In a laboratory experiment [thiocarbonyl-14C]thiram was incubated in a New York sandy loam soil in the dark at 20°C and 75% of field moisture capacity under aerobic conditions for 205 days (Morgenroth and Müller-Kallert, 1995). The initial thiram level in the soil was 20.3 mg ai/kg, chosen to represent a field application rate of 18 kg ai/ha. The system was continuously supplied with humidified air; evolved CO2 was trapped. The characteristics of the soil were pH 6.7, organic carbon 2.4%, cation exchange capacity 14.4 meq/100g dry soil, clay 14.8%, silt 29.6%, sand 55.6%.
Figure 2. Metabolism of thiram by plants.
The levels of thiram and identified and unidentified degradation products in the soil at various sampling times are shown in Table 19. The disappearance of the parent compound was not first-order, but the half-life in the initial period was about 2 days, with 85% disappearance in 7 days and 97% in 14 days. The identity of the major product M6.5 was shown by GC-MS and HPLC-MS to be dimethylcarbamoperoxothioic acid, (CH3)2N.C(:O).SOH. It reached its maximum level (1.8 mg/kg) at 4 days and exceeded the level of the parent compound after 42 days, but 99.8% of the parent had disappeared at that time.
Mineralisation of the residue was rapid with 9% of the applied 14C evolved as 14CO2 in 2 days and 50% within 21 days (Table 20). Bound or non-extractable residues reached a maximum of 48% of the applied dose at day 14 and thereafter declined slowly to 31% at day 205. The production of 14CO2 was much slower when most of the residue was in the bound form. Fractionation of the soil organic matter showed that much of the unextractable 14C was bound to the humin and humic acid fractions.
Table 19. Thiram and degradation products in a sandy loam soil during aerobic incubation in the dark for 205 days after an initial dosing with [14C]thiram at 20.3 mg ai/kg (Morgenroth and Müller-Kallert, 1995).
|
Days |
Thiram |
Metabolite, mg/kg |
||||||||||
|
M6.1 |
M6.2 |
M6.3 |
M6.4 |
M6.5 |
M6.6 |
M6.7 |
M6.8 |
TMTU |
TMU |
TMTM |
||
|
0 |
21 |
n |
n |
n |
n |
n |
n |
n |
n |
n |
n |
n |
|
1 |
13.4 |
n |
0.036 |
0.032 |
0.13 |
0.45 |
0.068 |
0.021 |
0.039 |
n |
n |
n |
|
2 |
9.5 |
n |
0.14 |
0.035 |
0.63 |
1.0 |
0.051 |
0.031 |
n |
n |
n |
n |
|
4 |
6.1 |
n |
n |
n |
n |
1.8 |
0.072 |
n |
n |
n |
n |
n |
|
7 |
3.1 |
n |
0.028 |
0.023 |
0.12 |
0.85 |
0.31 |
0.011 |
n |
n |
n |
n |
|
14 |
0.60 |
n |
0.074 |
0.015 |
n |
0.32 |
0.082 |
n |
0.012 |
0.069 |
n |
n |
|
21 |
0.34 |
n |
0.045 |
n |
0.017 |
0.17 |
0.11 |
n |
0.022 |
n |
n |
n |
|
42 |
0.048 |
n |
0.063 |
0.025 |
0.010 |
0.083 |
0.10 |
0.017 |
n |
0.005 |
n |
0.006 |
|
84 |
0.017 |
0.067 |
0.048 |
n |
n |
0.038 |
0.048 |
n |
n |
0.005 |
n |
n |
|
128 |
0.007 |
0.039 |
0.054 |
n |
n |
0.031 |
0.032 |
0.005 |
n |
0.002 |
n |
n |
|
205 |
0.006 |
0.022 |
0.043 |
n |
n |
0.022 |
0.023 |
0.001 |
n |
0.001 |
0.001 |
0.001 |
n not detected
TMTM: tetramethylthiuram monosulphide
TMU: 1,1,3,3-tetramethylurea
TMTU: 1,1,3,3-tetramethylthiourea.
Table 20. Mineralisation of thiram in a sandy loam soil during aerobic incubation in the dark for 205 days after an initial dosing with [14C]thiram at 20.3 mg ai/kg, as indicated by evolution of 14CO2. (Morgenroth and Müller-Kallert, 1995).
|
14CO2 4 as % of initial [14C]thiram after days of incubation | |||||||||
|
1 |
2 |
4 |
7 |
14 |
21 |
42 |
84 |
128 |
205 |
|
3.7 |
9.0 |
21.8 |
35.6 |
46.1 |
54.7 |
59.6 |
65.9 |
67.7 |
74.9 |
Burri (1995) applied [14C]thiram to a thin layer of the same sandy loam soil at a rate equivalent to 18 kg ai/ha and exposed it to an artificial light source with a spectrum simulating sunlight for 21 days with a 12 hour light-dark cycle at 20°C. In both control and irradiated samples the percentage of extractable 14C decreased with time, while the non-extractable 14C reached 30% after 3 days and then remained reasonably constant.
The half-life for the disappearance of thiram from the control in the dark was 15.9 days and in the soil under artificial light 3.7 days. The total volatile 14C produced by the soil under artificial light after 21 days amounted to 57% of the applied 14C (37% as CO2 and 20% probably as CS2) while 11% was evolved by the control. The volatile 14C compound captured in the trap matched CS2 in an HPLC system, but was not further identified. No other products of photolysis were positively identified but they were generally minor or transient.
Morgenroth (1995) measured the adsorption and desorption properties of [14C]thiram on four different soils, a sandy loam, loamy sand, silt loam and loam. Adsorption and desorption tests were carried out in 0.01M CaCl2 at 20°C. Adsorption reached equilibrium after about 4 hours with high proportions of thiram adsorbed: it was not directly related to the organic carbon contents of the soils. Only small proportions of thiram could be desorbed from the soils. Thiram was judged to be slightly mobile to immobile in the soils tested.
Figure 3. Degradation of thiram in soil
