Phosphorus Move By Leaching And Its Mechanism From Paddy Soil In The Taihu Lake Region Of South China

The main environment problem of the Taihu Lake, the third largest freshwater lake in china, is the water eutrophication. With the control of point sources pollution from industries, now nonpoint sources pollution (NPS) have been becoming the main pollution source, among which the phosphorus (P) losses from agricultural lands have been recognized to be more and more contribution to water eutrophication. In spite of many reports about the P losses in this region, mostly are focus on the surface runoff, the P losses by leaching is still little know. In order to answer questions above , a two-year field plot experiment was conducted on two types of paddy soils (the Permeable paddy soil in the Anzhen site and the Waterlogged paddy soil in the Changshu site). A soil column similar experiment was also conducted at the same way. Commericial NPK compound fertilizer and single superphosphate fertilizer were applied to furnish 0, 30,75, 150, 300kg applied P hm-2.The results were listed as follows:(1) Under usual P application rate(30 kg P hm-2, P30) in the permeable paddy soil at the Anzhen site, the average concentration of DP( dissolved P), PP( paniculate P) and TP( total P) in the leachate at 30cm depth in the rice season were 0.104 (0.078-0.137), 0.302 (0.196-0.348), respectively, and 0.406 (0.290-0.451) mg L-1; at the 60cm depth, they were 0.110(0.089-0.150 ), 0.225 (0.142-0.315) and 0.335 (0.263-0.409) mg L-1; for the 90cm, they were0.068(0.055-0.090),0.077 (0.035-0.125), 0.144(0.102-0.168)mg L-1. While in the Changshusite, at 30cm depth, they were 0.096 (0.081-0.110 ), 0.232 (0.164-0.352) and 0.328(0.262-0.442) mg L-1 ;60cm they were 0.087 (0.060-0.125 ), 0.154 (0.058-0.230) and 0.240(0.139-0.339) mgL-1;90cm they were 0.094 (0.068-0.124), 0.114 (0.049-0.171) and 0.208(0.124-0.266) mg L-1. In both two typies of paddy soils, all the DP concentrations were muchgreater than the threshold of 0.02mg L-1 caused the eutrophication.(2) Fertilizer P rate had a effect on P concentrations in the leachate from various depth, but only significantly in statistics for 30cm depth.(3) The P concentration in the vertical leachate, especially in Changshu site, was different from 30cm to 60cm.The P concentration at 30cm>60cm>90cm.(4) The P form in the leachate were mostly the PP(Particulate P).The value for PP/TP was 51.8%-79.8% and it had a getting down trend from the surface layers of soil to deeper levels.(5) Phosphorus concerntration in leaching water for each treatment seemed to decrease form the first sample in rice season. The greatest potential risk of P losses by leaching may occur in a month in rice season after P fertilizer application.(6) The DP and the TP concerntrations in P150, P300 plots in the waterlogged paddy soil in Changshu site were greater than that in the Permeable paddy soil in Anzhen site.(7) Under usual P application rate, the P losses by leaching at 30cm depth in the Changshu site was 469.7gP ha"1, and 378.5 gP ha"1 in the Anzhen site.(8) There was a change point of TP concertration respectively for the two paddy soil in 30cm soil depth. The change point was 33.0mg kg-1 in the pereable paddy in the the Anzhen site and 26.3mg kg"1 in the waterlogged paddy soil in the Changshu site.