Mineralization of soil-aged isoproturon and isoproturon metabolites by Sphingomonas sp strain SRS2

The aim of the study was to determine the effect of aging of the herbicide isoproturon and its metabolites monodesmethyl-isoproturon and 4-isopropyl-aniline in agricultural soil on their availability to the degrading bacterium Sphingomonas sp. strain SRS2. The ¹⁴C-ring-labeled isoproturon, monodesmethyl-isoproturon, and 4-isopropylaniline were added to sterilized soil and stored for 1, 49, 71, or 131 d before inoculation with strain SRS2. The availability of the compounds was estimated from the initial mineralization and the amount of ¹⁴CO ₂ recovered after 120 d of incubation. Aging in soil for 131 d reduced the initial mineralization of isoproturon and monodesmethyl-isoproturon and, in the case of isoproturon, also reduced the recovery of ¹⁴CO ₂. Initial mineralization and recovery of ¹⁴CO ₂ from aged 4-isopropyl-aniline were slightly reduced, but less ¹⁴CO ₂ was generally produced than with isoproturon or monodesmethyl-isoproturon. Thus, recovery of ¹⁴CO ₂ from ¹⁴C-isoproturon and ¹⁴C-monodesmethyl-isoproturon was 50.7 to 64.4% of the initially added ¹⁴C, while recovery from ¹⁴C-4-isopropyl-aniline was only 11.7 to 17.0%. Sorption measurements revealed similar Freundlich constants (K_f) for isoproturon and monodesmethyl-isoproturon, whereas K_f for 4-isopropyl-aniline was more than fivefold greater. The findings imply that in soil, partial degradation of isoproturon to 4-isopropyl-aniline may lead to reduced mineralization of the herbicide due to sorption of the aniline moiety.