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Toxicity to soil microorganisms

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Endpoint:
toxicity to soil microorganisms
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: Guideline study
Qualifier:
according to guideline
Guideline:
BBA Part VI, 1-1
Deviations:
no
GLP compliance:
no
Analytical monitoring:
no
Details on sampling:
Amounts of 25 or 50 g dry weight were removed from each soil sample immediately after treatment (day 0) and after 14, 28, 42, 56, 70 and 91 days thoroughly mixed with glucose and filled into plastic cylinders with a diameter of 3 cm and a height of 23 cm.
The cylinders were connected with a gas analyzer of Wösthoff Co., Bochum to measure carbon dioxide formation. The amount of glucose which guarantees the maximum respiration rate was determined for each soil in preliminary experiments. 3000 mg glucose per kg soil dry weight were required for respiration measurements in loamy sand and 4000 mg for measurements in silt
Details on preparation and application of test substrate:
The sieved soil was thoroughly mixed for 14 min. at 50 rpm in 20 L stainless steel containers together with ground quartz sand (10g/kg soil dry weight, control) or a mixture of Diuron (10.7 or 53.3 mg a.i./kg soil dry weight) by rolling on a gyrowheel mixer (Elte 650, J. Engelsmann AG, Ludwigshafen, FRG)
Test organisms (inoculum):
soil
Total exposure duration:
91 d
Test temperature:
20 ± 2°C
Moisture:
ca. 40 – 45 % of the water capacity.
Details on test conditions:
TEST SYSTEM
- Test container (type, material, size): 1 L preserving jars with glass lids
- Amount of soil: 750 g

SOURCE AND PROPERTIES OF SUBSTRATE (if soil)
Two soils were tested on soil respiration stimulated by amendment of glucose:

1. Loamy sand (Experimental Farm Laacherhof, field plot F)
69.1% sand
22.4% silt
8.5% clay.
TOC was 0.84 %.

2. Silt (Experiment Farm Höfchen, field plot 7a)
3.2% sand
89.4% silt
7.5% clay.
TOC was 1.23 %


EFFECT PARAMETERS MEASURED (with observation intervals if applicable) :
Carbon dioxide formation, soil respiration, mineralization of glucose
Sampling after 0, 14, 28, 42, 56, 70 and 91 days of incubation

VEHICLE CONTROL PERFORMED: no

SELECTION OF TEST CONCENTRATION:
The highest intended application rate of Diuron is 8.0 kg a.i./ha. The calculation was based on a soil depth of 5 cm and a soil density of 1.5 g/m3. Accordingly soil samples were treated with 10.7 and 53.3 mg of Diuron/kg soil dry weight
Nominal and measured concentrations:
control, 10.7 and 53.3 mg of Diuron/kg soil dry weight
Reference substance (positive control):
no
Duration:
91 d
Dose descriptor:
NOEC
Effect conc.:
10.7 mg/kg soil dw
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
respiration rate
Details on results:
The highest recommended rate of Diuron had no meaningful influence on the mineralization of glucose in a loamy sand. After amendment with glucose only insignificantly less carbon dioxide (0.7%) was released from the treated soil samples than from the control soils. On day 91, 24% more carbon dioxide was released than from the controls. Also in silt the highest recommended rate of Diuron had only an insignificant influence on the soil respiration after amendment with glucose. In comparison to the control, 0-11% less carbon dioxide were formed.
In the loamy sand the 5-fold overdose of Diuron caused a retardation of the soil respiration after amendment with glucose during the first 70 days by max. 36% in comparison to the control. This retardation had disappeared, however, on day 91 of the experiment (8% more carbon dioxide than in the control). In the silt the 5-fold overdose of Diuron caused a temporary reduction of the soil respiration after amendment with glucose (38-46% in comparison to the control soils) which persisted over the 91-day test period
Results with reference substance (positive control):
not applicable
Reported statistics and error estimates:
not stated

       Effect data

Deviation of soil respiration from control (100%) in %

 

Day

Loamy sand

mg a.i./kg soil dry weight

 

Silty sand

mg a.i./kg soil dry weight

 

 

10.7

53.3

10.7

53.3

0

1

0

0

-38

14

-2

-16

-1

-45

28

-7

-36

-3

-44

42

-4

-33

-5

-46

56

-2

-9

-7

-46

70

-4

-11

-7

-44

91

+24

+8

-11

-46

Executive summary:

A guideline study evaluating the influence of this test substance to soil respiration is available. The highest recommended rate of Diuron had no meaningful influence on the mineralisation of glucose in a loamy sand and only an insignificant influence in silt. Only after a 5 -fold overdose of Diuron did a temporary retardation of respiration occur in either substrate. Therefore, the test substance is not expected to have a relevant influence on the mineralisation of organic matter in soil when being applied to planted soil.

