Hexadecyltrimethoxysilane

Toxicity to soil microorganisms

Link to relevant study record(s)

Reference

Endpoint:

toxicity to soil microorganisms

Type of information:

experimental study

Adequacy of study:

key study

Study period:

16 Sep - 14 Oct 2019

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 216 (Soil Microorganisms: Nitrogen Transformation Test)

Version / remarks:

2000

Deviations:

yes

Remarks:

Due to technical reasons the room temperature exceeded 22 °C (maximum was 22.9 °C which corresponds to 2.2 % deviation) for altogether 55 hours (see for further comment under 'Overall remarks, attachments').

GLP compliance:

yes (incl. QA statement)

Remarks:

Staatliches Gewerbeaufsichtsamt Hildesheim, Germany

Analytical monitoring:

no

Vehicle:

no

Details on preparation and application of test substrate:

APPLICATION OF TEST SUBSTANCE TO SOIL
- Method: respective test item amounts were pipetted onto quartz sand (1 % of the total soil amount per treatment), mixed thoroughly and applied to the surface of the soil.

Test organisms (inoculum):

soil

Total exposure duration:

28 d

Test temperature:

18.6 - 22.9°C

Moisture:

41.0 - 43.8% (of Maximum Water Holding Capacity)

Organic carbon content (% dry weight):

0.578

Nitrogen content (% dry weight):

26.6

Details on test conditions:

TEST SYSTEM
- Testing facility: plastic boxes (volume 1.0 L, food grade) with perforated tops
- Amount of soil: 400 g dw
- No. of replicates per concentration: 3
- No. of replicates per control: 3

SOIL INCUBATION
- Method: bulk

SOURCE AND PROPERTIES OF SUBSTRATE (if soil)
- field fresh, silty sand soil (LUFA standard soil 2.3)
- Soil texture
- % sand: 50 - 75%
- Organic carbon content: 0.5 - 1.5 %
- pH (in water): 5.5 - 7.5
- Initial nitrate concentration for nitrogen transformation test (mg nitrate/kg dry weight): 3.50 ± 0.16 mg NO3-N/kg dw
- Maximum water holding capacity (in % dry weigth): at least 42%
- Cation exchange capacity: 7.3 ± 1.1 meq / 100 g
- Pretreatment of soil: manually cleared of large objects and then sieved to a particle size of up to 2 mm; the soil amounts were amended with powdered lucerne-green-grass-meal (0.5 % of soil dry weight) (total nitrogen content: 26.6 g/kg)
- Storage (condition, duration): stored for 60 days (2019-06-28 to 2019-08-27) in the dark at 4 - 6 °C in a climatic room. Subsequently, the soil was pre-incubated at room temperature (ca. 20 °C) for 20 days (2019-08-27 to 2019-09-16) to adapt the micro-organisms to test conditions before the start of the study
- Initial microbial biomass as % of total organic C: at least 1 %

EFFECT PARAMETERS MEASURED: inorganic nitrate content (day , 7, 14 and 28)

RANGE-FINDING STUDY
- Test concentrations: 10, 100, 1000 mg/kg dw
- Results used to determine the conditions for the definitive study: Inhibition of Nitrate-N Content after 28 days: 8% (10 mg/kg dw), 14% (100 mg/kg dw), 100% (1000 mg/kg dw)

Nominal and measured concentrations:

Nominal concentrations: 0 (control), 10, 31.6, 100, 316, 1000 mg/kg dw

Reference substance (positive control):

yes

Remarks:

50 mg/kg dw Cyanoguanidine (tested yearly)

Duration:

28 d

Dose descriptor:

NOEC

Effect conc.:

31.6 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

nitrate formation rate

Duration:

28 d

Dose descriptor:

EC50

Effect conc.:

254 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

nitrate formation rate

Remarks on result:

other: 95% confidence limit: 214.8 - 304.0 mg/kg dw

Details on results:

After 7, 14 and 28 days of exposure, a significant inhibition of the microbial nitrate transformation was observed for test item concentrations ≥ 100 mg/kg soil dry weight.

Results with reference substance (positive control):

The reference item toxicity was valid (difference to control ≥25 %) and a toxicity of 33 % (Inorganic-N Contents ) compared to the control after a test duration of 28 days was observed.

Reported statistics and error estimates:

One Way Analysis of Variances (ANOVA) and DUNNETT’S test, were carried out for the determination of statistically significant differences of the nitrate formation rate at day 7, 14 and 28 compared to control replicates. Normality test and an Equal Variance test were done first. P-values for both Normality and Equal Variance test are 0.05. The α-value for ANOVA and Dunnett’s test (acceptable probability of incorrectly concluding that there is a difference) is α = 0.05.

Table 1: Validity criteria for OECD 216.

