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Toxicity to soil microorganisms

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Reference
Endpoint:
toxicity to soil microorganisms
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Principles of method if other than guideline:
Degradation of C-14 guanidinium chloride in field soil samples was followed by trapping and counting of radioactivity (14-CO2). Test were made with sterile soil (no degradation), different concentrations of test substance, different additions carbon source (cellulose, glucose) and nitrogen.
GLP compliance:
not specified
Specific details on test material used for the study:
1µCi carbon-14 guanidine chloride (Amersham, 22 mCi/mMole) + unlabeled ganidine chloride
Analytical monitoring:
yes
Details on sampling:
1 x per day
Test organisms (inoculum):
soil
Total exposure duration:
23 d
Test temperature:
20 °C
Moisture:
60% soil mosture holding capacity
Details on test conditions:
The mineralization of guanidinium carbon was measured in the incubation and trapping system described by Marrinucci and Bartha (1979). Triplicate test flasks received 1 microCi carbon-14 guanidine chloride (Amersham, 22 mCi/mMole), sufficient distilled water to achieve 60% soil moisture holding capacity, and the equivalent of 20 g dry weight soil. Chemical additions including unlabeled guanidine chloride and the inhibitors mercuric chloride and sodium azide were made in conjunction with the moisture ad- justment. The organic carbon sources, cellulose and glucose, were added as sterile dry solids (4% by weight) and were mixed to homogeneity with a sterile glass rod.
Reference substance (positive control):
yes
Remarks:
Guanidinium chloride
Key result
Duration:
23 d
Dose descriptor:
NOEC
Effect conc.:
400 mg/kg soil ww
Nominal / measured:
nominal
Conc. based on:
test mat.
Remarks:
Guanidinium ion
Basis for effect:
other: Reduction of mineralization of Guanidinium Carbon
Key result
Duration:
23 d
Dose descriptor:
NOEC
Effect conc.:
636 mg/kg soil ww
Nominal / measured:
nominal
Conc. based on:
test mat.
Remarks:
Guanidine chloride
Basis for effect:
other: Reduction of mineralization of Guanidinium Carbon
Details on results:
Mineralization rates showed no tendency to decrease at any concetnration ( tested: 10, 25, 50, 100, 200, 400 mg/kg) until most of the Guanidinium was degraded.

Shown in Table are the effects of two carbon sources and three major nitrogenous components in nitroguanidine wastestreams on guanidinium carbon mineralization. Both glucose and to a much lesser extent its natural polymer cellulose enhanced 24 hour mineralization rates: production of radiolabeled carbon dioxide was nearly four times the unamended control in the presence of glucose and was increased by twenty percent in the presence of cellulose.

The wastewater components nitroguanidine, ammonium, and nitrate showed only a slight tendency to inhibit guanidinium mineralization whether alone at equimolar concentrations to guanidinium or in concert. Previous results have demonstrated that environmental microorganisms in surface water can mineralize guanidinium carbon slowly as a sole substrate and more rapidly in the presence of metabolizable carbon (Mitchell, 1987). While the relative contribution of such populations to the mineralization of the cation in soil can not be assessed in the present study, results with the added carbon sources suggest that the latter process can also be stimulated in soil.

Table Effects of Carbon and Nitrogen Compounds on the Mineralization of Guanidinium Carbon in Duffield Soil

Sample                            nM Mineralized/24Hours (1xsigma)       %Control (1xsigma)

Control(i)                            634 (4)                                                        100 (<1)

Carbon added as(ii):

Cellulose                            776 (26)                                                        122.4 (4)

Glucose                            2481 (273)                                                      391.4 (43)

Nitrogen added as (ii):

Ammonium                         579 (47)                                                         91.3 (7)

Nitrate                                596 (8)                                                           94 (1.3)

Nitroguanidine                     542 (22)                                                        85.5 (3.5)

Composite                           578 (44)                                                        91.7 (7)

i. Triplicate flasks each contained 1 µCi and 20.9 µM guanidinium at a concentration of 100 mg/Kg soil.

ii. Carbon and nitrogen compounds were 4% (w/w) and 20.9 µM respectively.

Validity criteria fulfilled:
not applicable
Conclusions:
For loading rates of 10 - 400 mg Guanidine /kg soil ww (15.9 - 636 mg Guanidine chloride /kg soil ww) mineralisation rates of Guanidine carbon to Carbon dioxide showed not tendency to decrease at any concentration.
Executive summary:

In a non guideline test with natural soil microorganism, degradation rates for mineralisation of 14C radiolabeled guanidine carbon were determined. Quantification was made by determination of trapped 14C labelled carbon dioxide. For loading rates of 10 - 400 mg Guanidine /kg soil ww (15.9 - 636 mg Guanidine chloride /kg soil ww) mineralisation rates of Guanidine carbon to Carbon dioxide showed not tendency to decrease at any concentration.

Description of key information

For loading rates of 10 - 400 mg Guanidine /kg soil ww (15.9 - 636 mg Guanidine chloride /kg soil ww) mineralisation rates of Guanidine carbon to Carbon dioxide showed not tendency to decrease at any concentration.

Key value for chemical safety assessment

Long-term EC10 or NOEC for soil microorganisms:
636 mg/kg soil dw

Additional information

In a non guideline test with natural soil microorganism, degradation rates for mineralisation of 14C radiolabeled guanidine carbon were determined. Quantification was made by determination of trapped 14C labelled carbon dioxide. For loading rates of 10 - 400 mg Guanidine /kg soil ww (15.9 - 636 mg Guanidine chloride /kg soil ww) mineralisation rates of Guanidine carbon to Carbon dioxide showed not tendency to decrease at any concentration.

(Remark: key value for chemical safety assesment NOEC 636 mg/kg is wet weight; pick list doesn't allow to choose ww but dw)