Downward movement of phosphorus in paddy soil installed in large-scale monolith lysimeters

Soils around Taihu Lake area, located in Yangtze River Delta (Jiangsu Province, China), have been continuously applied with phosphorus (P) fertilizer for at least 40 years and growing rice (Oryza sativa L.) under water submergence in summer, in rotation with wheat (Triticum aestivum L.) in winter. Because of the accelerated eutrophication of Taihu Lake, there is great concern as to whether P from rice cropping systems is one of the non-point pollutants responsible for the eutrophication. The experiment was carried out with five treatments and four random arranged replications under rice and wheat rotation on Gleyi-stagnic Anthrosols installed in large-scale monolith lysimeter from 2002 to 2003, and soil water penetration rate was adjusted to 3-5 mm d(-1), a typical rate in corresponding field soil. The leachate samples were collected at certain intervals for analysis of dissolved P. Soil samples were collected for P fractionation after rice was harvested in 2003. Data indicated that there was no correlation between concentration of dissolved P in leachates (at the 70 cm depth) and the P fertilization rates under present experiment condition, suggesting that the soil still had strong affinity to P despite long history of continuous P fertilization. However, considerable number of leachate samples, collected particularly in early submergence, contained P at concentrations that could cause eutrophication. Calculated leaching losses of P were 0.11 and 0.071 kg P ha(-1) in 2002 and 2003, respectively. More P fertilizer application resulted in more accumulation at top soil layer and deeper downward P movement in soil profile. Downward movement of Olsen-P, Al-P, and Ca2-P was observed in deeper soil layers than that of Fe-P and Ca8-P, and accumulation of Fe-P and Al-P were quantitatively more significantly than Ca2-P and Ca8-P.