Transformation pathways of C-14-chlorothalonil in tropical soils

Chlorothalonil (CTN) is a chlorinated wide-spectrum fungicide, heavily and widely applied throughout the world. This study was undertaken to directly evaluate the rates and forms of C-14-labeled CTN dissipation in three acid Brazilian soils (Typic Humaquept [GH], Typic Quartzipsamment [AQ], and Typi c Hapludox [LE]). Mineralization was not the major metabolic pathway of CTN -degrading microorganisms. However, CTN dissipation was fast in all soils a nd was mainly due to biodegradation (responsible for 50%, 54%, and 73% of C -14-CTN dissipation in the GH, LE, and AQ soils, respectively), as well as to formation of soil-bound C-14 residues (responsible for 46%, 34%, and 18% of C-14-CTN dissipation in the GH, LE, and AQ soils, respectively). Most s oil-bound C-14 residues were formed in the first day, but aging also contri buted to the formation of less reversible forms of CTN-soil complexes. In t hese acid soils, the most abundant metabolite formed from CTN degradation w as 3-carbamyl-2,4,5-trichlorobenzoic acid. A significant fraction of the CT N that had been assumed to be rapidly degradable in soils in previous repor ts has turned out to be soil-bound residues. Although bioavailability of an y compound is reduced when soil complexes are formed, further research is n eeded to evaluate accumulation and availability of CTN soil-bound residues over long-term applications, and the consequent detrimental effects on the environment and on soil quality and fertility.