BEHAVIOR OF DDT IN CHINESE TROPICAL SOILS

The behaviour of C-14-p,p'-DDT was studied in upland and paddy soils. Under field conditions in subtropical Hangzhou the initial amount decl ined to 67% after 43 weeks. In tropical upland soil at Guangzhou the o riginal amount was reduced to 46% after 34 weeks indicating faster dis sipation rates. Bound residues formed slowly and accounted for approxi mately 10% of the initial C-14 after 43 and 34 weeks in Hangzhou and G uangzhou respectively. Overall dissipation half-lives were 75 and 31 w eeks for Hangzou and Guangzhou respectively. The extractable residues contained predominantly DDT and smaller amounts of DDE, DDD and DDA. I n paddy flooded soil, 50% of the radioactivity dissipated after approx imately six weeks. Extractable residues declined gradually while bound residues slowly increased to represent >50% of the terminal residue a fter 25 weeks. The dominant product in the extracts after 25 weeks was DDD. It may be concluded that tropical climatic and soil conditions a t Guangzhou resulted in a relatively rapid dissipation of DDT, while f looded conditions were highly effective in removing the insecticide. U p to one third of bound C-14-residues formed in paddy flooded soil cou ld be released by incubation with a fresh soil inoculum (1:1) for four months. DDD was a major component in the released products.