Research On Phosphorus Migration Behavior In Paddy Field Under Irrigation-drainage Section In Jianghan Plain

Jianghan Plain is located in the middle reaches of the Yangtze River,there are large area of farmland formed by reclamating of lake wetlands.The characteristics of agricultural non-point source pollution in Jianghan Plain is very different from mountainous and hilly region,and the non-point source pollution is one of the main causes of the surrounding lakes eutrophication.The irrigation and drainage unit is the typical form of farmland in Jianghan Plain which is mainly composed of farmland and ditch which around farmland,and each unit is relatively isolated,both of irrigation and drainage are controlled by human.But in resent research,the study in rule of loss and the founction of retention considering the farmland and ditches are generally separated.Therefore,this study focus on the typical irrigation and drainage unit of Jianghan Plain,for example.It analyzed the characteristics of phosphorus transport and output in paddy field,and the ditch effect rules on phosphorus input by using both laboratory simulation culture and field monitoring.Finally,the paddy field and ditch are considered as a whole,we have confirmed the influence of the irrigation and drainage units to water environment.Meantime,this paper provide the basis for field management and the agricultural non-point source pollution control.The main conclusions are as follows:?1?In paddy-upland rotation planting patterns of Jianghan Plain irrigation and drainage unit.The available phosphorus has a low level with the content of farmland soil was mainly between 6-14 mg kg^-1,with an average of 9.8 mg kg^-1;Conventional fertilizers application was the major way of the farmland phosphorus input which bring 42.6 kg P hm^-2 total phosphorus every year accounting 91.7%of total phosphorus application.By calculating the phosphorus balance,the consumption of phosphorus was 3.36-8.11 kg hm^-2 per year in rice season,and the surplus of phosphorus was 7.46-17.62 kg hm^-2 in the dry season.The risk of phosphorus loss in rice season was less than dry season.?2?In rice season,the key period can control the loss of phosphorus in surface water within 12days after fertilization,and then the total phosphorus concentration become steady,fluctuating between0.5-1.5 mg L^-1 with an average of 1.04 mg L^-1,the water quality was worse than grade?.During the monitoring period,any field drainage operation had brought pollution risks of surrounding water.In paddy-upland rotation,the value of farmland soil phosphorus digestion risk was 94.5mg kg^-1,far above9.8 mg kg^-1 which was the average value of farmland soil available phosphorus.Within the 12 days of conventional fertilization treatment,the dissolved phosphorus was dominated accounting for 59%of total phosphorus while the gradually increasing particulate phosphorus concentration was 62.9%of total phosphorus after 12 days.The conclusion was that phosphorus in the surface water was mainly derived from the fertilizer.?3?In rice-wheat rotation systems,runoff mainly occurred in wheat season,but the volume of runoff was lower than rice season.And the average concentration of total phosphorus in runoff was 0.16mg L^-1 in rice season,worse than grade?standard,and it was less than 0.24 mg L^-1 in wheat season.There was no significant difference in the concentration of total phosphorus in runoff during the two corps growing pride between non fertilization treatment or conventional fertilization treatment,and the particulate phosphorus was the main form in runoff.Overall,total phosphorus in runoff was mainly from soil rather than fertilizer.?4?The water level of the ditch in irrigation and drainage unit was maintained at 30-70 cm during the whole growth period of rice.The average water level of irrigation was higher than rainfall.The drainage was main controlled by human,and the ditch will not overflow until heavy rainfall.The total phosphorus concentration was among 0.05-0.15 mg L^-1,with an average of 0.075 mg L^-1 and grade?or?of surface water quanlity.The ditch water was main dirved from irrigation,rainfall and runoff input,furthermore,the particulate phosphorus was the main form.?5?Rain water and irrigation water output through surface water and runoff,then enter ditch.The total phosphorus concentration of farm surface water increased at first and then decreased,as well as,water into ditch at a lower total phosphorus concentration than irrigation water.The irrigation and drainage unit had the function as phosphorus pool.Drainage from whole irrigation and drainage unit have a low risk while directly drainage from farm surface water and runoff will bring environment risks to surrounding water body.The results showed that the soil and sediment adsorption plays an important role in the purification of phosphorus in irrigation and drainage unit,which was based on the experiment of isothermal adsorption and adsorption kinetics.