Study On The Effectiveness And Mechanism Of Remediation Of Nitrate Contaminated Soil By Chroococcus Sp.

In recent years,people have applied large amounts of nitrogen fertilizers to the soil in pursuit of higher crop yields,resulting in serious nitrate pollution.Nitrogen-fixing cyanobacteria could improve fertility and act as biofertilizers for poor soils,while the non-nitrogen-fixing cyanobacteria could assimilate nitrate and nitrite nutrients from surrounding environments,which may reduce excess nitrogen in the soil.A typical non-nitrogen-fixing cyanobacterium that can grow in soil,Chroococcus sp.,was used for the remediation of nitrate polluted soil in the present study.Chroococcus sp.was inoculated into the soil containing different levels of nitrate,and the influence of Chroococcus sp.on the physicochemical properties and enzymatic activities of the soil was investigated.The effect of Chroococcus sp.on the bacterial community in nitrate polluted soil was studied by macrogenomics and macrotranscriptomics analysis,and the remediation effects and mechanisms of Chroococcus sp.on nitrate polluted soil was thoroughly discussed.Chroococcus sp.can effectively remove excess nitrate nitrogen from the soil by directly absorbing nitrogen nutrients.After 40 days of Petri dish experiments,the application of Chroococcus sp.at doses of 0.03-0.08 mg/cm2 removed 31.94-44.06%of NO3--N from the soil at initial NO3--N concentrations of 60-120 mg/kg,and the highest removal rate of nitrate nitrogen was observed at the application dose of 0.05 mg/cm2.The application of Chroococcus sp.significantly(p<0.05)increased the relative abundances of denitrifying bacteria such as Hyphomicrobium and Chitinophaga,thus elevating the activity of nitrate reductase and nitrite reductase by 1.05-1.23 and 1.08-1.21 times,respectively.Chroococcus sp.enhanced the denitrification process in the soil,which also contributed to the removal of NO3--N from the soil.Chroococcus sp.could enhance the content of available phosphorus in soil by 1.30-1.99 times through stimulating the activity of soil phosphatase(elevated by 1.06-1.14 times)and promoting the growth of acid-producing bacteria and phosphorolytic bacteria,and increasing the expression of functional genes related to phosphate and phosphite metabolism.The application of Chroococcus sp.could positively regulate the growth of Nocardioides maradonensis,Nocardioides maradonensis,Prevotella and Tumebacillus that are conducive to to soil fertility,and the activities of soil catalase,invertase and urease could be elevated by 1.06-1.38,1.09-1.72 and 1.09-1.42 times.Chroococcus sp.fixed carbon dioxide through photosynthesis and significantly increased(p<0.05)the organic matter(1.11-1.35 times)and polysaccharide content of the soil(1.55-2.36 times)and reduced the soil salinity.Chroococcus sp.also increased the expression abundance of genes related to the energy metabolism function of the soil microbial community,which cosequently promoted the growth of soil microorganisms.The application of Chroococcus sp.significantly increased the expression abundance of carbohydrate esterases,glycosyl transferases and polysaccharide lyases,and changed the form of carbohydrate utilization by soil bacteria,which is another explanation for the promotion of microbial growth in the soil.The results of this study proved that the application of Chroococcus sp.can be used as an economical and environmentally friendly biological method for the remediation of nitrate polluted soil.