Endpoint:
toxicity to soil microorganisms
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: Guideline study, no GLP
Qualifier:
according to guideline
Guideline:
BBA Part VI, 1-1
GLP compliance:
no
Analytical monitoring:
no
Details on sampling:
- Sampling method:
Immediately after treatment and after 7, 14, 21, 28, 42, 56, 70 and 91 days the soil was thoroughly mixed and portions of each soil equivalent to 10 g dry weight were weighed and extracted with 50 mL of 1N KCl for 60 min. on a mechanical shaker (about 200 rpm). The soil extracts were filtered to remove soil particles and analyzed for their content of ammonium-N, nitrite-N and nitrate plus nitrite-N on a Technicon Autoanalyzer II
Vehicle:
no
Details on preparation and application of test substrate:
APPLICATION OF TEST SUBSTANCE TO SOIL
Three portions of the sieved soil corresponding to 750 g dry weight were weighed and mixed for 15 min at 50 rpm in 20 liter stainless steel containers together with ground quartz sand (10 g/kg dry weight of soil, control) or a mixture of quartz sand and Diuron (10.7 or 53.3 mg of Diuron/kg dry weight of soil) using a gyro-wheel mixer (Elte 650, Engelsmann Co., Ludwigshafen, FRG). After treatment with sand or sand and Diuron, the soils were mixed for 15 minutes at 50 rpm with powdered ammonium sulphate (1,000 mg/kg) and quartz sand
Test organisms (inoculum):
soil
Total exposure duration:
91 d
Test temperature:
20 + 2°C
Moisture:
corresponded to 40-45% water capacity
Details on test conditions:
TEST SYSTEM
- Test container (type, material, size): 1 litre preserving jars, closed with glass lids (without rubber rings
- Amount of soil: 750 g dry weight

SOURCE AND PROPERTIES OF SUBSTRATE (if soil)
Two soils were tested on nitrification of added ammonium:

1. Loamy sand (Experimental Farm Laacherhof, field plot F)
69.1% sand
22.4% silt
8.5% clay.
TOC was 0.84 %.

2. Silt (Experiment Farm Höfchen, field plot 7a)
3.2% sand
89.4% silt
7.5% clay.
TOC was 1.23 %

EFFECT PARAMETERS MEASURED (with observation intervals if applicable) :
Nitrification of added ammonium, determination of ammonium, nitrite and nitrate plus nitrite
sampling after 0, 14, 28, 42, 56, 70 and 91 days of incubation

SELECTION OF TEST CONCENTRATION
The highest intended application rate of Diuron is 8.0 kg a.i./ha. The calculation was based on a soil depth of 5 cm and a soil density of 1.5 g/m3. Accordingly soil samples were treated with 10.7 and 53.3 mg of Diuron/kg soil dry weight (5-fold application)
Nominal and measured concentrations:
Nominal: control, 10.7 and 53.3 mg Diuron/kg dry weight soil
Reference substance (positive control):
no
Duration:
91 d
Dose descriptor:
NOEC
Effect conc.:
10.7 mg/kg soil dw
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
nitrate formation rate
Details on results:
The highest recommended rate of Diuron caused an insignificant retardation of the nitrification in the loamy sand (3-9 % less nitrate than in the untreated control). On day 91 after treatment the nitrate content was 9 % below the value of the control. In the silt the highest recommended rate of Diuron caused only a short-term reduction of the nitrate formation (day 7 and day 56) by 4 % in comparison to the control
The 5-fold higher rate of the herbicide caused a reduction of the nitrification in both soils (17-38 % in comparison to the control) which continued during the test period. In the loamy sand the nitrate content at the end of the test (day 91) was 25 % below the values of the control. In the silt the nitrate formation 91 days after treatment was reduced by 32 % in comparison to the control.
Nitrite formation was not detected in the silt. Traces of nitrite (0.31-0.58 mg nitrite-N/kg dry weight soil) were found in the loamy sand on days 28 and 42 after treatment with the normal rate of Diuron and on days 28, 42, 56 and 91 after treatment with the 5-fold rate of the herbicide. Based on these low amounts the nitrite formation is not considered, however, to be of any relevance.
Executive summary:

A guideline study evaluating the influence of this test substance on the nitrification in soil is available. The influence of the highest recommended rate of Diuron on the nitrification of added ammonium in loamy sand and silt was low (max. 9 % less nitrate than in the untreated control). Since a marked (about 30 %) reduction of the nitrification continuing over the test period of 91 days could only be observed after use of the 5-fold overdose of the herbicide, Diuron is not expected to significantly influence the nitrification in soil when being used as recommended

Description of key information

Not harmful to soil microorganisms

Key value for chemical safety assessment

Additional information

The toxicity towards soil microorganisms was investigated in two Guideline studies, according to BBA VI, 1-1 (Effects on the Activity of the Soil Microflora; BBA Guidelines for the Official Testing of Plant Protection Products). One study concerned the effects of Diuron on soil respiration (Anderson, 1989) and the other on soil nitrification (Blumenstock, 1989); both studies investigated effects in both loamy soil and silt.

At the highest recommended rate of Diuron, there was no significant influence on either soil respiration or nitrification in either test substrate. Only after a 5 -fold overdose of Diuron did a temporary retardation of respiration occur (Anderson, 1989) and about a 30% reduction in nitrification (Blumenstock, 1989). Therefore, Diuron is not expected to have an adverse effect on soil microorganisms when being used as recommended.