| Biologic retention technology is one of the limited selection technologies of sponge cities in China.As an important part of the biological retention system,plants play multiple roles in the system operation life cycle.Plant species not only affect the hydraulic performance of the system and the effect of pollutant control,but also directly affect their contribution to the nitrogen removal efficiency of the system.However,there is still a large unknown space for the effects of plants on the nitrogen transfer and transformation process in the system.Therefore,based on the characteristics of rainfall in Chongqing,this paper aims at the characteristics of the rainwater runoff in the mountain city,and studies the selection of plant and the nitrogen migration characteristics of the rainwater biological retention system.It is expected to provide a theoretical basis for the design and optimization of the plant selection and nitrogen removal characteristics of the rainwater retention system,and to provide technical support for the construction of the sponge city.Based on the commonly used plants in Chongqing’s greening department and market investigation,10 local plants were selected to build a rainwater biologic detention system.Through the investigation of the hydrological characteristics of the system(i.e.permeability coefficient,evaporation)and the effect of the removal of pollutants,the analytic hierarchy process(AHP)was used to evaluate the different plant systems,and the best plants were screened;Through the detection of nitrogen accumulation in soil and plants and the diversity of soil microbial diversity,the contribution of plants to nitrogen in the system was clearly defined,and the transformation process and convergence of nitrogen migration and transformation in the system of rainwater biological retention were explored.The main conclusions are as follows:(1)the permeability coefficient is influenced by the temperature,the existence of vegetation,the plant type and the vegetation characteristics.The vegetation can significantly improve the permeability of the system.The turf plants can easily form the clogging layer on the surface of the filter column and reduce the soil penetration rate because of their planting in the way of the sod.If the underground character of the plant is larger than that of the earth or the whole plant,the higher the dry weight,the total root length,the root depth and the relative underground growth rate of the root system,the lower the permeability coefficient of the system.The evapotranspiration of different plant systems is different.Among them,Canna indica is the strongest and Cyperus papyrus is the weakest.(2)there was no significant difference in the removal of TSS from different plants,but the removal of Arundo donax was the most stable,and plant planting could not improve the effect of the system on the removal of TSS;The effects of different plants on the removal of COD were significantly different,but all of them were higher than that of no plant systems.Among them,the removal ability of Clinopodiumurticifolium to COD was the strongest,and the median of the removal rate was 70.36%;Plants have significant effects on the nitrogen removal performance of the system,and the presence of plants can significantly increase the removal rate of NH4+-N,but there is no significant difference in the removal of NH4+-N between plants.The median of the removal efficiency of the other columns to NH4+-N is 81.06% except for the plants without plants;The removal efficiency of NO3--N was unstable.It was caused by the decay of plant roots and the death of microbes,resulting in the leaching of nitrogen in the system,resulting in the increase of NO3--N concentration in the effluent,the best removal of NO3--N from Clinopodiumurticifolium,the median of the removal rate of 74.48%,the worst removal of NO3--N in the non vegetation group and the median of only 5 of the removal rate.7%;The plant system has a high removal efficiency of TN,the median of removal efficiency is 74.09%,and the removal efficiency of TN is best with Clinopodiumurticifolium,the median of removal rate is 77.82%,the TN removal efficiency of non vegetation group is the worst,and the median of removal rate is only 67.41%;The plant has no significant effect on the TP performance,the difference between plants is not significant,the median of TP removal efficiency is 83.95%,the TP removal efficiency of Iris tectorum Maxim is the best,the median of removal rate is 87.19%,while the removal efficiency of Miscanthus sinensis is the worst and the median of the removal rate is 80.32%.Using the analytic hierarchy process(AHP)to synthesize the hydraulic performance and pollutant removal performance of plants,the comprehensive ordination of plants is determined: windmill > banana > iris > barracuda > vetiver > asparagus > Acanthopsis spp.> dogtooth > Cyperus papyrifera.(3)the accumulation of nitrogen in different parts of the plant was significantly different.In all plants,the accumulation of nitrogen in the plant stems was the highest,the total nitrogen accumulation in all plants was the highest,the accumulation of the root was not obvious,but the accumulation of leaves in the leaves(the total nitrogen content)was declining,and the main reason was that the photosynthesis of plants consumed leaves.The nutrient in it causes the consumption of nitrogen.The total nitrogen accumulation in plant roots was 0.215g/kg,the total nitrogen accumulation in stems was 1.885g/kg,and the total nitrogen accumulation in leaves was-1.52g/kg.(4)nitrogen in soil will migrate along with rainfall infiltration and produce 71.71% nitrogen accumulation before next rainfall.The accumulation of nitrogen mainly occurred in the soil of the planting layer,and the soil nitrogen in the plant system did not rise significantly,and the plant had a significant influence on the nitrogen in the soil,especially the decay of plant roots and the death of the microorganisms,which directly led to the increase of nitrogen in the planting layer.(5)the total microbial community in the system was rich in abundance,and the influence of the hydrodynamic force on the microbial community was significant.The ammonia oxidizing bacteria in the system after influent were obviously higher than that before the influent.The plant existence would affect the species of ammoniated bacteria in the system,the dominant bacteria in the planting layer were Sphingomonas,and the dominant bacteria in the submerged layer were Tolumonas.In the system,the denitrifying bacteria community is rich.There are significant differences in the microbial community between the windmill grass system and the plant free system.The dominant microbial species in the microorganism community of the windmill grass system is Dechloromonas,and the influent affects the species of the plant community greatly,and the planting layer and the submerged layer are different from the aerobic and anoxic environment.The species diversity of the biological community is distinct. |
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