| High-density and high-nutrient-input aquaculture methods are widely used in intensive aquaculture ponds.The excessive residual feed and metabolites are remained in intensive aquaculture pond sediments,leading them to cause nitrogen accumulation in intensive aquaculture ponds,which leads to the deterioration of the aquaculture water body and high nitrogen concentration in pond aquaculture tailwater.If not adequately treated,a large amount of nitrogen is discharged into receiving water bodies such as rivers,lakes,and reservoirs along with the pond aquaculture tailwater,which would inevitably cause ecological environmental issues like eutrophication in receiving water bodies.For the sustainable development of aquaculture,the up-to-standard discharge of pond aquaculture tailwater is essential.In recent years,pond water quality treatment technologies,such as ecological ditches and constructed wetlands,can achieve high nitrogen removal effect in high-temperature season,while the nitrogen removal effect is often poor in low-temperature season with low activity of bacteria and aquatic plants.Therefore,it is necessary to investigate and analyze the nitrogen removal pathway and efficiency of pond aquaculture tailwater in low-temperature season,so as to provide the theoretical basis for improving the pond aquaculture tailwater treatment technology in low-temperature season and making it discharge up to standard.In order to determine which is the main body of nitrogen removal in pond aquaculture tailwater,we investigated the nitrogen absorption contribution rate for low-temperature tolerant plants in low-temperature season,nitrogen cycle pathways,and functional bacteria in pond aquaculture tailwater by using metagenomic technique,mowing plant and other technologies.In addition,the efficient in-situ nitrogen removal treatment technology with nitrate diffusion nitrogen reduction technology as the main body was also developed,and the efficiency of nitrate diffusion nitrogen reduction technology in nitrogen removal of intensive pond water was evaluated in intensive aquaculture ponds,and the nitrogen removal mechanism of nitrate diffusion nitrogen reduction technology was explored.The main results are as follows:（1）Through the plant cutting experiment,this study found that the total nitrogen（TN）removal amount of the ecological ditch simulation system in low-temperature season was1633.68±253.60 mg/m2,the plant TN absorption amount was only 30.13±16.60 mg/m2,and the plant absorption contribution rate to nitrogen in the low-temperature season was1.85±0.92%.The lower plant TN absorption amount and plant absorption contribution rate to nitrogen suggested that microbial metabolism was the primary mechanism for nitrogen removal in pond aquaculture tailwater rather than plant assimilation and absorption in low-temperature season.（2）The microbial community composition of pond aquaculture tailwater in low-temperature season was determined using the metagenomic technique.The results showed that the phylum with the highest relative abundance in pond aquaculture tailwater of low-temperature season was Proteobacteria（66.95%）;the dominant order in pond aquaculture tailwater of low-temperature season was Burkholderiales（22.56%）;the genus with the highest relative abundance in pond aquaculture tailwater of low-temperature season was Pseudomonas（25.91%）.（3）The nitrogen cycle pathways and functional bacteria in pond aquaculture tailwater of low-temperature season were analyzed by using the metagenomic technique.The results showed that the main nitrogen cycle pathways in pond aquaculture tailwater of low-temperature season were nitrogen mineralization（42.71%）,nitrogen assimilation（38.63%）,nitrite oxidation（12.01%）,denitrification（3.40%）,nitrate reduction（1.83%）,nitrogen fixation（1.04%）,and dissimilation nitrate reduction to ammonium（DNRA）（0.37%）.Among them,the nitrogen cycle pathways involved in nitrogen removal are mainly nitrogen assimilation,nitrite oxidation,and denitrification.The functional bacteria driving nitrogen assimilation were mainly Rhizobiales,with the relative abundance of 14.31%;The functional bacteria driving nitrite oxidation were mainly Burkholderia,with the relative abundance of 43.69%;The functional bacteria driving denitrification were mainly Chitinophagales,with the relative abundance of 42.49%.In low-temperature season,nitrogen in pond aquaculture tailwater was temporarily stored in bacteria in the form of organic nitrogen through nitrogen assimilation on the one hand,and removed from the system in the form of nitrogen gas（N2）through denitrification on the other hand.（4）The recirculating aquaculture system（RAS）tailwater containing nitrate was slowly diffused into the intensive aquaculture pond sediments through the simulation experiment.The results showed that the nitrate diffusion nitrogen reduction technology significantly reduced ammonium-nitrogen（NH4+-N）concentration in interstitial water（the maximum NH4+-N removal rate was 56.13±5.43%）and total organic carbon（TOC）content（the maximum TOC removal rate was 19.64±7.87%）in intensive aquaculture pond sediments.Nitrifying bacteria and denitrifying bacteria abundance in intensive aquaculture pond sediments was significantly increased.Nitrate diffusion nitrogen reduction technology can effectively promote the organic matter（OM）decomposition by denitrifying bacteria,thus removing the anoxic restriction of nitrification in sediments,realizing the coupling of nitrification and denitrification,reducing the nitrogen accumulation in intensive aquaculture ponds. |
