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Environmental fate & pathways

Biodegradation in soil

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Description of key information

The results reported in the key study (Loehr & Matthews, 1992) show that thiourea is quickly degraded in natural soil samples when applied at concentrations that are not inhibiting and/or toxic towards soil microorganisms. In this case the determined half-lives for thiourea biodegradation range between 12.8 days to 18.7 days in acidic (intial thiourea concentration: 100 mg/kg soil dw) and basic soil (intial thiourea concentration: 660 mg/kg soil dw), respectively.  
Lashen & Starkey (1970) exposed soil samples at concentrations approximately in the same range as the concentrations that were determined to be non-inhibitory to microorganisms by Loehr & Matthews (1992). However, biodegradation of thiourea in the experiment by Lashen & Starkey (1970) was slower. This might be attributed to a different soil type and different soil microbial community that developed in the barnyard soil. In addition, previous exposure of the barnyard soil to agrochemicals cannot be excluded. Furthermore, high thiourea concentrations, e.g. 10 g/kg as used by Frederick et al., 1957, reduce the soil microbial population, subsequently slowing down or even inhibiting substance degradation.
The degradability of thiourea is therefore dependent on thiourea concentration, but also on soil type, pre-exposure of the soil and the soil microbial community.

Key value for chemical safety assessment

Half-life in soil:
18 d
at the temperature of:
20 °C

Additional information

For the assessment of the biodegradability of thiourea in soil, one key study (Loehr & Matthews, 1992, reliability 2) is available as well as two supporting studies with reliability scores of 3 and 4 (Lashen & Starkey, 1970; BUA Stoffbericht 179 citing Frederick et al., 1957).

Loehr & Matthews (1992) evaluated the loss of thiourea in one acidic and one basic soil (both previously unexposed to industrial chemicals or waste) in microcosm experiments under controlled conditions (20 °C, moisture 80 % of field capacity) for a total exposure duration of 64 days. Thiourea was applied at initial test substance concentrations of 100 mg/kg soil dw and 660 mg/kg soil dw (acidic and basic soil, respectively) that were determined to be non-toxic to the microorganisms present in the soil samples in a pre-test.

These concentrations correspond to 1/10 of the concentration that has been determined as acceptable (non-inhibitory) in the pre-test. Thiourea loss rates (attributed to biodegradation) were determined and used to estimate first and zero-order rate constants by least-squares regression. The half-life of thiourea biodegradation in acidic and basic soil under the conditions of the test was determined to be 18.7 days and 12.8 days, respectively.

In the study conducted by Lashen & Starkey (1970) thiourea concentrations of 0.02 M and 0.1 M were applied to 200 g barnyard soil with 0.5 % glucose. The soil samples were incubated for up to 15 weeks at 28 °C and 50 % of field capacity. Thiourea biodegradation was followed by determination of sulphate. Biodegradation of thiourea was inhibited at higher concentrations: At an initial concentration of 0.02 M thiourea 96 % of the test substance was degraded within 15 weeks, whereas only 28 % of the thiourea-S were recovered as sulphate after 15 weeks at an initial concentration of 0.1 M.

In order to compare the initial thiourea concentrations used in this study to other studies it is necessary to convert the given initial concentrations of 0.02 M and 0.1 M to mg/kg soil. In the study 200 g soil were used but no information is given whether this is the soil dry or wet weight. For assessment purposes it is assumed that this refers to dry weight. The bulk density of wet soil is 1700 kg/m³. The weight of the solid fraction of 1 m³ wet soil is 1500 kg which therefore corresponds to the bulk density of dry soil. Based on this density 200 g soil correspond to 0.07 L soil. A concentration of 0.02 M (mol/L soil) therefore corresponds to 210 mg thiourea in 200 g dry soil, or 1050 mg/kg soil dw. A concentration of 0.1 M corresponds to 5250 mg/kg soil dw.

Frederick et al. (1957; as cited in the BUA Stoffbericht 179) determined the biodegradation of thiourea in agricultural soil (sandy loam, pH 6.1) over 21 weeks at 28 °C, a field capacity of 40 %, and an initial thiourea concentration of 10 g/kg soil dw. 9 % degradation (determined via formation of sulphate) was observed. Application of 0.5 % CaCO3 and additional incubation for 21 weeks did not alter the degradation results.

In addition, bacterial and fungal populations were checked. The number of bacteria and actinomycetes was reduced from several million per g untreated soil to several thousand per g treated soil. The number of fungi was reduced by > 99 %.

Conclusion:

The experiment of Loehr & Matthews (1992) shows that thiourea is rapidly degraded in natural soil samples when applied at non-inhibitory concentrations. The half-lives determined for biodegradation of thiourea ranged between 12.8 and 18.7 days.

Lashen & Starkey (1970) exposed soil samples to similar thiourea concentrations. However, biodegradation of thiourea in the experiment by Lashen & Starkey (1970) was slower. This might be attributed to a different soil type and different soil microbial community that developed in the barnyard soil used by Lashen & Starkey (1970). In addition, previous exposure of the barnyard soil to agrochemicals cannot be excluded. The Lashen & Starkey (1970) study is of low reliability; besides poor documentation, the number and timing of samples is insufficient for determination of a meaningful degradation curve. Furthermore, high thiourea concentrations decrease the soil microbial population (Frederick et al., 1957), consequently slowing down or even inhibiting substance degradation.

The degradability of thiourea is therefore dependent on thiourea concentration, but also on soil type, pre-exposure of the soil and soil microbial community.

Overall, the results obtained by Loehr & Matthews (1992) are most reliable, and a degradation half-life of 18 days is considered to be relevant for the risk assessment.