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.