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EC number: 200-543-5
CAS number: 62-56-6
The amounts of volatilised nitrite-N
decreased with increasing thiourea concentrations. The addition of
thiourea at a rate of 10 % of the added ammonium-N almost completely
suppressed the oxidation of ammonium-N to nitrite-N and nitrate-N in
Kubota and Asami (1985) investigated the source of nitrous acid volatilisation from upland soils and applied a 100 and 200 mg/L thiourea-N solution, plus nutrient solution, to two air-dried upland soils. After 20 days of incubation, the amounts of thiourea-N were 5 or 10 % of the total-N volatilised in Taki and Matsuzawa soils, respectively. In both soils, thiourea-N concentrations of 200 mg/L almost completely supressed the oxidation of ammonium-N to nitrite-N and nitrate-N.
An inoculum of thiourea-treated soil produced substantional amount of sulphate in a few weeks in glucose-thiourea medium. A fungus was isolated from this mixed culture and produced sulphate in an aerated glucose-thiourea medium with thiourea as the only source of nitrogen and sulphur. The fungus that was isolated was identified as Penicillium species and resembled Penicillium rugulosum.
The Penicillium strain was incubated in basal medium containing glucose and 0.025 % (250 mg/L) and 0.05 % (w/v) thiourea (500 mg/l), respectively. It grew slowly in the presence of 0.10 % (1000 mg/l) thiourea and failed to grow at 0.20 % (2000 mg/L). Subculturing of the fungus led to acclimatisation of the strain to increasing thiourea concentrations.
Decomposition of thiourea up to concentrations of 0.1 % (w/v) led to production of sulphate and ammonia.
A Penicillium strain was isolated from thiourea-treated soil. The isolated strain resembled Penicillium rugulosum and was able to grow in the presence of 250 and 500 mg/L thiourea in basal medium. At thiourea concentrations of 1000 mg/L the Penicillium strain grew slowly and finally failed to grow at concentrations of 2000 mg/L. Acclimatisation to thiourea allowed growth of the subcultures at higher thiourea concentration. Furthermore decomposition of thiourea to sulphate and ammonium which served as S and N sources could be observed.
A NOEC in basal medium based on the growth and dry weight of the fungus was established to be 500 mg/L. This NOEC is not representative as it was established after sub-culturing of a pure culture.
A gradual decrease in mycelium dry weight as
well as sugar content with increasing concentrations of thiourea was
observed. At 750 ppm no growth was recorded in H. sativum, and
1000 ppm were fatal for both species.
In a short publication by Pandey et al. (1975), 750 mg/L of thiourea was reported to prevent growth in Helminthosporium sativum (fungi) and 1000 mg/L was stated to be fatal. The authors considered that gradual retardation in growth of fungal mycelia with increasing concentrations of thiourea may result from effects on the fungal metabolism and enzyme system. "No effect levels" could not be established since the growth inhibiting effect started at the initial concentration of 10 mg/L thiourea.
Growth and respiration of Rhizobium leguminosarum was not affected by thiourea.
Growth of Azotobacter chroococcum was inhibited by 8 % at 100 ppm thiourea, whereas respiration was unaffected.
Zacherl and Amberger (1990) studied the effect of 100 mg/L thiourea on soil microorganisms in culture medium by measuring growth inhibition and respiration of N-fixing bacteria. Only minor growth inhibition of Azotobacter chroococcum was observed; no effects were seen in Rhizobium leguminosarum. It was concluded that thiourea did not have a toxic effect on N-fixing bacteria in pure culture.
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.
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.
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