Migration And Transformation Of Nitrogen In Bioretention System By Nitrogen Isotope Tracing Technique

The nitrogen pollution of rainwater runoff is becoming more and more serious in China.Excessive nitrogen input in water is the primary factor leading to deterioration of water quality in urban lakes,rivers and lakes,nitrogen form is different.Its ecological function,the toxicological effects of aquatic organisms are also different,more soluble nitrogen migration,control more difficult.Therefore,the effective control of nitrogen pollution in urban rainfall runoff is essential.The biological retention system can effectively improve the water quality of rainwater runoff by using soil and plant through physical,chemical and biological effects and has been widely studied and applied.However,there is no uniform conclusion on how to effectively improve the denitrification efficiency of the system,mainly for the results of different studies on the removal of nitrogen is very large,the theoretical limit capacity and influencing factors of nitrogen control in biological detention facilities have not been determined.Therefore,this study uses ¹⁵ N isotope tracer technology to deepen the atomic level to study the migration and transformation of nitrogen,through indoor simulation of biological retention facilities.The addition of K¹⁵NO3 and ¹⁵NH4 Cl by water distribution resulted in the abundance of ¹⁵N-NO3-and ¹⁵N-NH₄⁺ in the experimental system was significantly higher than that in the ecosystem,the effects of different environmental conditions on the migration and transformation of nitrogen in biological retention system were analyzed by combining the changes of ¹⁵ N content in different forms of leachate,soil and plant.The main contents and results are as follows:?1?Study the mechanism and effect of plant in nitrogen migration and transformation in bio-retention.Danish grass was selected as the planting plant.Compared with planting and not planting Dan grass two kinds of conditions,to explore the role of plants in nitrogen removal in biological retention systems.The results showed that the direct absorption of NO3-in Danish grass had a partial contribution to the removal of NO3-,also it can help to improve denitrification indirectly to promote denitrification and improve runoff denitrification effect.The formation of biosphere around the root to increase the biomass of microorganisms,thereby promoting the microbial NO3-fixation;By increasing the oxygen content in the soil,thus affecting the biological retention system of nitrogen and nitrogen alienation reduction to ammonium?DNRA?and denitrification between the collaborative competition.The experiment also passed the summer and winter two different periods of ambient temperature,explore the effect of Temperature on the migration and transformation of nitrate in bio-retention system,obtain the effect of bio-retention system on the removal of nitrogen and nitrogen in runoff from different seasons.The results showed that the absorption of NO3-and the effect of denitrification in summer were higher than those in winter.The effect of DNRA was influenced by temperature,which was not conducive to the effect of DNRA in winter.The adsorption of NH₄⁺ in the system was lower than that in winter.?2?Through under different oxidation-reduction potential?Eh225m V¹00mV?0mV^-120 mV and-225mV^-340mV?,carbon sources(50mg/L?100mg/L?¹⁵0mg/L?200mg/L?250mg/L)and soil pH?pH=5⁹?value conditions were compared to analysis of the effect of DNRA in bio-retention system.The results showed that with the decrease of Eh,the effect of DNRA was more obvious,and the effect of DNRA was easy to be carried out in strong reducing soil.In the acidic soil environment,with the increase of pH value,the soil environment from acid to alkaline,DNRA effect of the more obvious,in the alkaline environment.With the increase of pH value,DNRA effect decreased,but the effect was more than acidic soil environment significantly,It can be seen,DNRA effect in the neutral alkaline environment.Using glucose as a carbon source,when the carbon source is too low,it may inhibit the growth of bacteria and can inhibit the DNRA;when the carbon source is relatively excessive,the denitrification showed a competitive advantage,thereby inhibiting the DNRA.?3?Through the different temperatures of summer and winter,the effect of temperature on the migration and transformation of ammonia nitrogen in biological retention system was discussed,and inferred the removal of ammonia nitrogen in rainwater runoff by bio-retention system at different seasonal temperatures.In the summer high temperature conditions,the fixation effect of microorganism on NH₄⁺ was higher than that of winter.The effect of nitrification on NH₄⁺ removal was higher than that in winter,which resulted in the decrease of ammonia nitrogen effluent concentration and the increase of nitrogen and nitrogen effluent concentration.In the bio-retention system,the volatilization of ammonia nitrogen is affected by the temperature,and it is not conducive to the volatilization of ammonia nitrogen under low temperature.The experiment also compared the different soil pH?pH=5⁹?affect the migration and transformation of ammonia in bio-retention system,the results show that the removal of ammonia nitrogen in the biological retention system is favorable in the neutral alkaline environment with pH=7⁹.With the increase of pH value,the amount of ammonia adsorption increased,pH value was positively correlated with soil ammonia content,the effect of nitrification in the biological retention system is more obvious,when the pH value exceeds a certain range,nitrification reaction will be inhibited;ammonia nitrogen volatilization by the impact of pH,with the pH value increased,ammonia nitrogen volatilization increased.In the acidic environment,is not conducive to the volatilization of ammonia.