Criterion from the guideline	Outcome	Validity criterion fulfilled
The variation between replicate control samples should be less than ± 15%.	4%	yes

Table 2: Inhibition of nitrate-N content in soil containing hexadecyltrimethoxysilane, relative to the control

Nitrate-N Content
Test item concentration	Inhibition [%] compared to control
[mg/kg soil dry weight]	0 d	7 d	14 d	28 d
10.0	-5	-30	-8	0
31.6	-5	-20	-9	-6
100	0	15	32	15
316	-9	44	80	47
1000	-5	70	100	100

 

 Table 3: Inhibition of nitrate-N formation rates in soil containing hexadecyltrimethoxysilane, relative to the control

Nitrate-N Formation Rate
Test item concentration	Inhibition [%] compared to control
[mg/kg soil dry weight]	7 d	14 d	28 d
10.0	-45	-9	1
31.6	-28	-10	-6
100	23	42	19
316	74	n.d.	59
1000	n.d.	n.d.	n.d.

negative values = increase

n.d.= Not determinable, due to negative Nitrate-N formation rates. For calculations of EC_x-values, inhibition was set as 100 %.

Table 4: Nitrate-N contents -day 0

Test item concentration [mg/kg soil dry weight]	Nitrate-N
	Repl.	MV±SD	CV
	[mg NO3-N/kg soil dry weight]		[%]
Control	3.59	3.50 ± 0.16	4
Control	3.32
Control	3.59
10.0	3.90	3.68 ± 0.20	6
10.0	3.50
10.0	3.63
31.6	3.37	3.69 ± 0.39	10
31.6	3.59
31.6	4.12
100	3.54	3.50 ± 0.04	1
100	3.50
100	3.46
316	3.90	3.80 ± 0.26	7
316	3.99
316	3.50
1000	3.94	3.66 ± 0.24	7
1000	3.50
1000	3.54

Repl.: Replicate MV: Mean value

SD: Standard deviation CV: Coefficient of variation

n.d.: not determinable, due to negative nitrate-N formation rates

Table 5: Nitrate-N contents and nitrate-N formation rates -day 14

Test item concentration [mg/kg soil dry weight]	Nitrate-N			Nitrate-N Formation Rate
	Repl.	MV±SD	CV	Repl.	MV±SD	CV
	[mg NO3-N/kg soil dry weight]		[%]	[mg NO3-N×(kg soil dry weight×d)^-1]		[%]
Control	14.7	14.6 ± 0.51	4	0.800	0.790 ± 0.04	5
	15.0			0.821
	14.0			0.750
10.0	16.7	15.8 ± 0.83	5	0.930	0.863 ± 0.06	7
	15.1			0.816
	15.5			0.844
31.6	15.7	15.9 ± 0.35	2	0.858	0.872 ± 0.03	3
	15.7			0.858
	16.3			0.901
100	8.38	9.96 ± 2.22	22	0.349	0.462 ± 0.16	34
	9.01			0.394
	12.5			0.643
316	2.57	2.93 ± 0.50	17	n.d.	–	-
	< LOQ			n.d.
	3.28			n.d.
1000	< LOQ	–	-	n.d.	–	-
	< LOQ			n.d.
	< LOQ			n.d.

Repl.: Replicate MV: Mean value

SD: Standard deviation CV: Coefficient of variation

n.d.: not determinable, due to negative nitrate-N formation rates

Validity criteria fulfilled:

yes

Remarks:

For further details please refer to “Any other information on results incl. tables".

Executive summary:

The effects of the test substance on the metabolic activity of soil micro-organisms were determined according to OECD Guideline 216 (2000). After 28 days, a significant inhibition of the microbial nitrate transformation was observed for test item concentrations ≥ 100 mg/kg. The corresponding no-observed effect concentration (NOEC) was 31.6 mg/kg dw.

Description of key information

NOEC (28 d) = 31.6 mg/kg soil dw (nominal, OECD 216)

Key value for chemical safety assessment

Long-term EC10 or NOEC for soil microorganisms:

31.6 mg/kg soil dw

Additional information

One GLP study is available testing the effects of the test substance on nitrogen tranformation in soil according to OECD 216. The test item was applied via quartz sand at 5 concentrations in the range between 10 and 1000 mg/kg soil dw. Measurements of nitrate-nitrogen concentrations in soil were made in all treatments and the control on the day of treatment application (day 0) and after 7, 14 and 28 days. After 7, 14 and 28 days of exposure to Hexadecyltrimethoxysilane, a significant inhibition of the microbial nitrate transformation was observed for test item concentrations ≥ 100 mg/kg soil dw. The corresponding no-observed effect concentration (NOEC) was 31.6 mg/kg soil dw. After 28 days of exposure the EC50-value for inhibition of the nitrate-N formation rate was 254 mg/kg soil dw.