| Rice is our country’s largest food crop,with an annual sown area of about 450 million acres,which plays an extremely important role in my country’s food security.At the same time,rice is also a big consumer of fertilizer and water;the nitrogen and phosphorus nutrients brought by fertilization during the growth of rice enter the surrounding water body through surface runoff and underground leakage under the action of rainfall and artificial drainage,which causes the water body to be eutrophic.One of the important reasons for the transformation.The irrigation and drainage unit composed of rice fields,ditches and ponds is a typical agricultural production unit in the middle reaches of the Yangtze River.Current researches on non-point source pollution in rice fields mostly separate the rice fields from the surrounding ditches and ponds,and study the law of nitrogen and phosphorus nutrient loss and the interception effect.At present,relevant researches are mostly focused on the removal of nitrogen and phosphorus by factors such as aquatic plants,sediment microorganisms,climatic conditions,hydrology and hydraulic power,and physical and chemical environmental factors;research on the reduction of nitrogen and phosphorus around the regulation and storage capacity of irrigation and drainage units less.To this end,this study takes the field-ditch-pond of the rice field irrigation and drainage unit in the middle reaches of the Yangtze River as the research object,and reveals the characteristics of nitrogen and phosphorus cascade changes in the irrigation and drainage unit through a combination of field in-situ monitoring,laboratory physical and chemical analysis,and scenario simulation.The interception potential of nitrogen and phosphorus in ditch ponds provides method support for the monitoring,management,and prevention of the loss of nitrogen and phosphorus in paddy fields.The main conclusions of the study are as follows:(1)During the monitoring period of the water volume of the rice field irrigation and drainage unit in 2019,the rainfall was 259.5mm,the total irrigation water volume of the rice field was 21960.6 m3,the irrigation volume per unit area of the rice field was 784.9mm;the total drainage volume was 2988.3m3.A total of three drainage incidents occurred during this period.The first was the field drainage.The total drainage of the field was 644.18m3,and the total drainage at the outlet of the irrigation and drainage unit was 1.79m3.The drainage of the field was almost completely intercepted by the ditches and ponds;the second was It is rainfall drainage.The total drainage volume of the field is 2048.08m3,the total drainage volume of the irrigation and drainage unit outlet is 2336.20 m3,and the interception of the drainage from the ditch and pond to the field is-288.12 m3.This rainfall is a heavy rainstorm(104.3mm).Part of the original water in the ditches and ponds was drained;the third time was also rainfall drainage.The total drainage volume of the field was 296.02m3,the total drainage volume of the irrigation and drainage unit outlet was 93.80 m3,and the rainfall this time was 56.9mm.The ditch and pond intercepted 202.22m3 of field outlet drainage,accounting for 68.3%of the total field drainage.Under the three drainage events,the total field-scale drainage volume was 2988.3m3,the total drainage unit-scale drainage volume was 2431.8 m3,and the total drainage volume intercepted by the irrigation and drainage unit ditches and ponds to the field was 556.5 m3,and the interception rate was 18.6%.Field drainage is not directly discharged into surrounding water bodies.Part of it will be intercepted by ditches and ponds.Ditches and ponds have a strong storage capacity for field drainage,especially for field drainage and rainfall runoff below heavy rain.(2)On-site continuous observation of the water quality of the irrigation and drainage unit found that the water quality of the ditch and pond changes regularly with the field water.Within 5 days after fertilization,the nitrogen and phosphorus concentrations of the water stored in ditches and ponds were significantly lower than the surface water,and the peaks of nutrient concentrations in paddy fields,ditches and ponds lag behind.According to the water balance of irrigation and drainage units,the loss of nitrogen and phosphorus in fields and irrigation and drainage units and the interception of nitrogen and phosphorus in ditches and ponds under the three drainage events during the rice growth period in 2019 are as follows:Total nitrogen(TN)loss in the field during the first soaking field drainage The amount is 2.02kg,the loss of total phosphorus(TP)is 0.11kg,the loss of TN in the irrigation and drainage unit is 0kg,the loss of TP is 0kg,and the interception rate of TN and TP in the drainage of the ditch and pond to