Experimental Study On Interception Effect Of Ecological Ditch On Nitrogen And Phosphorus In Paddy Field

In recent years,agricultural non-point source pollution dominated by nitrogen and phosphorus has become one of the largest sources of pollution in the water environment.The ecological ditch is an effective measure to control agricultural non-point source pollution.There is a nitrogen-phosphorus nutrient exchange between the overlying water and the sediment interstitial water in the ditch.The nitrogen and phosphorus in the sediment interstitial water is an important source of internal pollution.However,there are few studies on the transport of nitrogen and phosphorus between sediment interstitial water and overlying water under dynamic drainage conditions of ditch.At the same time,plant,microorganisms,matrix and hydraulic conditions in ecological ditch have unique characteristics.By combining the structure of sediment microbial community,the micro-morphology of low-grade weirs matrix and hydraulic conditions with the transport and distribution of nitrogen and phosphorus in sediment interstitial water and overlying water in ecological ditch,it is helpful to reveal the interception effect of ecological ditch on nitrogen and phosphorus.This paper takes the drainage ditches in paddy field in Xiaozhan Town,Jinnan District of Tianjin as the research object.First,the variation and distribution of nitrogen and phosphorus concentrations in overlying water and interstitial water of surface(25 cm)and deep(50 cm)soil were tracked monitoring and analyzed during the rice growing period from June 26,2017 to September 25,2017.Secondly,through exploring the purification effect and adsorption performance of nitrogen and phosphorus of different plant and adsorption matrix,a pilot model of ecological drainage ditch with mix plant-low grade weirs was constructed indoors,with blank soil ditch(no plant and weirs)as a control.The transport of nitrogen and phosphorus between sediment interstitial water and overlying water in the ecological ditch was studied under dynamic drainage conditions,and the microbial community structure of the sediment,the microscopic morphology of the low-grade weirs adsorption matrix in ecological ditch were observed.The interception effect of ecological ditch on nitrogen and phosphorus in paddy field was preliminarily revealed from the microstructure.Finally,the effects of influent flow and influent pollutant concentration on the transport of nitrogen and phosphorus in the overlying water and sediment interstitial water were explored.The main results are as follows:(1)The tracing monitor and analysis results on nitrogen and phosphorus in natural drainage ditches of paddy field in Xiaozhan Town,Jinnan District of Tianjin show that in the early stage of rice growth in Xiaozhan Town of Tianjin(June-July),due to the effect of fertilization and drainage,a large amount of nitrogen and phosphorus will accumulate in the soil interstitial water of the ditch,especially in the surface soil interstitial water(25 cm).The nitrogen and phosphorus present in the interstitial water will be transported to the overlying water,which will become an important source of internal pollution for the drainage ditch.(2)By constructing a pilot model of ecological drainage ditch with mix plant-low grade weirs and studying the interception effect of the ditch on nitrogen and phosphorus in paddy field,the results show that the ecological ditch can effectively reduce the internal nitrogen in the sediment interstitial water and nitrogen in overlying water.On the one hand,the oxygen transport of plant in ecological ditch gives it a higher dissolved oxygen content and improves the sediment microenvironment.The abundance and diversity of sediment microbes in ecological ditch are higher than those in control ditch,especially the relative abundance of Proteobacteria,which plays a central role in biological wastewater nitrogen removal.On the other hand,the adsorption matrix of low-grade weirs in the ecological ditch has a large specific surface area and pore structure,and a large number of mycelia are attached and grown,which enhances the adsorption and purification of pollutants in the water by ecological ditch.The ammonia nitrogen released from the sediment of the control ditch enters the interstitial water quickly,and then the ammonia nitrogen will be transported to the overlying water by the diffusion of concentration gradient between the interstitial water and the overlying water.The ecological ditch will weaken the diffusion of concentration gradient under the effect of plant and abundant microbial activities.Concentration gradient diffusion has little effect on nitrate and total nitrogen compared to ammonia nitrogen.However,the roots of the plant will produce acidic substances,which will promote the release of phosphorus from the sediment and increase the phosphorus concentration in the interstitial water of ecological ditch.(3)The analysis results from the influencing factors of nitrogen and phosphorus interception effect in ecological ditch demonstrate that the removal effect of total nitrogen,total phosphorus and ammonia nitrogen in ecological ditch are better under the same influent pollutant concentration and lower inflow rate.In the case of high influent pollutant concentration,a concentration gradient is formed between the overlying water and the interstitial water on the ecological ditch,so that the nitrogen and phosphorus in the overlying water are transported to the sediment interstitial water,which promotes the uptake of nitrogen and phosphorus in plant.The removal effect of total nitrogen and total phosphorus is significant.