Sodium dithionite

Toxicity to soil microorganisms

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

toxicity to soil microorganisms

Type of information:

read-across based on grouping of substances (category approach)

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: No standard test, but test well described and relevant ecological endpoint for evaluation the effect of thiosulfate on the N-cycle is evaluated (effect on urea hydrolysis).

Justification for type of information:

see attachment “Read-across concept – Human Health/Environment - Category approach for Inorganic sulfites/thiosulfates/dithionite" in section 13.

Principles of method if other than guideline:

ATS was added to urea-spiked soils, and urea levels were extracted and determined after 3 and 10 days using methods described by Douglas and Bremner (1970) and Mulvaney and Bremner (1979).

Details on preparation and application of test substrate:

The soils used were surface (0-15 cm) samples of Iowa soils selected to obtain a range in properties (Webster, Hayden, Harps, Storden). Each sample was air-dried and crushed to pass through a 2-mm screen

Test organisms (inoculum):

other: not specified

Total exposure duration:

10 d

Remarks:

samples were also analysed after 3 d

Test temperature:

20 degrees Celcius

Moisture:

2 mL in 5 gram of dried soil

Details on test conditions:

Five gram samples of air-dried soils were placed in 65-mL glass bottles and treated with 2 mL of water containing 10 mg of urea or with 2 mL of water containing 10 mg of urea and various amounts of the test compound. The bottles were stoppered and placed in an incubator at 20°C. After 3 and 10 days, triplicate bottles were removed from the incubator, and urea in the incubated soil samples was extracted and determined by a colorimetrc method.

Nominal and measured concentrations:

Nominal mg ATS/kg soil: 500, 1000, 2500, 5000

Duration:

3 d

Dose descriptor:

NOEC

Effect conc.:

1 000 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: retardation of urea hydrolysis

Remarks on result:

other: Webster, Harps, Storden soil

Duration:

3 d

Dose descriptor:

NOEC

Effect conc.:

>= 5 000 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: retardation of urea hydrolysis

Remarks on result:

other: Hayden soil

Duration:

3 d

Dose descriptor:

other: LOEC

Effect conc.:

2 500 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: retardation of urea hydrolysis

Remarks on result:

other: Webster, Harps, Storden soil

Duration:

10 d

Dose descriptor:

NOEC

Effect conc.:

2 500 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: retardation of urea hydrolysis

Remarks on result:

other: Webster soil

Duration:

10 d

Dose descriptor:

NOEC

Effect conc.:

>= 5 000 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: retardation of urea hydrolysis

Remarks on result:

other: Hayden, Harps, Storden soil

Duration:

10 d

Dose descriptor:

other: LOEC

Effect conc.:

5 000 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: retardation of urea hydrolysis

Remarks on result:

other: Webster soil

Details on results:

ATS significantlly retarded urea hydrolysis only when applied at rates as high as 2500 or 5000 mg/kg soil (NOEC: 2500 mg/kg soil).

Conclusions:

The effect of ammonium thiosulfate on urea hydrolysis was investigated in urea-spiked soils that were exposed to different concentration levels of ATS. ATS significantlly retarded urea hydrolysis only when applied at rates as high as 2500 or 5000 mg/kg soil (NOEC: 2500 mg/kg soil). These data can be used as supportive information for the assessment of thiosultfates in the terrestrial compartment.

Reference 2

Endpoint:

toxicity to soil microorganisms

Data waiving:

other justification

Justification for data waiving:

other:

Reference 3

Endpoint:

toxicity to soil microorganisms

Type of information:

read-across based on grouping of substances (category approach)

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: No standard guideline test for the evaluation of terrestrial micro-organisms toxicity, but study conducted in a scientific justifyable way

Justification for type of information:

see attachment “Read-across concept – Human Health/Environment - Category approach for Inorganic sulfites/thiosulfates/dithionite" in section 13.

Principles of method if other than guideline:

Soil was prepared (addition of urea ) and hydrolisis in different treatments was determined by comparing urea content at the start and at the end of different experiments
Clay content was 17 g/kg

GLP compliance:

not specified

Analytical monitoring:

yes

Details on preparation and application of test substrate:

Kahola silt loam soil (fine-silty, mixed, mesic cumulic Haploudoll) was used. Soil was collected from 2 depths (soil1: 0-2 cm ; soil2: 5-15 cm).

Test organisms (inoculum):

other: not specified

Test temperature:

20 degrees Celcius (exp. 1)
25 degrees Celcius (exp. 2, 3, 4)

Moisture:

water holding capacity was 225, 137 and 105 g/kg at -0.03, -0.1 and -1.5 MPa, respectively

Details on test conditions:

Tests in chambers with continuous air flow at 0.7 chamber volume/h; CO2 and NH3 were removed, and the incoming air was brought to 95% RH by bubbling through 2M KOH, 1M H2SO4, and deionized warer before entering the chamber.
Incubation containers were capped with polyfoam sponges moistened with NH3 trapping solution (75 mL H3PO4 + 250 mL glycerol/L).
After incubation the soil was extracted with 2M KCL containing 5 mg phenylmercuric acetate.
Urea, ammonium, thiosulfate, tetrathionate, nitrite and nitrate were analysed colorimetrically with a flow injector autoanalyzer.
The urea hydrolized was determined as the difference between urea applied and urea recovered.
Specific details for the different experiments are outlined in detail in the original paper; main differences were related to method of ATS administration (broadcast, surface band, subsurface band, uniform application).

Details on results:

Inhibition of urea hydrolysis by ammonium thiosulftate varies with application method, the rate of S2O3 applies, and environmental conditions. Band application of ATS was superior to broadcast application.
The maximum effect of S2O3 as a urease inhibitor was obtained with a 10:1 urea-N:S ratio. Tetrathionate applied at a 10:1 urea-N:S ratio resulted in the same or higher levels of soil urease inhibition as S2O3, indicating that S4O6 could be the urease inhibitor produced by ATS in soil.

Conclusions:

No specific endpoints (i.e., NOEC, EC10, EC50) were reported, and exposure periods were also not specified. The data in this publication, however, can serve as supportive information on the effects of thiosulfates on the N-cycle in the terrestrial compartment.

Additional information

In a supporting study by McCarty et al. (1990), the effect of ammonium thiosulfate on urea hydrolysis was investigated in urea-spiked soils that were exposed to different concentration levels of ammonium thiosulfate. Ammonium thiosulfate significantly retarded urea hydrolysis only when applied at rates as high as 2500 or 5000 mg/kg soil (NOEC: 2500 mg/kg soil), which are significantly above the OECD test limit of 1000 mg/kg soil. Thus, results indicate a low potential for the toxicity of thiosulfate substances on microbial nitrogen cycling in soil.