Impacts And Mechanism Of Fish-rice Co-culture On The Migration And Transformation Of Nitrogen And Phosphorus Around The Sediment-water Interfae

Rice is an important food crop in China,which has a significant capacity for water purification.Coupling of the rice cropping and pond culture system could have great potential for the restoration of aquaculture pond eutrophication and yield enhancement.The migration and transformation of nitrogen and phosphorus around the sediment-water interface is a key process that affecting the nutrients transformation in aquaculture pond.The purification of nutrients pollution in pond by rice cropping is mainly by adjusting this process.However,the mechanism is still unclear.Therefore,in this study,we carried out a field experiments with four treatments(yellowcat fish-rice coculture,yellowcat fish monoculture,freshwater shrimp-rice coculture,freshwater shrimp monoculture)to investigate the physical and chemical variables in sediment-water interface of fish-rice co-culture pond,nutrients diffusion flux in sediment-water interface and the existing forms of nitrogen and phosphorus in sediments by using the microelectrode system and cultivating in situ.The main purpose of study is to analyze rice growth's direct and indirect effect to the migration and transformation of nitrogen and phosphorus around the sediment-water interface and to improve scientific understanding for the purification mechanism of rice planting.The main results were as followed:(1)Fish-rice coculture significantly affected the vertical distribution of O2,Eh and pH around sediment-water interface.Comparing with yellowcat fish monoculture pond,yellowcat fish-rice co-culture pond slowed down the decrease of O2 content above the sediment-water interface about 2mm,and increase the O2 penetration depths by 71.4%.Additionally,co-culture increased the Eh value,and improved the aerobic condition in the sediment of pond.Yellowcat fish-rice co-culture also decreased the pH around sediment-water interface.The pH of co-culture under the sediment-water interface about 6mm was lower than that of monoculture pond by one unit.(2)Fish-rice coculture significantly reduced the fluxes of NO3'-N?NO2--N and NH4+-N,but no significant influence of DIP flux.NO3--N?NO2--N and NH4+-N were uptaken by the sediments in August,and NO3--N and NO2--N were released from sediments in September but NH4+-N was uptaken by the sediments in yellowcat and freshwater fish-rice coculture;NO3--N?NO2--N and NH4+-N were released from sediments from October to November in coculture ponds.While NO2"-N and NH4+-N were released from sediments in August,and NO3--N?NO2--N and NH4+-N were uptaken by the sediments in other months in yellowcat fish and freshwater shrimp monoculture pond.(3)The total concentration of nitrogen in the surface sediments in this study was 1204.00-1421.78 mg/kg.Organic N,accounting for 87.52-91.98%of the total N.was the major form;The average percentage of each fraction of P followed the sequence in the surface sediments:residual-P>NaOH-OP>NaOH-IP>HCl-P>NH4Cl-P.Among these forms of sedimentary phosphorus,the residual-P was the major form.(4)Fish-rice coculture can make the content of nitrogen and phosphorus in the sediments dynamic changes with the growth of rice by absorbing and using the inorganic N and active phosphorus.The content of NOx--N(NO3--N and NO2--N)in yellowcat and freshwater fish-rice coculture were higher than monoculture from August to September,and decreased in other months.While NH4+-N showed the opposite trend during the whole rice season.The content of NaOH-P in sediments in yellowcat and freshwater fish-rice coculture were lower than that in monoculture during the whole rice season.The content of HCl-P?NaOH-IP and NH4Cl-P in yellowcat and freshwater fish-rice coculture were higher than that in monoculture from August to September,and NH4Cl-P was increased;NaOH-IP and NH4Cl-P were decreased from October to November,but still higher than that in monoculture pond.