Cadmium Uptake by Paddy Rice and Soil Available Cd under Water Flooding as Affected by Nitrogen Fertilizer

In order to tackle the problem of Cd pollution in paddy soils and investigate soil available Cd as affected by nitrogen fertilizers, incubation and pot experiments were carried out to study effects of different nitrogen fertilizers and rates of urea on cadmium uptake by paddy rice and soil available Cd under water flooding condition. Results revealed that soil pH increased sharply after the soil was flooded, especially at the beginning of incubation, and gradually decreased with incubation time and finally tended to approach the neutral values. The patterns of soil pH change were just opposite to those of soil available Cd, a negative correlation observed between the two. Soil flooding made the soil available Cd drop by 58.2%-84.1%. Among 4 nitrogen fertilizers, the NH4Cl treatment significantly increased Cd content in rice more than the other three treatments including urea, (NH4)2SO4and NH4NO3and facilitated transfer of Cd from rice straw to grains; there was no significant difference in Cd contents in both straw and grains between the latter three nitrogen treatments. Optimal rate (N 0.2 g·kg-1) of urea could significantly lower the Cd concentration in rice grains, while either no N or high rate of urea treatment significantly enhanced the Cd concentration in grains. Results further revealed that on the Cd contaminated paddy soil, employing a Cd resistant rice cultivar in combination with optimized fertilizer and water management practices could lower the Cd content in rice grains far below the maximum permissible level of 0.2 mg·Cd kg-1set by the state for the pollution-free rice production.