Analysis Of The Stratified Retention And Transport Process Of Nitrogen Produced By Heavy Rainfall In Small Water Bodies

There are a large number of small water bodies in hilly and mountainous areas,which are small in area and widely distributed,which is of great significance for biodiversity protection,surrounding environment improvement and subsequent regulation.In heavy rainfall weather,such as rainstorm,many small water bodies can play a peak cutting flood detention and slow flow,interception and purification of pollution nutrients.With the continuous development of agriculture,the water quality of small water bodies is deteriorating,which seriously affects the important ecological function of nitrogen interception and purification.In recent years,along with the continuous improvement of our pollution control and management system,the related work has been promoted and deepened,and small watershed water bodies have been paid more and more attention gradually.Therefore,the study on the nitrogen transport process of small water body during rainstorm runoff and the identification of nitrogen interception and purification efficiency in the overburden water body can provide a reference for the development of small water body material transport model,which is of great significance for the treatment and protection of small water bodies in hills and mountains.Taking two ponds in a typical agricultural basin in Liuyin town,Beibei District,Chongqing Municipality as test ponds,the overhanging water of the ponds was treated by layers to simulate runoff production during rainstorm.Field experiments were conducted to analyze the hydraulic retention time and nitrogen retention capacity of the overhanging water at different depths.Laboratory experiments were carried out to study the nitrogen transport process at vertical depth of overlaid water in ponds,and the intensity of denitrification at each depth was analyzed.The influence factors of denitrification were explored by comparing the data of field experiment and laboratory experiment.The results show that:（1）Under different depths of overlying water,most nitrogen entering the pond flows slowly from the bottom and accumulates at the bottom of the pond with water flow,while a small part flows away quickly from the top to facilitate subsequent nitrogen removal.After the pond is fully mixed,the variation range of U_amb（NO₃^--N background concentration absorption rate）and V_f-amb（NO₃^--N background concentration absorption rate）in each layer is 2.50～9.13μg·（m²·s）^-1,and 0.12～0.40m·s^-1,respectively.The whole overburden water of the pond absorbs and changes nitrogen.The nitrogen in the bottom layer was continuously exchanged between the overlying water and sediments for nitrification and denitrification to reduce agricultural non-point source nitrogen pollution.（2）The average denitrification rates of reservoir bottom mud 7g,reservoir bottom mud 10g,reservoir bottom mud 15g and reservoir sediment 10g were 1.43×10^（-20）mol·d^-1,1.55×10^（-20）mol·d^-1 respectively.1.73 x 10^（-20）mol·d^-1 and 2.50 x 10^（-20）mol·d^-1.In the vertical structure of pond overlying water,the concentration of suspended sediment increases with the increase of depth,and the higher the average denitrification rate,the stronger the denitrification effect.The denitrification effect of suspended sediment overlying water is stronger than that of suspended sediment overlying water.In the overlying water of the pond,denitrification occurred mainly,followed by nitrate dissimilation reduction to ammonium（DNRA）,ammonification,anaerobic ammoxidation and nitrification,and nitrogen exchange between water samples and suspended sediment.（3）In overhanging water of small water bodies such as ponds,suspended sediment concentration,TN,NO₃^--N,NH₄⁺-N,TOC background concentration and increase and decrease,nir S-and nir K-denitrification genes and diversity,hydraulic retention time,nitrogen retention capacity,suspended sediment structure are all factors affecting denitrification,and are positively correlated.