| Agricultural non-point source pollution is one of the main sources of surface water pollution in China.Agricultural non-point source pollution of nitrogen and phosphorus is the main cause of water eutrophication.There are currently many control measures for agricultural non-point source pollution,such as artificial wetlands,vegetation filter strips,buffer strips,and other technologies.However,because these technologies occupy a large area and are difficult to be popularized and applied in a large scale,a new ecological engineering technology is needed to intercept agricultural non-point source pollution.Periphyton is a biological aggregate consisting of various organisms such as algae,bacteria,fungi,and protozoa,widely distributed on the surface of paddy soil.Surrounding organisms have a unique composition and diverse community structure,which can effectively absorb excessive pollutants such as nitrogen,phosphorus,and heavy metals in water bodies.They have been widely used in water pollution control,habitat construction,water quality purification,and soil remediation.However,the natural periphyton in the wild has shortcomings such as low biomass,unstable community structure,and limited purification ability.Therefore,it is necessary to artificially induce periphyton in rice fields.In this paper,the experimental field in Jurong City,Jiangsu Province,was taken as the research area,and the periphyton biological culture medium,sodium alginate and calcium chloride were used to prepare the periphyton biological artificial induction vector.Starting from rice transplanting,a 45 kg/ha artificial induction carrier of periphyton was applied to the experimental field to induce the growth of periphyton in the rice field.The interception effect of artificially induced periphyton on nitrogen,phosphorus,and heavy metals(Cu,As,Ni,and Cd)in the water and soil of the rice field was monitored throughout the entire rice growth period.From the rice turning green period in June 2022 to the rice ripening period in October 2022,samples of periphyton,soil,and water in the rice field were collected every 7 days to analyze various environmental factors and periphyton community structure,in order to evaluate the interception effect of artificially induced periphyton on nitrogen,phosphorus,and heavy metals in the rice field,as well as the improvement of downstream environmental risks.The main results obtained were as follows:(1)During the period from rice turning green to maturity,the nutrient concentration in the field water first increases,but then rapidly decreases with the absorption of nutrients by rice.Additionally,the nitrogen and phosphorus in the experimental field water are lower than those in the control field.This indicates that the artificially induced periphyton acted as a "buffer" and "slow-release device" for rice fertilizer at different stages.The concentration of nitrogen and phosphorus in the paddy soil first increases and then decreases,and the nutrient content in the experimental soil is higher than that in the control soil.This indicates that Zhou Cong organisms have become the "cover" on the surface of paddy soil.This proves that artificially induced periphyton organisms are more effective in intercepting the loss of nutrients in paddy soil.The concentration of heavy metals in the overlying water of the experimental field is lower than that of the control field,while in the surface soil of the rice field,the concentration of heavy metals in the experimental field is higher than that of the control field.This indicates that the artificially induced periphyton has a stronger ability to intercept heavy metals,reducing the risk of heavy metal pollution in downstream water bodies caused by rice field drainage.(2)In order to demonstrate the interception effect of artificially induced periphyton on nitrogen and phosphorus in rice field water,five water quality evaluation methods were used to evaluate the water quality of rice fields.Among them,the evaluation results of the single factor pollution index method and the single factor water quality labeling method showed that ammonium nitrogen and total nitrogen exceeded the standard values,and total nitrogen far exceeded the water quality standards specified in the Surface Water Environmental Quality Standard(GB3838-2002).The evaluation results of the comprehensive pollution index method show that the water quality levels of the experimental group and the control group are the same,but there is a significant difference in the comprehensive pollution index(?) between the two groups.The mean(?) of the experimental group was 2.304,while the control group was 3.274.Under the interception effect of artificially induced periphyton organisms,the water quality of rice fields improved by 29.6%.The evaluation results of the Nemero pollution index indicate that the water quality level of both the experimental group and the control group is inferior to Class V.Among them,the average Nemero pollution index of the experimental group was 9.93,while the control group was 12.55,resulting in a 20.9% reduction in pollution levels.The evaluation results of the comprehensive pollution index method and the Nemero pollution index method indicate that even if artificially induced periphytes cannot significantly improve the water quality level,they can improve the water quality of rice fields to varying degrees.The evaluation results of the comprehensive water quality labeling method are superior to the other four evaluation methods.The comprehensive water quality identification index method was used to evaluate the water quality of the experimental and control fields.The average CWQI of the experimental group was 5.79.This indicates that the water quality of the experimental group is Class V water,meeting the target water quality requirements,while the average CWQI of the control group is 6.80,which is not black,odorous,or inferior to Class V water.The comprehensive pollution situation has improved by 14.8%.This proves that artificially induced periphyton effectively reduces the pollution risk downstream of rice fields(3)The storage potential of artificially induced periphyton for nitrate nitrogen,ammonium nitrogen,total nitrogen,and total phosphorus is 37.7%,12.08%,12.21%,and 7.12% higher than that of natural periphyton,respectively.The storage potential of artificially induced periphyton for Cu,As,Ni,and Cd is 8.7%,15.7%,6.5%,and 19.5% higher than that of natural periphyton,respectively.In the early stage of rice transplanting,the relative abundance of 10 bacterial species,including Firmicutes,Teneristes,and Fusobacteria,in artificially induced periphyton is much higher than that of natural periphyton in the control field at the same period.In artificially induced periphyton treatment,the changes in community structure during the early and late stages of rice growth are smaller than those of natural periphyton.This indicates that the variation in the composition of artificially induced periphyton communities on a time scale is smaller,which is why it is necessary to maintain a stronger ability to intercept nonpoint source pollution in rice fields.Compared with natural periphytes,artificially induced periphytes contain more enriched green bay bacteria,proteobacteria,and bacteroides.In a word,artificially induced periphyton can effectively intercept the migration of nitrogen,phosphorus and heavy metals in paddy soil to the overlying water,which not only reduces the concentration of nitrogen,phosphorus and heavy metals in paddy water,reduces the risk of water pollution caused by farmland drainage,but also extends the retention time of nutrients in paddy soil,and improves the nutrient utilization efficiency of paddy soil. |
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