Rare earths are applied widely in Chinese agriculture to improve crops nutri tion and incidentally in fertilizers, as yet little is known of their effect on the biological function of the soil. An incubation experiment was performed to a ssess lanthanum toxicity on microbial ATP content and dehydrogenase and urease a ctivities in paddy soil, which was assayed after adding La for 4h, 1d, 1, 4, 8, and 16 weeks. The relative ED50 values were calculated by two kinetic models (mo del 1 and model 2) and one sigmoidal dose-response model (model 3). Model 1 wa s successful in calculating the ED50 values for the ATP content, dehydrogenase a nd urease activities. The ED50 values relating to ATP content and dehydrogenase and urease activities were 1086.5 and 897.8 and 1112.5 mg La kg-1 soil, respec tively. The ED50 values of ATP content and dehydrogenase activity increase with incubation time, but the ED50 value of urease activity decreases. After 16 weeks , La amendment has no inhibition of urease activity. The present dosage of mixed rare earths, about 0.15mg kg-1 soil year-1, applied in agriculture can hard ly affect ATP content and dehydrogenase and urease activities in the paddy soil even over a long period.
