Study On The Characteristics Of Nitrogen And Phosphorus Pollution Of Facility Agriculture In Qionghai Lake Basin

Qionghai,the second largest freshwater lake in Sichuan Province,which plays an important role in the ecological environment and socio-economic development of Xichang city.The agricultural non-point source pollution in Qionghai lake has a great impact on the water quality.In this study,Chuanxing Town,a typical facility agriculture planting area located on the north of Qionghai Lake,was selected as the research area.The soil of facility farmlands with different planting years and ordinary farmland,as well as surface water and groundwater samples were collected,and the contents and morphological components of nitrogen and phosphorus in soil and water samples were measured.The pollution ways and effects of long-term planting facility agriculture on soil and water environment around Qionghai Lake basin were studied.Through the simulated rainfall leaching experiment,the loss characteristics of nitrogen and phosphorus nutrients in the soil of facility farmland were explored,and the risk of nutrient loss and pollution in the long-term cultivated facility agriculture was revealed.Combined with the hydrodynamic model,the reasons for the temporal and spatial distribution characteristics of the main pollutants in Qionghai sea were simulated,and the influence of the pollution inputs of surrounding agricultural sources on the water quality of Qionghai sea was further clarified.The main results were as follows:(1)Accumulation characteristics of soil nitrogen and phosphorus in facility agricultural planting areas: the average contents of nitrogen and phosphorus in facility farmland soil were 1.5 and 2.5 times higher than those in ordinary farmland soil respectively.At the initial stage of planting,the accumulation of nitrogen and phosphorus in the soil of facility farmland was small.After 15 ～ 20 years of planting,the average content of total nitrogen and total phosphorus in the soil reaches the maximum of 0.47 g/kg and 0.69 g/kg.The distribution of nitrogen and phosphorus in the soil profile of facility farmland showed obvious surface accumulation,which was concentrated in the 0～30 cm soil layer,while there was also a large amount of available nitrogen accumulation in the 60～120 cm soil layer.With the increase of planting years,nitrogen and phosphorus in the soil of facility farmland continue to accumulate,increasing the risk of shallow groundwater pollution.(2)Characteristics of nitrogen and phosphorus pollution in water body of protected agricultural planting area: In the surface water,the pollution degree of Guanba river was relatively light,while the concentration of nitrogen and phosphorus in the sewage ditches around the facility vineyards and the intercepting ditches around Qionghai Lake was relatively high.In the groundwater,the nitrate concentration of facility farmland was high,and some of them had exceeded the class V water quality standard.The average nitrate concentration in the groundwater of facility farmland was 1.2,3.5 and 4.1 times that of sewage ditch groundwater,ordinary farmland groundwater and well water respectively.The leaching loss of nutrients in the soil of protected vineyards was mainly nitrate nitrogen,and the pollution to groundwater become more and more serious with the increase of planting years.Compared with the surrounding facilities and agricultural planting areas,the altitude and terrain of Qionghai is low,and the pollution of groundwater would increase the pollution risk of Qionghai water quality.(3)Characteristics of soil nitrogen and phosphorus loss in protected agricultural planting areas: Total nitrogen in the soil of facility farmland was easy to leach.Under the condition of initial irrigation,a large amount of total nitrogen was lost,while the loss rate of phosphorus was relatively slow.The average loss rate of nitrogen and phosphorus in the soil of facility farmland was 5.67 and 2.46 respectively.While the existing accumulation of nitrogen and phosphorus in the soil of facility farmland was high,if the land use mode changes in the future,the leaching loss of nitrogen and phosphorus in the soil may cause serious pollution to the groundwater environment when it is scoured by a large amount of precipitation.(4)Distribution characteristics of nitrogen and phosphorus in Qionghai Lake:Qionghai basically maintained the class II water quality of surface water,the nutritional status was at the medium nutritional level,and the main pollution factors were ammonia nitrogen,total phosphorus and total nitrogen.The water quality in the lake area was closely related to the water quality of tributaries,and the correlation degree with the water quality of Guanba river was the highest,and the correlation degree of TN was higher than TP.The agricultural source in the upper reaches of the surrounding area was the main pollution source.Due to the large amount of fertilizer and irrigation in the process of agricultural planting in the surrounding facilities in the dry season,the water quality of Qionghai sea had significant seasonal differences,and the water quality in the dry season was significantly worse than that in the wet season.The nutritional status in the northwest of the lake area was higher than that in the southeast.Combined with the hydrodynamic model simulation results,it showed that under the condition of south wind,the pollutants input by tributaries were more likely to migrate to the western waters of the lake area,which may cause the regional water quality exceeding the standard and even eutrophication.