Ethanone, 2,2-dichloro-1-(2,3-dihydro-3-methyl-4H-1,4-benzoxazin-4-yl)-

Biodegradation in soil

Description of key information

Five studies of biodegradation in soil have been performed for benoxacor, covering 4 soil types and various experimental conditions (aerobic, anaerobic and sterile). It is proposed that the most reliable values for use in the chemical safety assessment are those reported by Ellgehausen 1994b and Morgenroth 1997 for aerobic soil incubated at 20 °C (1.2, 5.3, 0.5 and 0.7 days). The overall geometric mean DT50 is 1.2 days. Converting to the EU average outdoor temperature of 12 °C (in accordance with ECHA R.16 guidance) gives a normalised DT50 of 2.3 days.

Key value for chemical safety assessment

Half-life in soil:

1.2 d

at the temperature of:

20 °C

Additional information

Five studies of biodegradation in soil have been performed for benoxacor.

A study of biodegradation of benoxacor in two agricultural soils (Morgenroth, 1997) was investigated under aerobic, anaerobic and sterile conditions. The study was carried out according to BBA Guideline Part IV, 4-1 (Dec 1986) and others and is considered to be reliable with restrictions (Klimisch score 2).14C-benoxacor was applied at a concentration of 0.22 mg/kg and incubated for a period of 365 days.14C-benoxacor was rapidly degraded under aerobic conditions, mainly by binding to the soils and mineralization, with a half-life of 0.5-0.7 days. Anaerobic incubation slows degradation; half-life 4.0-4.4 days. Under sterile conditions degradation via binding to soil and mineralization is blocked leading to the conclusion that rapid degradation of benoxacor in soil is enabled by aerobic soil microorganisms. This study was conducted according to guideline methods and GLP.

The inherent biodegradability, in soil, of 14C labelled benoxacor was also tested in a GLP compliant study (Ellgehausen, 1994b) conducted to non-standard guidelines and considered to be reliable with restrictions (Klimisch score 2). Benoxacor was rapidly degraded with a half-life of 1.2 and 5.3 days in a sandy loam and sand soil, respectively.

The influence of moisture, temperature and dose rate on the degradation of benoxacor was investigated in a further study (Ellgehausen, 1994a). The study was performed according to national guidelines, was GLP compliant and is considered to be reliable without restriction (Klimisch score 1). Under normal conditions (20 °C and 60% FC) the half-life was found to be 0.9 days, at 10 °C the half-life increased to 3.6 days. At 20 °C and reduced moisture content the half-life was 2.1 days. Finally, under normal conditions and an application rate 10 times the recommended field dose the half-life was 1.1 days. In all cases degradation of benoxacor was rapid.

Two additional studies were performed with14C-benoxacor in a sand loam soil by Cranor (1986a and 1986b). Aerobic DT50 values were determined to be 48.7 and 21.6 days, whilst a half-life of 70.4 days was calculated for benoxacor under anaerobic conditions. However, it should be noted that the studies were conducted at an application rate almost 100-fold greater than the maximum that would occur in reality. Thus, it is proposed that the resulting soil degradation rates are unrepresentative of realistic conditions.In addition, the kinetic data are the result of a recalculation of the raw data since the DT50 values calculated in the original report were based on a linear regression of a half logarithmic plot leading to inaccurate results. The soil had a mean moisture content of only 2.9% (64% FC). Under anaerobic conditions (Cranor 1986b) the calculation of the DT50 value was based on a set of only three data points. Recalculation was not possible, because of the limited raw data.

The results of the laboratory studies on degradation of benoxacor in soil are summarised in the table below:

Incubation	Temp (°C)	Organic carbon (%)	Moisture % (w/w)	Rate  (kg / ha)	Soil type	DT50(days)	DT90(days)	Reference
Aerobic	25	0.6	2.9 (64 % FC)[1]	6.8	Loamy sand	22[2]	93[3]	Cranor, 1986a
Aerobic	20	2.0	19.4 (40% MWC)[4]	0.1	Sand	5.3	31	Ellgehausen 1994b
Aerobic	20	1.1	20.3 (40% MWC)	0.1	Sandy loam	1.2	5
Aerobic	20	2.1	28.4 (60% FC)	0.1	Sandy loam	0.9	4	Ellgehausen 1994a
Aerobic	10	2.1	28.4 (60% FC)	0.1	Sandy loam	3.6	16
Aerobic	20	2.1	14.2 (30% FC)	0.1	Sandy loam	2.1	9
Aerobic	20	2.1	28.4 (60% FC)	1	Sandy loam	1.1	5
Aerobic	20	1.8	18.8 ( 75% FC)	0.1	Loamy sand	0.5	2	Morgenroth 1996
Aerobic	20	1.4	29.9 ( 75% FC)	0.1	Silt loam	0.7	6.0
Anaerobic	20	1.8	flooded	0.1	Loamy sand	4	30
Anaerobic	20	1.4	flooded	0.1	Silt loam	4	96
Sterile	20	1.8	29.9 (75% FC)	0.1	Silt loam	>200	> 200

 

The studies demonstrate that the dissipation rate of benoxacor is very fast and not significantly influenced by soil moisture, temperature, soil type or use rate. This is shown by the degradation half life range of 0.5 to 5.3 days in aerobic soils at representative dose rates. It is proposed that the most reliable values for use in the chemical safety assessment are those reported by Ellgehausen 1994b and Morgenroth 1997 for aerobic soil incubated at 20 °C (1.2, 5.3, 0.5 and 0.7 days). The overall geometric mean DT50 is 1.2 days. Converting to the EU average outdoor temperature of 12 °C (in accordance with ECHA R.16 guidance) gives a normalised DT50 of 2.3 days.

The route of degradation of benoxacor is dominated by the formation of bound residues and the subsequent mineralisation to carbon dioxide. Only minor levels of metabolites are formed, none reaching 10% of the applied dose.The bound residues reach their maximum under aerobic conditions after about one week. Thereafter the concentration of bound residues steadily decreases while the mineralisation rate increases. Accordingly an accumulation of bound residues is unlikely to occur.

Anaerobic conditions increase the DT50 value by a factor of two, the DT90 value by a factor of six. Under anaerobic conditions, the dominating process for the route of degradation is the formation of bound residues. The mineralisation process is not significant under these conditions. Only minor levels of metabolites were formed, none reaching 10% of the applied dose. It is worthwhile to note that anaerobic conditions are usually found only in the deeper layers of the soil. Given the very short degradation half life in aerobic soil, the chance of benoxacor reaching deeper soil layers is low. 

Sterility of the soil nearly stops the degradation of benoxacor (DT50> 200 days). Microbially active soil is thus a prerequisite for the degradation of benoxacor. However, assuming application to a viable natural soil, it would not be expected that benoxacor or any metabolites would accumulate, due to the fast degradation and the transient nature of the metabolites.

The rapid degradation of benoxacor is confirmed by field trials (Mostert, 1993), where no benoxacor residues are found 80 days after application. The field studies regarding the degradation of benoxacor are not included in the IUCLID dossier (since sufficient information regarding degradation is already provided by the laboratory studies). However, further details can be provided, if necessary.

[1] FC = Field Capacity

[2] This value was recalculated since the DT50 of 48.7 days calculated in the original study report was considered to be inaccurate because the application rate of this study was 68 times higher than the recommended application rate

[3]Value recalculated by extrapolation of raw data

[4] MWC = Maximum Water Capacity