The ED50 values for assessing lanthanum toxicity on microbial ATP content and dehydrogenase and urease activities of paddy soil

Rare earths are applied widely in Chinese agriculture to improve crops nutrition 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 assess lanthanum toxicity on microbial ATP content and dehydrogenase and urease activities in paddy soil, which was assayed after adding La for 4h, Id, 1, 4, 8, and 16 weeks. The relative ED50 values were calculated by two kinetic models (model 1 and model 2) and one sigmoidal dose-response model (model 3). Model 1 was successful in calculating the ED50 values for the ATP content, dehydrogenase and 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, respectively. 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 0.15 mg kg(-1) soil year(-1), applied in agriculture can hardly affect ATP content and dehydrogenase and urease activities in the paddy soil even over a long period.