Regulation Of Rice-fish Co-culture On The Nutrient And Oxygen Content In Pond Water

Pond aquaculture has occupied an important position in China.In order to increase production,it has been commonly adopted of high-density and intensive cultivation mode in pond aquaculture of our country,resulting in serious problems of eutrophication and anoxia in aquaculture water.Rice is the major food crop in China,it has significant absorption capacity for water nutrients and the function of radial oxygen loss.So rice-fish coculture can help reduce the eutrophication and hypoxia of aquaculture water.However,there is still a lack of systematic and thorough research on the process of oxygen cycling in the system of rice and fish coculture.In this study,we choose yellow catfish-rice coculture and freshwater shrimp-rice coculture two modes,through plot experiments,dynamically monitoring the change of water nutrient including nitrogen,phosphorus and potassium,mainly oxygen production and oxygen consumption process in the pond water in the rice-fish coculture and fish monoculture ponds.The effects of rice fish coculture on nutrient reduction in aquaculture ponds and the regulation of rice fish coculture on the main oxygen production and oxygen consumption were compared and analyzed.The results are as follows:（1）Rice-fish coculture can significantly reduce the nutrient content in pond water,while yellow catfish-rice coculture has a better nutrient removal capacity than freshwater shrimp-rice coculture.The final content of TN,TP in the water of yellow catfish-rice coculture were reduced by 58.1%,80.7%compared to yellow catfish monoculture.The final content of TN in the water of freshwater shrimp-rice coculture were reduced by 56.4%compared to freshwater shrimp monoculture.The water of yellow catfish fish monoculture did not meet the second emission standards"discharge requirements of freshwater pond aquaculture water",while the water of yellow catfish-rice coculture was close to the first emission standard.Yellow catfish-rice coculture had a better absorption of NH₄⁺-N,NO₂^--N in the pond water,and the content of NH₄⁺-N,NO₂^--N in the water of the yellow catfish-rice coculture were reduced by 89.9%,62.8%compared to yellow catfish-monoculture.The potassium content in the water was significantly reduced by yellow catfish-coculture too.（2）Rice-fish coculture had no significant effect on the diurnal variation and seasonal variation of DO value in the water,but it can increase oxygen content in the sediment through radical oxygen loss.There has the same diurnal and seasonal variation of DO content in the rice-planting and non rice-planting pond water.It had a close DO content in the water of yellow catfish-rice coculture and yellow catfish-monoculture,but the lowest DO content of rice-fish coculture was delayed by1 h as compared with that of the yellow catfish monoculture.The seasonal change of DO content had the same trend in yellow catfish-rice coculture and freshwater shrimp-rice coculture compared to fish monoculture respectively,but there had no significant difference.Using the microelectrode system,the peak of O₂ was detected at the bottom soil interface of yellow catfish coculture in the depth of 12mm,30.5mm,44mm into water-sediment interface,indicating that rice could release O₂ to the sediment through the radial oxygen loss.（3）rice-fish coculture had no significant effect on net photosynthetic oxygen production in pond water,but significantly reduced water respiration and sediment oxygen demand in the pond aquacuture.In the rice growing season,the average net production of the bottom water of the yellow catfish-rice coculture was increased by88.3%compared with yellow catfish monoculture.The average water respiration rate of the surface and bottom water,the sediment oxygen demand were decreased by63.3%,90.6%and 36.2%respectively in the yellow catfish-rice coculture compared with yellow catfish monoculture,while the sediment oxygen demand of freshwater shrimp-rice coculture was decreased significantly by 38.2%compared with freshwater shrimp monoculture.During aquaculture,the respiratory rate of yellow catfish（0.312mg-O₂/g/h）was 17.7%significantly higher than monoculture fish ponds（0.265mg-O₂/g/h）,while the respiration rates of freshwater shrimp were 0.406 mg-O₂/g/h and 0.410 mg-O₂/g/h,respectively,with no significant difference.