Study On Transformation And Control Of Phosphorus From Agricultural Non-point Source In Small Watershed

Rural non-point source pollution is serious in Poyang Lake small watershed,environmental problems have intensified.It is of great urgency to control agricultural pollution,because the pollution from agriculture is wider and more serious compared to others.Agricultural pollution is principally transported from agricultural land.The harm is more serious than point source pollutions and hard to be monitored and controlled,because an indefinite quantity of agricultural pollutants are discharged during uncertain period through uncertain process.1.The transformation process of phosphorus ’source’:Rural life and farming is the main source of rural non-point source pollution through the background investigation and analysis of pollution sources with the method of participatory rural appraisal(PRA)in Poyang Lake small watershed.Farming in the demonstration area is the foremost main sources of rural non-point source pollution,the proportion of total and phosphorus load more than 50%and 49%,respectively.The results from the experiment of influence of phosphate application and incubation time on content of inorganic phosphorus,CaCl2-P,Bio-available Phosphorus(BAP)and Dissolved Reactive Phosphorus(DRP)in paddy soil show the fertilization can greatly increase the loss of runoff.According to the segmentation regression model,Olsen-P content corresponding for critical point of soil environmental sensitive can be infered as 98.09 mg/kg with the linear fit of the content of Olsen-P and CaCl2-P.A similar result can be showed with linear fitting to the content of Olsen-P and Pw,which is 99.50 mg/kg.Risk of P loss from soil will be increased when the application amount of P is more than 138.16kg/ha.The key point of source controlling is controlling rate of fertilizer application.2.The transformation process of phosphorus ’flow’:Plant buffer zone and ecological field ditches are the important ways of the transformation process of phosphorus ’flow’.Bermudagrass can be chosen as plants in the buffer,because its developed roots can loose soil,increase the permeability and greatly improve the capacity of interception with plant in buffer zone.Phosphorus can migrate from sediments to water because of wetting and drying cycle,but the quantity of migration decrease with the wetting and drying increasing,which shows that microbial has gradually adapted to environmental changes.So the measure of slow velocity to extend the time of contact between the soil and water,and drainage in advance can be adopted for microbes to adapt to the changes in dry-wet alternate to reduce total phosphorus loss.The experimental results with X-ray photoelectron spectroscopy(XPS)binding energy data show that no electron transfer occurs insignificantly at different pH values in sediments,i.e.no electron transfer reaction occurs on surface.This reveals that the phosphorus absorption form HPO42-in sediment substitutes OH-state on the sediment surface.The amount of phosphorus migration and transformation is the minimum under the neutral condition,and the maximum under the alkaline condition.Under aerobic atmosphere,the data of XPS binding energy shows the surface chargeof iron after oxygen aeration reduces,the surface charge of oxygen increases,i.e.Fe2+is oxidized into Fe3+,the phosphorus is absorbed increasingly;the experimental results with XPS binding energy data disclose there exists electron transfer reaction significantly,that is the illumination affects the process of phosphorus migration and transformation through dissolved oxygen(DO)and the pH impacted on the biological processes under the presence of sunlight in sediments,indirectly.The influence of illumination on the sediment phosphorus migration and transformation is insignificantly regular.Conclusively,the amount of migration and transformation in sunlight-free group is greater than the sunlight group.The important ways on controlling the loss from the transformation process of phosphorus ’flow’ are rational combination within plant buffer zoom and ditches(slower flow,longer contact)and the enhancement of ecosystem.3.The transformation process of phosphorus ’sink’:The expression of influence on the transformation process of phosphorus focus on the collection,retention and becoming P reserve pools eventually in wetlands and biological ponds.The virtuous cycle of phosphorus will be realized by giving full play to synergy of the substrate,microorganism and plant.The equilibrium phosphorus concentration(EPC0)of the interface between substrate and water in constructed wetland in demonstration area can be inferred as 0.050mg/L through the balance of the wetland phosphorus concentration experiment,which means the wetland has fine interception ability of phosphorus.New constructed wetland has a strong resistance to pollution load capacity of phosphorus through the load resistance experiment.The interception of phosphorus in wetland has been more increased with adding carbon source than without carbon source.The final effect of plant carbon source is better than that of liquid organic carbon source,glucose.The growths of the plants have a certain influence on the removal rate of phosphorus.There is a linear relation between growth heights of plants and the removal rate of phosphorus(R2 = 0.71),which shows that plant growth affects the migration process of pollutants,pollutants migration and transformation process affects the growth of plants in turn.Plants harvest also promotes the transformation and migration of phosphorus in the wetland,which suggests that plants can be used as important supplement of phosphorus interception from the wetland.The integrated mode of source Controlling-Intercepting-Recycling(CIR)is suggested to solve the problems of rural non-point source pollution.CIR mode includes three processes:emissions control from the source via changing bad water-use habits and tillage methods;sewage interception by planting buffer zone,ditch wetland;biogas produced by methane tank with livestock breeding wastewater and municipal waste,together with economic hydrophyte planted in wetland and fish cultured in the facultative pond so as to realize the waste resource recycling.The results of 16 months testing show that this mode is of not just a highly efficient decontamination,but an economic benefit.It is worthwhile to popularize in the rural areas in the Poyang Lake Watershed.Nitrogen and phosphorus loss can be intercepted effectively by integrated CIR mode(Controlling-Intercepting-Recycling,CIR),in which source control and end treatments are combined,buffer,ditch wetland,artificial wetland,biological ponds are added into a group rationally.Pollutant cutting quantity per area of the whole system is 755.711kg of CODcr,135.31kg of SS,4.23kg of TP,and 36.38kg of TN respectively,while the operation cost is less than 0.10 yuan/m3.The constructed wetland has the highest removal capacity and impact resistance,through which total phosphorus can be cut down more than 0.46g per unit area.Overall,decontamination ability of ditch wetland is not low,but the capacity of resisting impact load is weak,easily affected by external factor.Decontamination ability can be enhanced by improving the eco-system.