Nitrapyrin

Toxicity to soil microorganisms

Description of key information

The degradation product inhibited the conversion of ammonium to nitrite in soil at 1000 ppm but not at 100 ppm, Goring et al. (1967).

Key value for chemical safety assessment

Short-term EC50 for soil microorganisms:

3.1 mg/kg soil dw

Additional information

In the key study, as reported by Roberts et al. (2003), a 28 day N transformation test was developed according to the OECD guideline 216. In the laboratory-based test, a suitable soil was amended with powdered plant meal as an organic N source. Soil samples of 1 kg were treated with five concentrations of test material, in the range 1.0–100 mg/kg dry weight and incubated for 28 days at 20 ± 2 °C. A dose response was produced and the N mineralisation EC50 (95% C.L.) for the test material was determined to be 3.1 (1.9–4.3) mg/kg dry soil.

A further study is available on the substance, and was reported by Roberts et al. (2010), the findings from the study are regarded as supporting information.

The aim of the study was to identify the moisture content range, within which the measured concentration of nitrate-N was sufficiently above background levels in soil. This moisture content range, which will be different for soils of different texture types, focused on a sandy loam soil fulfilling the criteria of the OECD 216 guideline (OECD, 2000). The nitrate-N yield and the EC50 (the concentration causing 50% inhibition of nitrogen transformation) of the test material, were determined at varying soil moisture contents. The results indicate that the current OECD recommended upper limit for the soil moisture content (60% MHC) may be too high. The yield of nitrate-N and the EC50 for the test material, were relatively constant between 20 and 40% MHC

In addition to the data that are available on the substance itself, two reports are available outlining the effect of the substance, its degradation product, or a formulation containing the substance at around 24 %, on soil microorganisms.

 

In the first report, Goring et al (1967) presented data to demonstrate the selective nature of the test material in soil. Since the chemical is converted to its degradation product, data on the toxicity of this compound to microorganisms and biological soil processes was also presented.

Under the conditions of the study the degradation product was found to be inactive against 43 of the 48 organisms evaluated at 1000 ppm in the medium. With 5 of the organisms it was inactive at 100 ppm but gave varying degrees of activity from partial to complete inhibition at 1000 ppm. The test material was found to be inactive on 4 organisms at a concentration of 256 ppm in the medium. The degradation product was essentially inactive on the total fungal and microbial soil populations at 1000 ppm in the soil, and at this concentration did not significantly inhibit carbon dioxide production by the soil. The test material was inactive on the soil microorganisms at 100 ppm and had no effect on carbon dioxide production at 95 ppm. However, significant effects were obtained in both instances at concentrations of 1000 and 950 ppm, respectively. The degradation product inhibited the conversion of ammonium to nitrite in soil at 1000 ppm but not at 100 ppm.

 

In the second report, O’Melia (1966) evaluated the effect of the substance, its formulation inerts, and the degradation product, on the general soil fungi and bacterial populations. Overall, it was concluded that:

- The test material appears to have no effect on general soil fungi at concentrations of 40 ppm and below, but tends to decrease numbers of colonies at 400 ppm and above. The data would indicate that the diluents themselves are primarily responsible for this reaction.

- The degradation product of the substance has no effect on the total fungi colonies up to and including 1,000 ppm.

- The test material and the diluents alone increase the soil bacteria population when concentrations of 400 ppm and above of the materials are used. Lower concentrations have only a small or negligible effect.

- The degradation product of the substance has little effect on the general soil bacterial at 100 ppm and below but tends to increase the number of colonies at 1,000 ppm.

- Rates of substance (as test material) normally used with fertilisers would not be expected to have any significant effect on the total soil population.

Further supporting information is available in the form of three publications. The studies reported in all three publications were conducted to sound scientific principles and were therefore assigned a reliability score of 2 in line with the criteria of Klimisch et al. (1997).

The first publication was reported by Facherl & Amberger (1990).

Facherl & Amberger found that the nitrification inhibitor dicyandiamide [DCD] did not inhibit growth and respiration of N-fixing bacteria (Rhizobium leguminosarum and Azotobacter chroococcum) in cell suspensions with concentrations of 400 ppm DCD. Growth of Rhizobium leguminosarum was inhibited by 17 % with 100 ppm test material but respiration was not affected. Growth ofAzotobacter chroococcumwas inhibited by 10 ppm (10 %) and 100ppm test material (50 %); in the latter case, respiration was also impaired (36 %). Thiourea only caused a minor growth inhibition of Azotobacter chroococcum with 100 ppm (8 %) and had no effect on Rhizobium leguminosarum.

 

In the second publication, reported by Belser & Schmidt (1981) five strains of Nitrosomonas and one each of Nitrosospira and Nitrosolobus were examined for sensitivity to the test material nitrification inhibitor. Considerable variation in sensitivity was observed, with some strains about five times more resistant than others. Sensitivity to test material varied more with strain than with genus.

 

In the third publication, reported by Goring (1961) the effect of the test material in controlling soil fungi and bacteria was determined by injecting acetone compositions into 50 g samples of moist soil in 4 ounce ointment jars. The jars were immediately capped, and the contents were thoroughly mixed by tumbling for 30 minutes. The jars were incubated for 3 days at 70 °F. Samples of the soil were then diluted in water and plated on rose bengal-streptomycin agar for fungi, and peptone-dextrose agar for bacteria. Counts of fungi and bacteria in the treated soil samples were compared with those in appropriate check samples. Under the conditions of the study concentrations of test material in the soil up to at least 1200 ppm gave no appreciable control of the general soil fungi and bacterial populations.