Nitrogen And Phosphorus Budgets In The Circulating Aquaculture System Of Constructed Wetland And Occurrence Modes

The experiment takes vertical subsurface flow constructed wetland-pond circulating aquiculture system as the research object,based on a series of research results since the establishment of the circulating water aquaculture system of the research group.Measure the input and output of nitrogen and phosphorus in the constructed wetland-pond circulating aquaculture system during the trial period,calculate the nitrogen and phosphorus budgets,clarify the decontamination path of artificial wetlands;Evaluate the purification effect of constructed wetland on aquaculture tailwater,and analyze the reasons for the fluctuation of water purification efficiency in different periods from the perspective of nitrogen and phosphorus budgets;Measure the changes of nitrogen and phosphorus occurrence forms and the diffusion flux of nutrients at the water-sediment interface at different periods of time.The main findings are as follows:Feed input is the most important way for nitrogen and phosphorus input of the circulating aquaculture system.The input of nitrogen and phosphorus is 49.49kg and 10.79 kg,accounting for 65.61%of the total nitrogen input and 81.44%of the total phosphorus input,which is significantly higher than other projects;Followed by water layer input,the nitrogen and phosphorus input accounted for 19.42%and 7.57%of the total input,respectively.The contents of nitrogen and phosphorus contributed by farming organisms and plants are relatively low.The fingerlings accounted for 6.58%and 6.90%,and the nitrogen and phosphorus content of the plants was less than 1.50%of the total.Sediment accumulation is the most important way of nitrogen and phosphorus expenditure.The total amount of nitrogen and phosphorus accumulated is 21.96kg and 5.82 kg,accounting for 28.60%and 42.23%of the total output;Farming organisms are the secondary route of output.The total nitrogen and phosphorus contained in the fish body accounted for 26.05%and 30.77%of the total output.The substrate accumulation in the constructed wetland and the nitrogen and phosphorus contained in the plant accounted for the total output.15.81%and 16.11%;Nitrogen and phosphorus contained in the bottom sediment leakage of aquaculture ponds accounted for the least proportion of all output projects,accounting for 2.33%and 1.31%of the total output.In the later stage of the experiment,the percentages of total nitrogen and phosphorus in cultured organisms and plants increased significantly compared with the initial stage.The mass fraction of total nitrogen in the culture pond sediment is 3.97～5.57mg·g-1,and the mass fraction of exchangeable nitrogen is 44.99～103.47μg·g-1,which accounts for 0.97%-2.58%of the total nitrogen content.The highest proportion of exchangeable nitrogen is NH4+-N,and the lowest is NO2--N.Phosphorus forms are divided into NH4Cl-P,Fe-P,Al-P,Ca-P,Res-P,and OP.The mass fraction of TP is 2.36-3.29mg·g-1.OP is the main component of total phosphorus,accounting for 49.81%～66.84%of TP.The content of NH4Cl-P in the sediment was the lowest among all forms,and the Fe-P and Al-P accounted for 15.22%～21.78%of the total phosphorus.The content of Ca-P is 86.51～100.89μg·g-1,and the content of Res-P is 482.51～595.34μg·g-1.There is no significant difference in the proportion of its content in inorganic phosphorus in different periods.During the experiment,the diffusion flux of NH4+-N was 86.24～289.30mg·m-2·d-1,and the diffusion direction was that the sediment was released to the overlying water.The diffusion fluxes of NO3--N and NO2--N are-22.35～15.84mg·m-2·d-1 and-2.79～1.98 mg·m-2·d-1.The diffusion flux of DIP is-24.35～31.31mg·m-2·d-1.The sedimentation from the overlying water to the sediment at the beginning of the experiment,and the diffusion from the sediment to the overlying water at the end of the experiment.Based on the above research results,compared with the traditional pond culture model,constructed wetland has relatively reduced the accumulation of nitrogen and phosphorus nutrients in the sediment of the pond,reduced the possibility of endogenous pollution outbreaks in the pond,and removed unused nitrogen and phosphorus in the pond by means of harvesting above-ground parts of wetland plants and replacing supersaturated substrates.The contribution of constructed wetland substrates,microorganisms,and plants in the process of nitrogen and phosphorus removal affects the removal rate of nutrients in different periods.This experiment lays the foundation for further research,it is expected to provide theoretical support for the long-term stable operation of constructed wetlands and the establishment of ecologically beneficial pond breeding models.