thiourea; thiocarbamide

Toxicity to soil microorganisms

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

toxicity to soil microorganisms

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

no data

Reliability:

4 (not assignable)

Rationale for reliability incl. deficiencies:

documentation insufficient for assessment

Qualifier:

no guideline followed

Principles of method if other than guideline:

To identify the source of nitrous acid volatized from upland soil, the authors carried out two experiments and demonstrated that nitrous acid originated mainly from ammonia through an oxidation process.

GLP compliance:

not specified

Analytical monitoring:

not specified

Details on sampling:

No data

Vehicle:

not specified

Details on preparation and application of test substrate:

Not reported

Test organisms (inoculum):

soil

Total exposure duration:

20 d

Test temperature:

30 °C

Moisture:

Distilled water was added to bring the soil moisture content to 60.%.

Details on test conditions:

See "any other information on materials and methods incl. tables"

Nominal and measured concentrations:

Thiourea was added as 5 and 10 % of added ammonium-N

Reference substance (positive control):

no

Remarks on result:

other: No NOEC could be determined as all concentrations showed an effect.

Details on results:

See "any other information on results incl tables".

Results with reference substance (positive control):

No data

Reported statistics and error estimates:

No data

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 both soils.

Validity criteria fulfilled:

not specified

Executive summary:

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.

Reference 2

Endpoint:

toxicity to soil microorganisms

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

no data

Reliability:

4 (not assignable)

Rationale for reliability incl. deficiencies:

documentation insufficient for assessment

Reason / purpose for cross-reference:

reference to same study

Qualifier:

no guideline followed

Principles of method if other than guideline:

Decomposition of thiourea by a penicillium species isolated from soil and production of sulphate and ammonia was shown.

GLP compliance:

not specified

Analytical monitoring:

yes

Vehicle:

no

Test organisms (inoculum):

soil

Total exposure duration:

7 d

Test temperature:

28 °C

Moisture:

The moisture content was brought to 50 % of the moisture-holding capacity, and water was added periodically to maintain the moisture level during incubation at 28 °C for several weeks.

Details on test conditions:

Decomposition of thiourea was tested in 200 g portions of barnyard soil at 0.02 and 0.1 M glucose (0.5 %).
Pure cultures were grown in 250 mL flasks containing 100 mL basal medium and shaken at 28 °C (240 to 280 1/min).
Basal medium:
Glucose, 20 g;
Thiourea, 0.25, 0.50 or 1.00 g
K2HP04, 1.5 g
KH2P04, 0.75 g
CaCI, . 2H20, 0.05 g
MgCl2.6H20, 0.08 g; FeCI3.6H2O, 0.01 g
Dist. H2O to 1000 ml, pH 7.0.

Nominal and measured concentrations:

Thiourea was added in concentration of 0.025, 0.05, 0.10 and 0.20 %.

Reference substance (positive control):

no

Key result

Duration:

7 d

Dose descriptor:

NOEC

Effect conc.:

500 other: mg/L medium

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: growth; dry weight of fungus

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.

Validity criteria fulfilled:

not specified

Conclusions:

Thiourea inhibited growth of a Penicillium species. After acclimatisation of the fungus thiourea was decomposed to sulphate and ammonia.

Executive summary:

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.

Reference 3

Endpoint:

toxicity to soil microorganisms

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

no data

Reliability:

3 (not reliable)

Rationale for reliability incl. deficiencies:

significant methodological deficiencies

Qualifier:

no guideline followed

Principles of method if other than guideline:

After incubation of two different fungi species with different concentrations of thiourea the dry weight and sugar content of the mycelial felt was recorded.

GLP compliance:

not specified

Analytical monitoring:

not specified

Details on preparation and application of test substrate:

Thiourea concentrations were prepared using Czapek's solution medium.

Test organisms (inoculum):

other: Helminthosporium sativum, Fusarium oxysporum

Total exposure duration:

15 d

Test temperature:

26 ± 1 °C

Moisture:

n.a.; test was conducted in Czapek's solution medium

Details on test conditions:

See section "any other information on materials and methods incl. tables"

Nominal and measured concentrations:

Nominal concentrations of thiourea: 10, 25, 50, 75, 100, 250, 500, 750 and 1000 ppm

Reference substance (positive control):

not specified

Remarks on result:

other: "No effect levels" could not be established since the inhibiting effect of thiourea started at 10 ppm.

Details on results:

See section "any other information on materials and methods incl. tables".

Results with reference substance (positive control):

None

Reported statistics and error estimates:

None

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.

Validity criteria fulfilled:

not specified

Conclusions:

Thiourea inhibited growth of the fungi species Helminthosporium sativum and Fusarium oxysporum.

Executive summary:

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.

Reference 4

Endpoint:

toxicity to soil microorganisms

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

no data

Reliability:

4 (not assignable)

Rationale for reliability incl. deficiencies:

documentation insufficient for assessment

Qualifier:

no guideline available

Principles of method if other than guideline:

The growth and respiratory inhibitory effects of thiourea on Rhizobium and Azotobacter chrooccum werte determined using a Warburg respirometer and measuring the optical density of the bacterial cultures at 578 nm.

GLP compliance:

not specified

Analytical monitoring:

not specified

Vehicle:

not specified

Test organisms (inoculum):

other: Bacterial cultures of Azotobacter chroococcum and Rhizobium leguminosarum

Total exposure duration:

20 h

Remarks:

The bacterial growth was monitored by measuring the OD578 after 20 h growth.

Test temperature:

25 °C for determination of bacterial growth; 28 °C for determination of respiration in the Warburg apparatus.

Moisture:

n.a.; pure cultures were tested in nutrient solution

Details on test conditions:

See section "any other information on materials and method incl. tables"

Nominal and measured concentrations:

Nominal concentration of thiourea tested: 10 and 100 mg/L

Reference substance (positive control):

no

Key result

Duration:

20 h

Dose descriptor:

NOEC

Effect conc.:

100 other: ppm

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

respiration rate

Details on results:

See "any other information on results incl. tables".

Results with reference substance (positive control):

No reference substance

Reported statistics and error estimates:

None

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.

Validity criteria fulfilled:

not specified

Conclusions:

In this study a concentration of 100 ppm thiourea caused a minor growth inhibition of Azotobacter chroococcum and did not affect growth of Rhizobium leguminosarum.

Executive summary:

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.

Description of key information

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.

Additional information

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.