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Environmental fate & pathways

Biodegradation in soil

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Reference
Endpoint:
biodegradation in soil: simulation testing
Type of information:
experimental study
Adequacy of study:
key study
Study period:
1995-10-05 to 1996-05-30
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
guideline study with acceptable restrictions
Remarks:
Results obtained are replicable and well documented. The deviation to OECD test guideline 304A is restricted to usage of different concentrations and duration of the test.
Qualifier:
equivalent or similar to guideline
Guideline:
OECD Guideline 304 A (Inherent Biodegradability in Soil)
Deviations:
yes
Remarks:
e.g. amount of radioactivity applied/assessed, duration of the study
Principles of method if other than guideline:
In general the principle of the method applied is identical to the method stated in OECD guideline 304A. Only details of implementation are not in all aspects identical.
GLP compliance:
not specified
Test type:
laboratory
Specific details on test material used for the study:
Form: dissolved in water (about 12.2 MBq/5 ml)
Further Test material: three agricultural soils: German standard soils BBA 2.1 (sand), BBA 2.2 (loamy sand) and a silt loam from Bayer farm Laacherhof.
The start concentration was 0.92 µg PBTC/100 g DW of soil (0.92 ppm).
Radiolabelling:
yes
Oxygen conditions:
aerobic
Soil classification:
other: German standard soils BBA 2.1 (sand), BBA 2.2 (loamy sand) and silt loam from Bayer farm Laacherhof
Year:
1995
Soil no.:
#1
Soil type:
sand
% Org. C:
2.48
CEC:
5 meq/100 g soil d.w.
Soil no.:
#2
Soil type:
loamy sand
% Org. C:
10
CEC:
10 meq/100 g soil d.w.
Soil no.:
#3
Soil type:
silt loam
% Org. C:
0.9
CEC:
8 meq/100 g soil d.w.
Details on soil characteristics:
German standard soils 1.) BBA 2.1 (sand), 2.) BBA 2.2 (loamy sand) and 3.) silt loam from Bayer farm Laacherhof
Soil No.:
#1
Duration:
ca. 133 d
Soil No.:
#2
Duration:
ca. 133 d
Soil No.:
#3
Duration:
ca. 133 d
Soil No.:
#1
Initial conc.:
0.92 ppm
Based on:
test mat.
Soil No.:
#2
Initial conc.:
0.92 ppm
Based on:
test mat.
Soil No.:
#3
Initial conc.:
0.92 ppm
Based on:
test mat.
Parameter followed for biodegradation estimation:
CO2 evolution
Soil No.:
#1
Temp.:
20
Humidity:
14.94 g water in 100 g dry soil
Microbial biomass:
74 mg microbial C/kg DW soil (initial), 21 mg microbial C/kg DW soil (end)
Soil No.:
#2
Temp.:
20
Humidity:
22.38 g water in 100 g dry soil
Microbial biomass:
406 mg microbial C/kg DW soil (initial), 181 mg microbial C/kg DW soil (end)
Soil No.:
#3
Temp.:
20
Humidity:
17.50 g water in 100 g dry soil
Microbial biomass:
531 mg microbial C/kg DW soil (initial), 249 mg microbial C/kg DW soil (end)
Details on experimental conditions:
About one week before degradation test start the soils were gently air-dried so that they could be screened to <= 2 mm. Subsequently, the soil moisture (% of dry weight, DW) was determined. Each 100 g DW were weighed into 300 ml Erlenmeyer flasks and then adjusted to a soil moisture of 50% of the maximum water holding capacity (WHC max.) each by using demineralised water. The volume of 1 ml water later added via the application solution was taken into account (for data see Table 2). The test batches were weighed, closed with cot-ton wool and pre-incubated at 20°C in the dark until the beginning of the test (4 days later). For respective data see following Table 2. Determinations of the microbial population of soils were performed by the laboratory of Dr. J.P.E. Anderson, Bayer AG, PF-E/OE, bldg. 6620, Monheim at the start and at the end of the experiment
Soil No.:
#1
% Recovery:
104.6
Remarks on result:
other: soil BBA 2.1 (sand)
Soil No.:
#2
% Recovery:
101.4
Remarks on result:
other: soil BBA 2.2 (loamy sand)
Soil No.:
#3
% Recovery:
102.8
Remarks on result:
other: silt loam, Bayer farm Laacherhof
Parent/product:
parent
Soil No.:
#1
% Degr.:
ca. 21
Parameter:
CO2 evolution
Sampling time:
133 d
Parent/product:
parent
Soil No.:
#2
% Degr.:
ca. 27
Parameter:
CO2 evolution
Sampling time:
133 d
Parent/product:
parent
Soil No.:
#3
% Degr.:
ca. 15
Parameter:
CO2 evolution
Sampling time:
133 d
Key result
Soil No.:
#1
DT50:
ca. 142 d
Type:
(pseudo-)first order (= half-life)
Temp.:
20 °C
Soil No.:
#2
DT50:
ca. 102 d
Type:
(pseudo-)first order (= half-life)
Temp.:
20 °C
Soil No.:
#3
DT50:
ca. 107 d
Type:
(pseudo-)first order (= half-life)
Temp.:
20 °C
Transformation products:
not measured
Details on transformation products:
NA
Evaporation of parent compound:
not measured
Volatile metabolites:
not measured
Residues:
not measured
Details on results:
The degradability of [3,4-14C]PBTC, 2-phosphonobutane-1,2,4-tricarboxylic acid, was investigated in three agricultural soils: 1.) BBA 2.1 (sand), 2.) BBA 2.2 (loamy sand), 3.) silt loam from Bayer farm Laacherhof. Start concentration was 0.92 µg PBTC/100 g DW of soil (0.92 ppm). Temperature and soil moisture during testing period (133 days) were 20°C and ca. 50% of the resp. maximum water holding capacity. The recoveries (material balances) ranged from 101.7% to 105.6%. The [14C]PBTC was metabolised to 14CO2, the main degradation product, accounting for 21.3% , 27.4%, 15.5% of the applied radioactivity in the soils 1.), 2.) and 3.) after 133 days, respectively. During the incubation period a constant increase of 14CO2 was measured. However, decreased 14CO2-formation rates with increasing time parallel to the decrease of the active biomass of soils were noted.
After 133 days low portions of PBTC were recovered by two extractions using aqueous CaCl2 solution (2.1%, 1.4% and 0.2% for soils BBA 2.1, BBA 2.2 and Laacherhof, respectively) indicating a correlation to the textural class of soil. The main portion of radioactivity (pre-dominantly as PBTC) was extracted by extensive HCl extraction. The portion of not-extracted (bound) residue resulting from the [14C]PBTC treatment amounted to 16.8%, 31.8% and 42.1% for the soils BBA 2.1, BBA 2.2 and Laacherhof, respectively. Correlation to the textural class of soil (lowest bound residues in the sand, highest in the silt loam) was observed. The predominant portion of radioactivity as well as of PBTC remaining in soil after 133 days of incubation was not easy to extract indicating a low mobility or leaching potential of PBTC in soils.
The time for disappearance of 50% of PBTC (DT50 value) calculated (1st order) from the results of HPLC (on realistic worst case assumption for peak evaluation) was 142 days, 102 days and 107 days for the soils BBA 2.1, BBA 2.2 and Laacherhof, respectively.
Due to known limitations of laboratory test systems (not all the processes relevant for degradation under outdoor conditions are reflected) the degradation rates reported here do not necessarily reflect the real situation in a natural environment. Nevertheless, it was shown that the PBTC is moderately degradable and is thoroughly metabolised to CO2 in soil.
Results with reference substance:
NA

