTU750.At the fungal level,Mo fertilization reduced the relative abundance of Gracilipodida and OTU40.Seasonal changes had a more evident effect on the community structure,diversity and functional gene abundance of soil bacterial and fungal.In early spring,the bacterial and fungal unique OTUs were induced,the specificity of soil microbial was increased,and the abundance of AOA gene was higher.In early summer,the gene copy numbers of bacterial 16 S rRNA,AOB,narG,nirK,nirS,and nosZ were all higher.At the bacterial level,early spring induced the appearance of Verrucomicrobia,decreased the relative abundances of OTU1134 and OTU1617.At the fungal level,the relative abundances of Zygomycota,OTU153,and OTU197 were significantly higher in early summer than in early spring,in contrast,the relative abundances of OTU168 and OTU184 were significantly lower in early summer.3.In the rhizobox cultivated experiment,the N transformation in rhizosphere and non-rhizosphere soil are two different systems In rhizosphere soil,soil pH increased with increasing rates of Mo application,NO₃^--N accumulated at the rates of 0.15 and 0.3mg/kg Mo,and decreased under 1 mg/kg Mo conditions;catalase activity decreased significantly;PDA was negative numbers and significantly decreased;narG and nosZ genes copy numbers were added.In non-rhizosphere soil,with the strength of Mo addition,NO₃^--N concrntration and NR activity decreased significantly;the abundance of narG gene increased obviously.Soil pH,NO₃^--N concrntration,catalase activity,apparent nitrification rate?ANR?,nosZ gene abundance were significantly higher in rhizosphere than those in non-rhizosphere soil;On the contrary,soil NH₄⁺-N and total N concrntrations,Nir activity,PDA,the abundance of AOB,nirK and nirS genes were significantly higher in non-rhizosphere than soils rhizosphere soil.Therefore,appropriate Mo additions?0.3 mg/kg?could promote the transfer of NO₃^--N and MBN from non-rhizosphere to rhizosphere soil,increase crop biomass,N content,and N uptake,and weaken the denitrification potential in both rhizosphere and non-rhizosphere soils.4.In the soil cultured experiment,in the acid soil of pH5.3,1 mg/kg Mo addition significantly increased N₂O emission rate,and inhibited the copy number of AOA,AOB,narG,nosZ,nirK and nirS genes after 24 h;Mo additions significantly decreased soil NH₄⁺-N content before 48 h but increased it at 72 h.In the alkaline soil of pH7.5,the total N₂O emission was higher than that in pH5.3,and the emission peak was appeared at 24 h,except for 24 h,Mo addition inhibited N₂O emission rate at other cultivated time points.The effects of two Mo application rates on functional genes abundance were varied at different incubation times.The above results show that the main processes of soil N accumulation and environmental N emission caused by Mo deficiency are:the lower crop biomass,the less uptake of the N,the higher soil N component contents especially the reactive N,and the stronger nitrification and denitrification.The application of Mo fertilizer can increase the uptake of soil N by increasing crop biomass,promoting NO₃^--N and MBN transfer from non-rhizosphere to rhizosphere soil;can reduce soil N residue by decreasing soil NH₄⁺-N,NO₃^--N,MBN,and total N contents;can impair nitrification by inhibiting the amount of ammonia-oxidizing microorganisms;can weaken denitrification in acidic soil by reducing substrate concentration and regulating the amount of denitrifying microorganisms.Therefore,the application of moderate Mo fertilizer in Mo deficient soil is of great significance to improve plant N use efficiency and reduce of environmental nitrogen emissions.