Study On The Law And Mechanism Of Nitrogen Transfer And Transformation In Paddy Field Eco-ditch Wetland System

The non-point source pollution of rice fields has become a hot spot of research because of its wide surface and large degree of influence.Paddy field non-point source pollution control research can be divided into three parts:rice fields source control,ecological ditch(eco-ditch)interception and wetland purification.However,the process and mechanism of non-point source pollution(especially nitrogen pollution) in rice fields,eco-ditch,wetland and their rice eco-ditch wetland system are not completely clear.The model can simulate the different migration and transformation processes of nitrogen and overcome many shortcomings in the field experiment.However,the existing rice-water balance model does not simulate all of the nitrogen migration and transformation process in rice fields,and the system model of nitrogen transfer and transformation process has not been constructed yet.Based on the above deficiencies and problems in the study of nitrogen transfer and transformation in rice field eco-ditch wetland system,the experimental study on nitrogen transfer and transformation law and mechanism of rice field eco-ditch wetland system in Erhai irrigation area of central Yunnan Plateau were carried out from 2016 to 2019.The main research contents and conclusions are as follows:(1)In order to find out the law and mechanism of nitrogen transfer and transformation in rice field,the experiment of comprehensive regulation mode of water and nitrogen in rice field was carried out in four rice season continuously.The results showed that,compared with the flood irrigation mode,the intermittent irrigation mode could reduce the amount of irrigation,drainage and leakage in the field,and the effect of nitrogen application rate and fertilization frequency on them was not significant.Intermittent irrigation mode,reduced fertilization and increased fertilization times could reduce nitrogen loss in paddy field drainage,leaching,topsoil and tillage.Intermittent irrigation could also inhibit the growth of duckweed,reduce nitrogen fixation by microorganisms,increase soil mineralization and nitrogen denitrification.Increasing the fertilization times could also reduce nitrogen leaching and drainage losses in paddy fields,but increase ammonia volatilization losses.(2)Based on the results of field experiments,a comprehensive model of daily water and nitrogen balance in rice field was established.The water and nitrogen balance model of paddy field can better simulate nitrogen migration and transformation processes under the different water and nitrogen treatments including urea hydrolysis,irrigation nitrogen,microbial nitrogen fixation,wet nitrogen deposition,soil organic nitrogen mineralization,field drainage,leaching,ammonia volatilization,nitrification,denitrification,plant uptake,nitrogen absorption and release of duckweed.The verification results showed that there was no significant difference between the simulated nitrogen values and the measured values except for denitrification,and the correlation was higher.The nitrogen migration and transformation rules and mechanisms of rice fields under different water and nitrogen regulation modes were revealed comprehensively..(3)In order to study the law and mechanism of nitrogen removal from drainage of paddy field by eco-ditch under different hydraulic conditions and plant types,experiments of nitrogen removal and tracing in different flow patterns were carried out in Pontederia cordata,Canna and Zizania ditches.The results showed that,the nitrogen removal effect of non-point pollution in paddy field were the best under the static and dynamic flow of Pontederia cordata ditch.The removal rate of NH₄⁺-N and TN under dynamic flow decreased significantly with the increase of water level.The removal rate of NO₃^--N increased with the increase of water level.In general,under low water level and low flow rate the removal rate of TN was large.The nitrogen removal of the eco-ditch was the result of the interaction of V_f and u*h.When u*h was fixed,the larger the V_f,the smaller the S_W and the higher the removal rate.The hydraulic retention time had the greatest impact on the nitrogen removal rate of the eco-ditch,and was also affected by the lateral recharge and the size of the transient storage area.The hydraulic retention time of the eco-ditches mainly came from the main channel rather than the transient storage area.(4)In order to study the nitrogen removal law and mechanisms of wetland under different plant types and operation modes,the experiments of hydraulic performance and main influencing factors were carried out in Pontederia cordata,Thalia dealbata Fraser and Scirpus validus Vahl wetlands.The results showed that,the TN removal in wetland could achieve satisfactory results after a week of hydraulic stagnation.In the continuous water inflow and outflow mode,the removal effect of the Pontederia cordata wetland was the best.The nitrogen removal effect of the wetland was affected by the DO of the water body and could be reflected in the pH change of the water body at different depths.The DO and pH decreased with the water body depth increase.According to the comprehensive maximum root length,underground biomass and root porosity,the roots of Pontederia cordata were in line with the best nitrogen removal effect.The decomposing rate of different plants was about 50% for 30 days,and duckweed was the highest.Zizania aquatica was a preferred plant for nitrogen removal in eco-ditch and wetland because of its large nitrogen uptake during the growth process and the slow release of nitrogen after decomposing.The nitrogen remaining ratios of Calamus,Pontederia cordata and Thalia dealbata decomposed were relatively close and large,and their decomposition was a long-term process.(5)The nitrogen removal model and the synergistic linkage strategy of paddy field eco-ditch wetland showed that,the removal effect of nitrogen simulation by the system was better,and the simulation effect of eco-ditch and wetland were better than that of paddy field runoff.The rice field with W1N1F2 treatment could obtain the source advantage of the systematic nitrogen removal,which made the nitrogen removal rate of the whole system higher.Reducing the flow of eco-ditch and wetland was also an important measure to enhance the nitrogen removal rate of the system.The key to the synergistic removal strategy of nitrogen in paddy field eco-ditch wetland could be summarized as:Paddy fields adopt water-saving irrigation,reducing nitrogen fertilizer,and appropriate multiple fertilization;and extend the hydraulic retention time of eco-ditch and wetland through design and management measures.