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Toxicity to soil microorganisms

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The toxicity of thiourea to soil microorganisms was investigated in several studies. It could be shown that thiourea is toxic to soil microorganisms and inhibits growth as well as nitrification. Acclimatisation to thiourea occurs over time and thiourea can be decomposed.
However, all studies were conducted in solution cultures and not in standardized soil samples and soil microbial communities. Therefore, conclusions for soil microbial communities cannot be made.

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The toxicity of thiourea to soil microorganisms was shown among others in the study by Lashen and Starkey (1970). A strain resembling Penicillium rugulosum was able to grow in liquid culture medium in the presence of 250 and 500 mg/l thiourea. Concentrations of 1000 mg/l thiourea led to reduced growth of a Penicillium strain whereas 2000 mg/l completely inhibited growth. During sub-culturing of the strain acclimatisation to thiourea could be observed and decomposition of thiourea to sulphate and ammonium was monitored. Because the inhibiting effects of thiourea were elucidated after the sub-culturing of a pure culture no representative effect level could be established.

Zacherl and Amberger (1990) studied the toxicity of 100 mg/l thiourea to nitrogen-fixing microorganisms in liquid culture. Only minor growth inhibition was observed in one species (Azotobacter chroococcum). It was concluded that the results obtained with pure bacterial cultures in the lab cannot be directly transferred to field conditions.

Pandey et al. (1976) studied the toxicity of thiourea at concentrations of 10-1000 mg/l to the two fungi species Helminthosporium sativa and Fusarium oxysporum, measuring growth in liquid culture. A gradual decrease in mycelial dry weight and sugar content was observed with increasing thiourea concentrations.

In a study by Kubota and Asami (1985) it was shown that concentrations of 100 and 200 mg/l thiourea have an inhibiting effect on nitritification. It was not possibile to derive an effect level.

In summary, soil microorganisms when grown in liquid culture appear to be sensitive to thiourea. Inhibitory effects on growth were observed in liquid culture media containing thiourea at concentrations between 10 and 1000 mg/l. Acclimatisation to thiourea, accompanied by its decomposition, was observed. Nevertheless a representative effect level could not be established as some of the studies were conducted with pure cultures in liquid media and cannot be transferred directly to field conditions.

However, additional studies on effects to microorganisms in soil (guideline compliant) are not considered to be required, since the risk characterisation for the terrestrial compartment does not indicate a need for additional data.