Sources And Characteristics Of Phosphorus Loss In Non-point Source Pollution From Tangxun Lake Watershed

Eutrophication of lake has become one of the most serious water-related environmental problems.Phosphorus is a key limiting factor for eutrophication.With the continuous effective control of point source phosphorus input,non-point source pollution has gradually become the main source of phosphorus in water.Therefore,accurately understanding the sources and characteristics of phosphorus loss from non-point source pollution in watersheds has become an important prerequisite for targeted control of non-point source pollution.In this paper,the sources of non-point pollution phosphorus in the basin are analyzed by determining the distribution characteristics of phosphate oxygen isotope abundance at different points in the Tangxun Lake,combined with the survey results of the current status of the water system and water environment in the Tangxun Lake Basin.On this basis,through simulated rainfall experiments,the loss characteristics and eutrophication risk of non-point source phosphorus polluted mainly by surface runoff were revealed.The main conclusions are as follows:（1）The concentrations of total phosphorus（TP）and inorganic phosphorus（IP）in the waters of the eastern and western lakes of Tangxun Lake are obviously different.The concentrations of TP and IP in the waters of eastern lake area are higher than those in the western lake area.Although the waters in the eastern and western lake area are connected by culverts,the exchange of waters between the two lake areas is very limited.The oxygen isotope ratios of phosphate(δ¹⁸O_PO4)of the two lake areas are also significantly different.Theδ¹⁸O_PO4 in the waters of the eastern lake area is higher than that of the western lake area.Under the premise of very limited water exchange between the eastern and western lake area,combined with the current status of land use and water supply and drainage system construction around the eastern and western lake areas,the analysis shows that the non-point source pollution caused by a large amount of agricultural land in the southern part of the eastern lake area is the main reason for the higher TP and IP concentrations in the eastern water.（2）The simulated rainfall experiment results show that changing the rainfall intensity(60 mm·h^-1,90 mm·h^-1 and 120 mm·h^-1)or slope gradient（5°,10°and 15°）,the surface runoff outputs of yellow brown soil with inorganic fertilizer,organic fertilizer,fermented straw and no fertilization treatment increase with the increase of rainfall intensity,and show a decreasing trend as the slope gradient increases;the sediment yields increase as the rainfall intensity and slope gradient increase.The form of phosphorus loss by surface runoff of yellow brown soil is mainly particulate phosphorus（PP）,and inorganic fertilizer,organic fertilizer and fermented straw all obviously increase the amount of phosphorus loss by surface runoff.Under the conditions with the same rainfall intensity and different slope gradients,the proportions of the average runoff of PP to TP increase with the increase of the slope gradient;under the conditions with the same slope gradient and different rainfall intensities,the proportions of the average runoff of PP to TP increase with the increase of the rainfall intensity.The results of graded extraction of different forms of phosphorus in the sediment indicate that the content of different forms of phosphorus in sediments using inorganic fertilizers,organic fertilizers and fermented straws increases significantly,and the sediment applying organic fertilizers has the highest organic phosphorus（OP）content.The sum of OP and Fe/Al-P in the sediment with organic fertilizer accounts for 83.87%of TP,and the sum of OP and Fe/Al-P in the sediment with the inorganic fertilizer accounts for 97.33%of TP.This shows that the application of inorganic fertilizer leads to the loss of more biologically effective phosphorus,but the loss of phosphorus caused by the application of organic fertilizer cannot be ignored either.