Transfer And Loss Of Phosphorus From Agriculture Non-point Source Of Xiangxi River Watershed

Water blooms were appeared in some tributaries of the Three Gorges Reservoir of China after impoundment. And nitrogen(N) and phosphorus(P) loss caused by excess use of fertilizer and pesticide, transport in surface runoff and soil erosion, is considered a major contributor to agricultural non-point source(ANPS) pollution and water eutrophication in the Three Gorges Reservoir Area(TGRA). This research focused on demonstrating P from ANPS transfers mechanisms and loss characteristic in Xiangxi River Watershed(XXRW), which is the largest tributary of the Yangtze River in Hubei Province. Based on data analysis of investigation, four main agriculture soils in XXRW, purple soil(PS), yellow soil(YS), calcareous soil(CS), and yellow brown soil(YBS), were exploited to approach P loss process by simulation experiments in laboratory. Moreover, N and P loss characteristic were analyzed through setting up runoff plots in TGRA under natural rainfall and local agricultural practices. Furthermore, ANPS prevention and control measures were proposed accordingly. These results provide useful and valuable information for decision makers and planners to take sustainable measures for control of ANPS pollution in TGRA. The main research contents and results of this thesis are shown as follows:(1) Four types of soils in XXRW were used to learn P retention/release and sorption/desorption characteristics and mechanism under constant temperature. The results indicate that P release amounts of different types of soil were shown as YS > CS > YBS > PS under specified environmental conditions, such as P application, flooding depths, acid rain pH, and organic acids. When the amounts P application is more, flooding depths is deeper, acid rain or organic acid pH is lower, the more amounts P release. Then, STP of 4 soils all tend to decrease under different conditions, while the change of amounts of various inorganic P forms is complicated and uncertain. The amount of Ca2-P, S/L-P, Ca8-P, Al-P, and Fe-P form varied more greatly and complicatedly than that of O-P and Ca10-P form. For another hand, P adsorption processes of 4 soils have been well fitted by Freundlich and Langmuir isothermal equations, and so do Elovich equation and two-constant equation. Moreover, the amounts of P adsorption and desorption increases as temperature increase, and these are spontaneous endothermic reactions. Furthermore, organic acids can decrease P adsorption of 4 soils, and the decrease amounts depend on the type of organic acids and the amount added. Additionally, composite indicators of soil test P, sorption P indicators and “change point” indicate that 4 types of soil P loss risk are shown as PS ? YS > CS > YBS.(2) The characteristics and mechanism of P loss through leaching and surface runoff were studied by using indoor soil column and soil pan device respectively under simulated artificial rainfall. The WTP concentration of leachate gradually decreases and reaches some constants along with rainfall repeats. And most of WTP in leachate from soil column is DP, in which DRP accounts for above 80%. In addition, the vertical distribution of STP concentration in soil column profile showed that STP concentration of the top layer soil is higher than the concentration of the underlying soil. Moreover, IP accounts for above 60% in each layer soil, and Fe/Al-P of each layer soil is more active for migrating and transforming than OP and Ca-P. On the other hand, the amounts of P loss in the soil pan from surface runoff and sediment increase as rainfall intensity increased. Nevertheless, the maximal P loss is under the condition of the soil pan slope about 20°, while there is no significant relationship among WTP concentrate, SeTP concentration and rainfall intensities, soil pan slopes. However, the concentrations of WTP and SeTP from soil pan under higher P application are higher than these under average P application and no fertilization. And the amount of sediment P, accounted for above 70% of total P loss, is more than runoff P under different rainfall intensities, soil pan slopes and P application amounts.(3) The concentrations of N and P from surface runoff of in-situ plots under natural rainfall were monitored and analyzed in 2012 to study ANPS loss characteristic in TGRA. Monitoring results indicated that the slope of maximal loss amounts and loss rates of N and P from runoff and sediment is about 15°. Moreover, the loss loads of N and P from surface runoff are much less than those from sediment. And the total N load is more than total P load in runoff while the total N load is less than total P load in sediment. For another, the sequence of surface runoff N and P loss loads from different soil types is PS > YS > CS while the sequence of N and P loss rates is YS ? PS > CS. In fact, the STN and AN enrichment ratio of CS are higher than PS and YS while the STP and AP enrichment ratios of PS are higher than CS and YS. Furthermore, the order of 5 planting patterns for N and P loss amounts is R-S > V-P > B-C-S > C-W > C-G. And the sequence of N and P loss rates is C-W > V-P ? R-S > B-C-S > C-G. Then, the N and P enrichment ratios of R-S and B-C-S are higher than C-G and V-P.