Calcium hydrogen phosphonate

Toxicity to microorganisms

Link to relevant study record(s)

Reference

Endpoint:

activated sludge respiration inhibition testing

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Study on micro-organisms in soil. However, there is no reason why the sensitivity of soil and aquatic micro-organisms should be considered different

Principles of method if other than guideline:

Field study measuring soil microbial enzyme activities and soil mesofauna feeding activity

GLP compliance:

no

Details on sampling:

Monthly samples were taken in September and October 2994 and from April 2005 to November 2005.
Measurement of microbial enzyme activities and faunal feeding activity was performed in samples taken from the soil upper layer (3-15 cm).

Details on test solutions:

In September 2004, an aqueous solution of KH2PO3 (7.5 g P per m2) with mud flat soil (42% DM, 2.5 kg per m2) were mixed to a volume of 4 L per m2. The finished dispersion was spread evenly on the soil by a watering can an afterwards superficially rake in.
Control plots received only 4 L tap water per m2.
In April 2005, the application of 7.5 g P per m2 was repeated.

Test organisms (species):

other: Mud flat soil from the North Sea shoreline near Husum, Germany

Test type:

static

Test temperature:

ambient temperature throughout the year

Nominal and measured concentrations:

Dosing at 7.5 g phosphite per m2 resulted in an initial concentration of 841 mg/kg dw in the first soil samples. After the second application, measurements yielded maximum circa 60 mg phosphite per kg dw.

Details on test conditions:

The existing grass sward was removed before the first soil treatment. Afterwards a new grass cover (Lolium perenne) was established.

Key result

Duration:

420 d

Dose descriptor:

other: no significant inhibition

Effect conc.:

7.5 other: g/m²

Nominal / measured:

meas. (not specified)

Conc. based on:

test mat.

Basis for effect:

other: enzyme activities

Remarks on result:

other: Initial concentration peaked at 840 mg/kg soil, after repeated application the maximum reached 60 mg/kg dw soil.

Details on results:

Phosphite concentrations increased in the phosphiet treated plots and after the first phosphite application, the phosphate concentrations increased, after a lag phase. After the second phosphite treatment in April 2005 no delay was observed, oxidation of phosphite restarted immediately. This could be a sign of a well-adapted microbial community.

The microbial enzyme activity of dehydrogenase and alkaline phosphatase were significantly increased in the phosphite treated soils, by 21 and 28%, respectively.

The cellulase activity was decreased by 16% (not significant), while the protease activity remained contstant.

Validity criteria fulfilled:

not applicable

Conclusions:

Phosphite applied in a mud flat soil suspension to soil was oxidised to phosphate after a lag phase. No significant correlations were found between microbial enzyme activitues and soil phosphite content. It was concluded that phosphite did not harm the biocoenosis of micro-organisms. This study demonstrates the insensitivity of a generic microbial population to phosphonic acid. The organic material originated from an aquatic environment (mud flat soil) and thus the study can be used to fulfil the current data requirement.

Executive summary:

The effect of phosphite on the microbial biocoenosis of an arable soil was investigated in a field trial. The response of soil micro-organisms was studied. After application of phoshite in the form of KH2PO3, combined with nud-flat soil, to the upper layer of a grass grown plot, an adaptation of the metabolic acitvity of the active soil biocoenosis caused at least a partly oxidation of phosphite to phosphate. Initial concentrations reached up to 840 mg/kg soil dw, whereas after repeated dosing the concentration reached 60 mg/kg soil dw as apparently the microflora was capable to tranform phosphite into phosphates.

Microbial enzyme activities demonstrated no significant correlation to soil contents of phosphite and phosphate with exception of dehydrogenase (correlated to phosphate) which was increased. Thus no toxic effects were observed. This study demonstrates the insensitivity of a generic microbial population to phosphonic acid. The organic material originated from an aquatic environment (mud flat soil) and thus the study can be used to fulfil the current data requirement.

Description of key information

Potassium phosphite (KH2PO3) caused no enzyme inhibition on the microflora from arable soil/mud flat soil (North Seashore) mixture. Dose rates were > 840 and > 60 mg/kg soil dw. No toxicity to microorganisms is expected.