Agriculture Non-Point Source Phosphorous Pollution Under Different Land Use In Sichuan Western Mountain Small Watershed

The accelerated eutrophication of most freshwaters was limited by phosphorus input. Nonpoint sources of P in agricultural runoff now had been identified as one of the main causes of freshwater eutrophication contributed a large portion of freshwater inputs, due to easierid entification and recent control of point sources. Agricultural nonpoint pollution, being reason of its universality, was becoming important water pollution after point source pollution, and more and more researchers from all kinds of disciplines were paying the most extensive attention to the problem. Understanding the characteristics of phosphorus accumulation in agricultural soils and the impact of land use patterns on the sorption and desorption of P was necessary to develop management practices for the protection of surface water quality in eutrophication afflicted watershed. The objective of our research was to study the impact of land use pattern on the P status of agricultural soil and its effects on water quality in Mingshan river watershed. In the mean time, to evaluate P loss risk in Mingshan river watershed, Phosphorus Index system which was developed and now widely used in U. S. A., was applied and modified here. This method ranked vulnerability to P loss by accounting for source (soil test P, application rate of P fertilizer) and transported factors (erosion, runoff, distance to stream, slope), and finally computed through GIS platform.(1) Results showed that decades of P fertilization at rates exceeding the amount removed by crops had resulted in widespread accumulation of P in agricultural soils in the sub-watershed of Mingshan River. This effect had manifested itself mainly in Total P and Olsen-P which were increased in degree under different land uses. The experiment results always indicated that the ordering of Q_m of soil under different land use, according to magnitude, was tea garden'＞paddy field's＞wood land's＞dry land's, and the ordering of MBC was paddy field's＞tea garden's＞wood land's＞dry land's, and the ordering of DPS was dry land's＞paddy field's＞tea garden's＞wood land's, and the ordering of EPC₀ was dry land's＞paddy field's＞wood land's＞tea garden's.(2) To research the removal capability of phosphorus from soil to water with its mechanism, this paper was concentrated on selecting relative indexes such as RDP, Olsen-P,Q_m,EPC₀,DPS,SPR, to assess the effect of soil phosphorus on environment. The result showed that DPS had the largest contribution to assessment, next were Olsen-P, EPC₀, SPR, RDP and Q_m.(3) Estimating the demand of soil for phosphorus under different land use types, it was concluded that, by 10⁴kg/hm² of soil weight, the mean demand of dry land was 20.30kg/hm², and the mean demand of dry land was 30.75 kg/hm², and the mean demand of paddy field was 28.88 kg/hm².(4) At the scale of the whole research watershed, the percentage of regions with relatively high risk of P loss was less than 5%, and medium risk area occupied about 20% in the whole study area. Regions with high or medium risk located besides the rivers, where most high or relatively high soil available P or fertilizer P application rate, or intense soil erosion were observed.(5) Because P loss was affected by many factors, the regions with intense soil erosion and the regions with high-risk possibility of P loss were not always identical. Only when high-risk source factors such as high soil available P, high Fertilizer P application rate and high-risk transportation factors such as intense soil erosion, short distance to the streams, appeared at the same region, could the high-risk areas of P loss be observedFrom the foregoing, specific mitigation options including nutrient budgeting, input management, soil conservation and land use management were also proposed for the improvement of surface water quality in the area.