Biodegradation of norflurazon in a bog soil

Microbial degradation of the pre-emergent herbicide norflurazon (4-chloro-5-(methylamino)-2-(α,α,α-trifluoro - m-tolyl)-3(2H)-pyridazinone) was evaluated in three horizons (Oi, A, B) within the profile of an acidic bog soil in laboratory experiments. Organic matter, nutrient content and microbial biomass all decreased with depth. Mineralization of norflurazon was biologically-mediated, with negligible abiotic degradation. The rate (0.17% 14CO2 d−1) and extent (4.03% of initial 14C) of norflurazon mineralization were comparable in the Oi and A horizons, and significantly lower in the B horizon. Sorption of norflurazon decreased with soil organic matter content and depth in the profile, with more of the herbicide sorbed in the Oi than in the A or B horizons. Incorporation of norflurazon carbon into microbial biomass increased with depth within the soil profile, with up to 9.09% of added norflurazon 14C found in microbial biomass in the B horizon. The relative contribution of bacteria and fungi to norflurazon mineralization varied within the soil profile, with bacteria contributing most in the B horizon (89%) and least in the A horizon (42%). These results were similar to glucose-carbon mineralization in the A horizon only. Analysis of extracts of 14C-norflurazon-amended soil by HPLC with UV-vis and 14C detection showed a small decrease in norflurazon concentration over 9 weeks and appearance of a second chromatographic peak—presumably a metabolite—in the A and Oi horizons, but not in the B horizon. The biodegradation of norflurazon in bog soil appears to be a slow process that is significantly affected by position within the soil profile.