Dependence of accelerated degradation of atrazine on soil pH in French and Canadian soils

A series of agricultural soils varying in their atrazine treatment history were sampled from 12 sites in France and two sites in Canada. The soils varied widely with respect to soil chemical, physical and microbiological (total microbial biomass, kinetics of C and N mineralization) properties. Soils treated with as few as two successive atrazine field applications mineralized [U- ring- 14C]atrazine significantly more rapidly in 35 d laboratory incubations than did soils which had never received atrazine. Longer treatment history tended to favour more rapid mineralization in the so-called “adapted” soils. Up to 80% of the initially applied 14C-atrazine was mineralized at the end of the incubations in these adapted soils. Of the properties tested, soil pH was the most significantly related to atrazine mineralized. In soils with pH lower than 6.5, less than 25% of the initial 14C-atrazine was mineralized even after repeated application in field conditions. Atrazine retention in soil did not influence its mineralization rate. Both hydroxylated and dealkylated atrazine metabolites were detected, but no clear pattern of metabolite production could be determined. Large amounts of bound residues were formed in soils that mineralized little atrazine.