No remarks.

Conclusions:
Degradability of radioactive labelled [14C]PBTC was investigated in three soils. Main degradation product was 14CO2, on average between 15.5 and 27.4 % after 133 d. DT50 between 102 and 142 days were calculated. The recoveries (material balances) for the different test vessels ranged from 101.7 to 105.6 %. It was shown that PBTC is moderately degradable and thoroughly metabolised to CO2 in soil. However, with a worst-case DT50 >142 d the substance must be classified as persistant (P).
Executive summary:

The degradability of [3,4-14C]PBTC, 2-phosphonobutane-1,2,4-tricarboxylic acid, active agent in BAYHIBIT AM, was investigated in three agricultural soils. The test soils maintained under aerobic conditions were German standard soils 1.) BBA 2.1 (sand), 2.) BBA 2.2 (loamy sand), 3.) silt loam from Bayer farm Laacherhof. Start concentration was 0.92 µg PBTC/100 g DW of soil (0.92 ppm). Temperature and soil moisture during total testing period of 133 days were 20°C and about 50% of the respective maximum water holding capacity. The recoveries (material balances) for the different test vessels ranged from 101.7% to 105.6%.


The [14C]PBTC was thoroughly metabolised to 14CO2, the main degradation product, accounting for 21.3% , 27.4%, 15.5% of the applied radioactivity in the soils 1.), 2.) and 3.) after 133 days, respectively. During the incubation period a constant increase of 14CO2 was measured. However, the formation rates of 14CO2 decreased with increasing time parallel to the decrease of the active biomass of soils.


After 133 days low portions of PBTC were recovered by two extractions using aqueous CaCl2 solution (2.1%, 1.4% and 0.2% for soils BBA 2.1, BBA 2.2 and Laacherhof, respectively) indicating a correlation to the textural class of soil. The main portion of radioactivity (pre-dominantly as PBTC) was extracted by extensive HCl extraction. The portion of not-extracted (bound) residue resulting from the [14C]PBTC treatment amounted to 16.8%, 31.8% and 42.1% for the soils BBA 2.1, BBA 2.2 and Laacherhof, respectively. Correlation to the textural class of soil (lowest bound residues in the sand, highest in the silt loam) was observed. The predominant portion of radioactivity as well as of PBTC remaining in soil after 133 days of incubation was not easy to extract indicating a low mobility or leaching potential of PBTC in soils.


The time for disappearance of 50% of PBTC (DT50 value) calculated (1st order) from the results of HPLC (on realistic worst case assumption for peak evaluation) was 142 days, 102 days and 107 days for the soils BBA 2.1, BBA 2.2 and Laacherhof, respectively.


Due to known limitations of laboratory test systems (not all the processes relevant for degradation under outdoor conditions are reflected) the degradation rates reported here do not necessarily reflect the real situation in a natural environment. It was shown that the PBTC is moderately degradable and is thoroughly metabolised to CO2 in soil. However, with a worst-case DT50 of 142 d the substance must be classified as persistant (P).

Description of key information

Degradability of radioactive labelled [14C]PBTC was investigated in three soils. Main degradation product was 14CO2, on average between 15.5 and 27.4 % after 133 d. DT50 between 102 and 142 days were calculated. The recoveries (material balances) for the different test vessels ranged from 101.7 to 105.6 %. It was shown that PBTC is moderately degradable and thoroughly metabolised to CO2 in soil. However, with a worst-case DT50 of 142 d the substance must be classified as persistant (P).

Key value for chemical safety assessment

Half-life in soil:
142 d
at the temperature of:
20 °C

Additional information