Migration And Transformation Of Nitrogen And Phosphorus In Pond Integrated Rice-fish Coculture

Co-culture of rice and aquatic animals,which imply growing aquatic animals and rice together in aquaculture pond,has been suggested as a strategy to improve the utilization of land and water resources to provide both grains and meat to humans,while reducing the risks of environmental pollution associated with rice and fish monoculture production.Rice fish（RF）integrated system is anticipated to be more fitting to uptake and accumulate surplus nutrients of pond aquaculture and provide additional food and generate extra income while remediating ecosystem and mitigate the extra nutrient,which would otherwise become a pollution to the environment.Based an on-farm experiment we determined the optimization of the complex integrated RF system resource utilization efficiency,transport,and transformation of the basic biogenic element（Nitrogen and Phosphorus）in the entire rice-fish integrated pond culture system.The present study outcomes proved that the RF coculture in pond system has maintained the nutrients level in pond water column and bottom sediment below the environmental pollution level,and aquaculture discharge standard index.The integrated RF coculture pond system maximized the efficiency usage of nutrient and mitigated the accumulation of N and P nutrients in the pond water body,reduce the possibility of environmental pollution,and remove part of the nitrogen and phosphorus that are not effectively used in the pond by cultured animals by harvesting extra grain of rice yields.During the growing season of rice;rice enriched flooding water column with atmospheric O₂through its aerenchyma absorption to the roots by its rhizosphere secretion.Also,RF co-culture regulated the p H of water column by the secretion of organic acids of rice root rhizosphere,thus mitigate the p H in the sediment and water column in the integrated culture.The mean p H values（7.5–8.002）,together with high dissolved oxygen（9.5–10.03 mg/L）suggested that rice fixation pathway was dominated within the pond and altered the oxygen in the aquatic phase of the rice,and the decline of DO after mature stage indicated that reduced photosynthesis rate and autotrophic pathway lost importance at the end of the experiment.The concentrations of TN,ammonium nitrogen,nitrate nitrogen,nitrite nitrogen,total phosphorus and orthophosphate within the pond water body were significantly lower than the first-grade standard of discharge requirements for freshwater pond aquaculture water.The mean concentration in water column of total nitrogen were(2.548±2.08,ammonium nitrogen（0.777±0.522）,nitrate nitrogen（0.054±0.071）,nitrite nitrogen（0.033±0.015）,total phosphorus（0.126±0.097）and orthophosphate（0.054±0.05）.The balance and form of nutrients was strongly influenced by the temperature,DO and p H level.In the sediment of bottom soil,TN and TP decreased over time due to rice and plankton uptake while exchangeable nutrient due to breeding activities increased during the experiment period.The total nitrogen content in the bottom sediments of aquaculture RF ponds was 0.627±0.327 mg/g and the inorganic nitrogen accounted for 41.37%NO₃^-N>24.43%NH₄⁺-N>6.05%NO₂^--N of the nitrogen nutrient content.The content of total phosphorus in the sediment of RF pond was between 0.07±0.023 and 0.048±0.01 mg/g,which decreased over time.Organic phosphorus was not detectable because it was extremely low in sediment and was not accounted for the proportion of biological phosphorus.In this study we sought the nutrient accumulation in rice plant and fish in this system,results showed that fish and rice plant accumulated a considerable proportion of total nitrogen and total phosphorus in the pond integrated system.22%total nitrogen nutrient（316.04 kg）were removed by rice,and it accumulated 27.37%total phosphorus（57.51 kg）and fish accumulated 16%of total nitrogen（100.72 kg）in the system and a big proportion of the total phosphorus 47.81%（48.84 kg）of the entire rice fishpond system.Therefore,rice and fish contributed to the mitigation of nutrients levels.Rice plants promoted aerobic activities in the pond and decomposition of uneaten feed and fish excreta,and fish promoted the fluctuation of water in fishpond and increased the release of nutrients of the applied organic fertilizer.This gave an insight on why the content of TN was significantly lower in the bottom soil 11.2%and 47.33%in water column of pond,however TN was higher at some sampling time.Furthermore,total phosphorus contents were low in water column and bottom soil,（13.02%and 5.64%）in water and soil,respectively.This study concluded that RF integrated culture is a self-sufficient system and nutrient use were ecologically effective and reduced environmental pollution,however future studies are recommended to explore the fate of nutrients cycle and removal capacity of rice crop in remediation of pond water and soil nutrients.Also,to quantify the amount of feed and fertilizer needed for effective crop production.