the field is 100%;In the secondary rainfall drainage,the loss of TN in the field block was 5.14kg,the loss of TP was 0.53kg,the loss of TN in the irrigation and drainage unit was 5.44kg,the loss of TP was 0.72kg,and the interception rate of TN and TP in the drainage of the ditch and pond.Respectively-5.8%and-35.8%;in the third rainfall drainage,the loss of TN in the field was 0.96kg,the loss of TP was 0.04kg,the loss of TN in the irrigation and drainage unit was 0.22kg,and the loss of TP was 0.02kg.The interception rates of TN and TP in field drainage are 77.1%and 50%,respectively.In the three drainage events,the total TN loss of the field block was 8.12 kg,the total TP loss was 0.68 kg,the total TN loss of the irrigation and drainage unit was 5.66 kg,and the total TP loss was 0.74 kg.The interception of TN is 2.46kg,the interception of TP is-0.06kg,and the interception rates are 30.3%and-8.8%,respectively.Ditch ponds have the ability to intercept nitrogen and phosphorus in field drainage;in some extreme rainfall situations,the original water stored in the ponds will be discharged to prevent waterlogging,which cannot guarantee the nitrogen and phosphorus interception capacity of the ponds.In general,the interception of ditches and ponds reduces the risk of nitrogen and phosphorus loss from paddy field drainage.(3)The surface water is the source of nitrogen and phosphorus loss in the paddy field of the entire irrigation and drainage unit.The dynamic changes of the nitrogen and phosphorus concentration in the paddy field surface water under different planting methods and different fertilization methods are analyzed.The results show that there are two different planting methods:artificial planting and machine transplanting.The changing law of nitrogen and phosphorus concentration in the surface water of rice fields is basically the same,and its fluctuation is mainly affected by irrigation and drainage events.Conventional fertilization and organic fertilizer partial replacement of compound fertilizers have certain differences in the nitrogen and phosphorus concentration changes in paddy field surface water under the two fertilization methods.The main manifestation is that the nitrogen concentration in paddy field surface water of conventional fertilization is slightly higher than that of organic fertilizer.Partially replace the rice fields where compound fertilizers are applied.(4)In order to judge the water environmental risk of rice field drainage after fertilization,taking total nitrogen as an example,the rapid detection method of paddy field surface water quality was discussed.Taking dissolved oxygen,conductivity,pH,redox potential and other factors that can be obtained quickly and in real time as independent variables,and TN as the dependent variable,a multiple linear regression model is obtained to serve the monitoring and management of the risk of nutrient loss in paddy field surface water.According to the "Urban Wastewater Treatment Plant Pollutant Discharge Standard"(GB 18918-2002)the TN limit of 15 mg·L-1(Class A standard)and the "Surface Water Environmental Quality Standard"(GB 3838-2002)TN The limit of 2 mg·L-1(Class V)is used to classify the TN concentration,and the accuracy of the model prediction is 80%.This method is used to quickly determine the environmental risk of field water to surrounding water bodies,which basically meets the needs of water environmental management.(5)Scenario analysis was conducted using the water quantity and water quality response model software of the paddy field irrigation and drainage unit,and the fertilization measures of "front nitrogen shift" and the proportion of ponds(the ratio of the control area/irrigation and drainage unit’s pond surface area to the total field area)were discussed.The impact of changes in structural parameters such as depth of ditch and depth of ponds on the reduction of nitrogen and phosphorus loss.The structural parameters of the irrigation and drainage unit directly affect the storage capacity of the ditch and pond for the drainage of the field.When the risk of nitrogen and phosphorus loss in the paddy field is greater,the storage capacity of the ditch and pond can be vacated in advance to accept the high nitrogen and phosphorus concentration of the field for drainage,and used Follow-up circular irrigation to strengthen the nitrogen and phosphorus reduction effect of the irrigation and drainage unit.Based on the scenario simulation of the structural parameters and fertilization measures of the irrigation and drainage unit,the results show that the nitrogen and phosphorus reduction potential of the irrigation and drainage unit increases with the expansion of the pond storage capacity.Lost load is reduced by 30.3%and 42.1% respectively. |
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