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Environmental fate & pathways

Biodegradation in soil

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Description of key information

The degradation of glufosinate-ammonium was studied in ten soils covering a wide range of soil types from loamy sand to clay loam soil. Standard conditions (20 °C, 40% relative soil moisture) as well as lower temperature and higher soil moisture (60 %) were used.

In summary, glufosinate-ammonium mineralises rapidly under standard aerobic laboratory conditions in soil with the formation of considerable amounts of CO2. The degradation of glufosinate-ammonium is rapid even at low temperature. The fast degradation of glufosinate-ammonium in soil was confirmed under field conditions. The main soil metabolites PPO (AE F065594), MPP (AE F061517) and MPA (AE F064619) have also been shown to rapidly degrade in soil under laboratory and field conditions.

Key value for chemical safety assessment

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

Additional information

For a better understanding of the environmental profile of glufosinate-ammonium, information on aerobic degradation in soil are shortly summarised as well, but no study summaries are included.

 

Route of degradation

Laboratory studies on the route of degradation in aerobic soil were performed by applying14C-glufosinate-ammonium to ten European soils at 20°C and incubation for 120 days in maximum. One soil was incubated at 10°C and a test moisture of 40% MWHC after application of 3,4-14C-labeled glufosinate-ammonium. Information was amended by tests performed after application of metabolites14C-AE F061517 (MPP),14C-AE F064619 (MPA) and14C-AE F065594 (PPO) separately dosed to aerobic soil at 20°C and in case of MPP at 10°C as well and incubation for 120, 122 and 7 days in maximum, respectively.

 

The degradation of glufosinate-ammonium was found to proceed predominantlyviabiotic degradation processes. A similar metabolic pattern was observed in all soils: multiple degradation steps (see below for metabolite PPO) resulted in the formation of 3-methylphosphinico-propionic acid (MPP, AE F061517) as the most prominent metabolite in the pathway. In a next step, the formal but multistep loss of a methylene group in the aliphatic chain by oxidative processes resulted in the formation of 2-methylphosphinico-acetic acid (MPA, AE F064619). Metabolites AE F061517 (MPP) and AE F064619 (MPA) were observed at maximum values of 46.9% AR and 26.4%, respectively. Metabolite 4-methylphosphinico-2-oxo-butanoic acid (PPO, AE F065594) was detected as a precursor of AE F061517 (MPP) indicating that oxidative deamination were the initial steps of degradation of the active substance in soil. The transient character of PPO was indicated by a maximum occurrence of 6.2% by day 1 and thus at levels significantly below 10% AR.

 

The degradation of glufosinate-ammonium and its metabolites was accompanied by the formation of non-extractable residues (NER) and mineralization to14CO2. Dependent of position of radiolabel applied the level of NER under aerobic conditions reached 6.1 to 9.7% AR after 28 days (1-14C-label) or 20.5 to 29.5% AR after 120 days of incubation (3,4-14C-label). The level of mineralization ranged from 50.5 to 66.8% AR after 28 days (1-14C-label) or from 19.7 to 62.3% after 120 days (3,4-14C-label). The formation of NER and mineralization underlined the biotic nature of degradation of glufosinate-ammonium residues in soil.

 

Rate of degradation:

The degradation data have been kinetically evaluated on the basis of a common metabolic pathway to derive a compartmental model for use in the evaluation (Preuss & Mikolasch, 2014; M-521412-02-1). The evaluation followed FOCUS Guidance on kinetic evaluation (FOCUS, 2011). The evaluation results in half-lives for the degradation of glufosinate-ammonium and its metabolites MPP (AE F061517), MPA (AE F064619) and PPO (AE F065594) in aerobic soil. The resulting degradation half-lives were normalised to reference conditions of 20 °C and 100% field capacity.Geometric mean normalized (20°C, pF2) half-lives for glufosinate-ammonium are summarized in table 11.The geometric mean of normalised half-lives was 8.8 days for metabolite AE F061517 (MPP), 12.2 days for AE F064619 (MPA) and 0.4 days for metabolite AE F065594 (PPO).

 

Table 11: Normalised laboratory DT50-values for glufosinate-ammonium in aerobic soil in the laboratory

Compound

Glufosinate-ammonium

[days]

Normalised (20°C, pF2) DT50 , range

3.2 - 14.8

Geometric mean

6.5

 

According to Echa guidance R.16, equation R.16 -9, t

he DT50 of 6.5 day at 20°C resulted in a half-life of 12.33 d or a degradation rate of 0.056 d-1 at 12°C.

The degradation of glufosinate-ammonium was evaluated in addition under field conditions for three locations in the US in total. Following repeated spraying of formulated glufosinate-ammonium onto bare soil samples were analyzed for residues of glufosinate-ammonium, AE F061517 (MPP) and AE F064619 (MPA) for 574 days in maximum.

The evaluation of the field data according to FOCUS kinetic guidance revealed best fit, non-normalized half-lives ranging from 4.5 to 14.1 days for glufosinate-ammonium.

For comparison with laboratory degradation data the kinetic evaluation of field data according to FOCUS kinetic guidance (Preuss, T.;Herrmann, M.;Hoerold, C.;2015; M-521416-01) revealed normalized (20 °C, pF2) geometric mean half-lives of 7.1 days for glufosinate-ammonium, 13.4 days for AE F061517 (MPP) and 19.0 days for AE F064619 (MPA).

 

Overall conclusion degradation in soil:

The degradation of glufosinate-ammonium was studied in ten soils covering a wide range of soil types from loamy sand to clay loam soil. Standard conditions (20 °C, 40% relative soil moisture) as well as lower temperature and higher soil moisture (60 %) were used.

In summary, glufosinate-ammonium mineralises rapidly under standard aerobic laboratory conditions in soil with the formation of considerable amounts of CO2. The degradation of glufosinate-ammonium is rapid even at low temperature. The main soil metabolites PPO (AE F065594), MPP (AE F061517) and MPA (AE F064619) have also been shown to rapidly degrade in soil under laboratory and field conditions. Field studies are not included in IUCLID since the data from laboratory and field are not greatly different and thus, field data provide no new or further information. Furthermore, the field studies were conducted with an formulation product and a product is not relevant for the proposes of REACH registration.