Study On Effect Of Soil Properties On Nitrogen Removal Of Bioretention System

Bioretention system is a common low-impact development facility for stormwater runoff.It has great potential to control runoff and purify water quality through the coupling of plants,soil and microorganisms,effectively reducing the impact of urban development on urban hydrology.In the process of removing pollutants from stormwater runoff,various physical,chemical and biological reactions are accompanied.Based on the soil,plants,and microorganisms in the bioretention system,this paper used indoor simulated columns for experiments to explore the effects of soil texture,compaction,organic matter content,and plant species on the hydrological performance and water purification of the bioretention system.Stable ¹⁵N isotope tracing technique was used to quantify the contribution of soil and plant on nitrogen removal,and qPCR technique was used to analyze the characteristics of nitrogen removal functional genes in soil layer.1.The experiment was based on the simulated columns of the bioretention system of six soil texture.The results showed that the hydrological performance of the bioretention system was affected by the soil texture,and systems with high sand content in the soil layer had strong permeability but weak in water retention.The systems with low sand content in the soil layer had better effect of removing TN,TP and COD.When stormwater entered the system,the nitrification was stronger than the denitrification,and the denitrification rate was maximum around 13h.The surface soil removed pollutants more efficiently than the bottom soil,but phosphorus leaching occurred.In the process of nitrogen removal,the order of the ability of soil,microorganisms and plants to remove nitrogen was:soil adsorption>microbial assimilation>plant uptake.Soil texture affected soil adsorption and microbial assimilation,which in turn affected the contribution of soil nitrogen removal.The removal rate of the system with low sand content in the soil layer was higher than that in the system with high sand content.2.The experiment was based on the simulated columns of the bioretention system of three compacted states,two organic matter content in soil layers and planting two plant species.The results showed that the hydrological performance of the bioretention system was affected by soil compaction,organic matter content and plant species.Plants with shallow root,soil compaction and low organic matter content were not conducive to the infiltration of the bioretention system.Compacted soil was not conducive to the removal of TN,TP,COD,especially TP,and not conducive to nitrification.Although it was conducive to denitrification,it may result in insufficient sources of NO₃^--N due to insufficient nitrification.Moreover,surface compaction was more difficult to remove nitrogen than deep compaction.Except for NO₃^--N,buxus?shrubs?was better than Ophiopogon japonicus?herbs?in removing pollutants.System with high organic matter content had a certain effect on nitrogen and phosphorus removal,but adding excessive organic matter or organic matter that could leach nutrients may make the soil as a new source of pollution.In the process of nitrogen removal,the order of the ability of soil,microorganisms and plants to remove nitrogen was:soil adsorption>microbial assimilation>plant uptake.Nitrogen removal in soil was affected by compaction,compaction location,plant species,and organic matter content.Plant roots in compacted areas were not conducive to nitrogen removal.Planting shrubs and soil with high organic matter content were conducive to nitrogen removal.3.The effects of different properties of the soil layer and plant species on nitrogen removal in bioretention system were analyzed in two experiments.The results showed that as the depth of soil increased,except for several compaction groups,the total number of soil bacteria?16S rRNA?,nitrifying bacteria?amoA,nxrB?,denitrifying bacteria?narG,nirK,qnorB,nosZ?and DNRA bacteria?nrfA?decreased.Because nutrients and pollutants carried by stormwater runoff accumulated in surface layer of soil,resulting in insufficient nitrogen and carbon sources in the deep soil,which limited the growth of bacterial communities.The nitrification gene abundance in soils with low sand content decreased,and the denitrification and DNRA gene abundance increased.The nitrification gene abundance in compacted soils decreased,and the denitrification and DNRA gene abundance increased,and the adjacent position of the compacted part were also affected.The nitrification,denitrification,and DNRA gene abundance in soil with high organic matter content all increased.The nitrification gene abundance in the soil planted with buxus was higher than that in Ophiopogon japonicus,and denitrification,DNRA gene abundance was slightly lower.The nitrification reaction couldn't occur sufficiently because of the low DO content and insufficient nitrogen source in deep soil.The content of DO in the surface soil was high,but due to anaerobic micro-zones and other factors,the denitrification reaction could occur at surface layer of soil at a higher rate than that in deep soil.DNRA bacteria preferred to survive in a highly reducing soil environment.The enhancement of the nitrification capacity brought by plant roots exceeded the detrimental effects of denitrification,which could provide more NO₃^--N sources for denitrification in deep soil.The rotted roots in the soil brought about accumulation of organic matter,which brought more anaerobic micro-areas and nutrients,promoted denitrification reactions,and increased the denitrification ability of deep soil.