| The Taihu Lake is the third largest freshwater lake in China and plays an important role in the economic development in the Taihu Lake region. However the water quality in the Taihu Lake is becoming more and more deteriorative in recent years because the environment protection did not keep the pace with the rapid development of agriculture and industry in this region. The main problem in the Taihu Lake is the eutrophication limited by phosphorus (P) from point sources and non-point sources. With the improvement of contoling point sources, non-point sources P, especially from the agricultural runoff have been identified as one of the main causes for the accelerated eutrophication of waterbodies in Taihu Lake and contribute a large share to the wate quality deterioration, and thus have been paid more and more attention to. In spite of the many reports about P losses in this region, It is still little know how many P are lost exactly per year from the paddy soils in the Taihu Lake region, what the contribution is among all kinds of non-point sources, and how the soil types and the agricultural management practices affect the P runoff losses etc. In order to answer questions above and to provide available data for decision-makers, a field plot experiment was conducted on two types of paddy soils (the Permeable paddy soil in Anzhen site and the Waterlogged paddy soil in Changshu site) in the Taihu Lake Region from June 2000 through 2001 to November 2002. The objectives of the present paper were to evaluate the P losses by runoff from paddy soils in this region, and to identify the impacts of the soil types and the agricultural management practices as well as the land use pattern on the P runoff losses.The results showed that:1. It was observed that P was surplus in the agricultural soil ecosystems in the Taihu Lake region.2. Fertilizer P rate had a significantly effect on P concentrations in runoff. Either DP, PP, or TP.3. Particulate P was the dominant portion of total P lost by runoff even in the paddy soils within the flat plain of the Taihu Lake Region where there is almost no undulation in topography.4. Phosphorus concentration in runoff for each treatment seemed to decrease from the first runoff event to the last event both in rice season and in wheat season. The greatest potential risk of P loss by runoff may occur in a month in rice season and one or two months in wheat season after P fertilizer application.5.Dissolved P and TP concentration in runoff from the waterlogged paddy soil were greater than from the permeable paddy soil, respectively if the runoff event occurred at the same period.6. It was also observed that the paddy soil type had an obvious effect on the number of runoff event. The numbers of runoff event in Anzhen site were more than in Changshu site in rice reason, but on the contrary in wheat season.7. Of the average annual TP load of 603.2 g P ha-1 yr-1 by runoff from P30/P20 plot in year2000/01, while in year 2001/02, it was about 1148.9 g P ha-1 yr-1 lost by runoff. However, for the waterlogged paddy soil, it was observed 679.5 g P ha-1 yr-1 lost in year 2000/01, while about 1350.3 g P ha-1 yr-1 was lost in year 2001/02.8. Results revealed that under present rice -wheat double cropping system in the Taihu Lake region, P losses to water in two types of paddy soil were much higher in winter wheat season than in rice season. It was recorded that runoff events and runoff volumes were evident more in wheat season than in rice season, although precipitation in wheat season is somewhat less than in rice season.9. The average annual TP load by runoff from the waterlogged paddy soil was greater than from the permeable paddy soil, although such the difference varied with seasons.10. It was roughly estimated that there were about 7000 tons non-point sources P lost entering to water, of which an approximate of 620 tons TP was from 5 typical paddy soils in this region. Therefore, the agricultural non-point sources P accounted for about 8.9% in all of the non-p... |
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