The route of degradation was studied under aerobic and anaerobic conditions in sandy loam soil (pH 6.2, 0.9% oc) using [14C-phenyl]-labelled fenitrothion. It is proposed that in soil, aerobic degradation of fenitrothion occurs via cleavage of the P-O-aryl linkage, cleavage of P-O-methyl linkage and oxidation of P=S group to P=O group. The metabolites formed are then mineralised to CO2 via ring opening of the phenyl ring. Under anaerobic conditions, breakdown is via cleavage of the P-O-aryl and the P-O-methyl linkages and reduction of NO2 group to NH2 group with subsequent acetylation or formylation. The notifier’s proposed metabolic pathway in aerobic soil is presented at Figure B.8.2. During photolysis, fenitrothion is broken down by oxidation of the P=S group to the P=O group and cleavage of the P-O-aryl linkage, with the aromatic carbon eventually photodegraded to CO2. The rate of degradation of [14C-phenyl]-fenitrothion in soil under dark aerobic conditions was investigated in sandy loam at 25􀁲1􀁱C. In sandy loam soil (pH 6.2, 0.9% oc) fenitrothion degraded relatively rapidly, with the majority of degradation (at least 90%) occurring by one week. A first order DT50 value for fenitrothion was calculated as 36 days, using data for the whole year (r2 = 0.84). However, this is longer than the usual recommended duration for such studies of 120 days. Also microbial activity in the soil for this period was not reported and may not have been maintained. The degradation curve fitted using data from 365 days also falls short of the first 4 datapoints, (due to placing equal weight on the later data), so is probably an overestimate of the initial rate of degradation. Plotting a degradation curve using only the data points up to 7-10 days after treatment gave a better fit over the initial data points and a first order DT50 of ca. 2 days, (r2 = 0.98). Recovery of applied radioactivity during the study ranged from 83.7-111.6%, with poorer recovery seen after 3 days (83.7-94.2%). The mean mass balance over the study duration of a year was 90.8% AR. Only one major metabolite was formed in significant amounts, 3- methyl-4-nitrophenol (NMC). This metabolite was detected on day 0, reached a maximum of 20.5% AR at day 3, but then decreased to <10% AR within one week and <1% AR within one month. Five minor metabolites were also identified at low levels (<1%AR) and only during the first 2 months. These were FNO, DM-FNT, DM-FNO, FA-FNT and NMA. Unknown compounds and applied radioactivity remaining at the TLC origin were at very low levels (<5%AR). There was substantial mineralisation of fenitrothion to 14CO2, with more than half (54.4%AR) of applied fenitrothion being converted to CO2 after 122 days. Formation of 14CO2 continued to increase to ca. 67% by end of the study year. An additional small amount (ca. < 4%) of other volatile organic products were also detected, though not identified. Unextracted radioactivity reached peak levels before 120 days (35.4%AR after 21 days), then declined indicating some further degradation occurred after incorporation into unextracted residues. For the 1 and 6 month samples, it was shown that the majority of unextracted radioactivity was associated with the humin and fulvic acid fractions (ca. 10 and 7-9%AR, respectively).