Transformation pathways of 14c-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 14C-labeled CTN dissipation in three acid Brazilian soils (Typic Humaquept [GH], Typic Quartzipsamment [AQ], and Typic Hapludox [LE]). Mineralization was not the major metabolic pathway of CTN-degrading microorganisms. However, CTN dissipation was fast in all soils and was mainly due to biodegradation (responsible for 50%, 54%, and 73% of 14C-CTN dissipation in the GH, LE, and AQ soils, respectively), as well as to formation of soil-bound 14C residues (responsible for 46%, 34%, and 18% of 14C-CTN dissipation in the GH, LE, and AQ soils, respectively). Most soil-bound 14C residues were formed in the first day, but aging also contributed to the formation of less reversible forms of CTN-soil complexes. In these acid soils, the most abundant metabolite formed from CTN degradation was 3-carbamyl-2,4,5-trichlorobenzoic acid. A significant fraction of the CTN that had been assumed to be rapidly degradable in soils in previous reports has turned out to be soil-bound residues. Although bioavailability of any compound is reduced when soil complexes are formed, further research is needed 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.