Nitrapyrin

Biodegradation in soil

Description of key information

DT50 = 5 - 42 days (6 soils at 25°C and 75% 1/3 bar soil moisture), Unger et al. (1981)

DT50 (California (loam) soil at 25 °C ) = 2 days; DT50 (Mississippi (loam) 35 °C) = 1.5 days,  Regoli et al. (1976)

DT50 (anaerobic loam, silty clay loam)~ 3 hours, Oliver et al (1982)

Key value for chemical safety assessment

Half-life in soil:

34.78 d

at the temperature of:

20 °C

Additional information

Available Data

Two key studies, and one supporting study, are available investigating the degradation of the test material in soil. All three studies were conducted to sound scientific principles and were assigned a reliability score of 2.

In the first key study, reported by Regoli et al. (1976) the aerobic degradation of ¹⁴C-radiolabelled test material was studied in two loam soils (Davis and Wayside soils) at an application rate of 1.0 ppm to the Davis soil incubated at 25 °C for 195 days and 0.5 ppm to the Wayside soil incubated at 35 °C for 161 days.

The test systems consisted of individual incubation flasks, each containing 50g treated soil, maintained in the dark at a moisture content of 100% of 1/3 bar at 25 °C and 35 °C. A system with an oxygen manifold was used in order to maintain aerobic conditions and ¹⁴CO₂ was trapped in NaOH. Samples were taken at day 0 and after 3, 7, 14, 28, 56, 100 and 195 days of incubation for the Davis soil and after 2, 4, 10, 18, 39, 80 and 161 days for the Wayside soil.

The soil samples were sequentially extracted with acidified ether and hot NaOH and radioactivity in the extracts was measured by LSC. Thin layer chromatography was used to determine the distribution of the radioactivity (silica, ethyl acetate:methanol:acetic acid (8:1:1 vol)). It should be noted that TLC did not separate 6-CPA from DCMP, as discussed in the 1981 study. Unextractable soil residues were combusted and radioactivity quantified by LSC. Trapped radioactivity was confirmed as¹⁴CO₂ by precipitation with BaCl₂.

Total recovery ranged from 97.4% AR to 101.6% AR. The levels of test material continuously decreased over the incubation period, from approximately 99.7 – 99.0% AR at day 0 to 0.6 % and 6.5 % AR at the end of the study in the Davis and Wayside soil, respectively. Mineralisation was observed with evolved ¹⁴CO₂ accounting for maximums of 42.1% AR (Davis 25 °C, day 195) to 70.8 % AR (Wayside 35 °C, day 161). The degradation product 6-chloropicolinic acid decreased from a maximum of 67.2 % AR (Davis soil, day 56) and 37.6% AR (Wayside soil, day 18) to 50 % AR and 13.8 % AR in the Davis and Wayside soil, respectively, at the end of the study. The DT₅₀ for the test material was estimated to be 17 and 6 days, at 25 °C and 35 °C, respectively and the DT₉₀ was 66 and 88 days, at 25 °C and 35 °C. Remaining radioactivity in the NaOH and the unextractable radioactivity in soil were concluded to be humic acid. It was concluded that degradation occurred via cleavage of the trichloromethyl group to form 6-chloropicolinic acid, which was then further degraded to CO₂.

In the second key study, reported by Unger et al. (1981), the aerobic degradation of the test material was examined in six soils that had been treated with 14C-test material and incubated at 25°C and 75% 1/3 bar soil moisture. As well as test material, the metabolites 6-chloropicolinic acid (6 -CPA) and 2 -chloro-6 -dichloromethylpyridine (DCMP) were also analysed for by HPLC, which separated 6 -CPA from DCMP.

Under the conditions of the study the parent test material degraded rapidly in six soils with estimated half-lives ranging from 5 to 42 days. The half-lives appeared to increase with an increase in soil organic carbon. The degradation of the test material resulted in the formation of an intermediate metabolite, 2-chloro-6-(dichloromethyl)pyridine and the end product of hydrolysis, 6-chloropicolinic acid. The final breakdown product was carbon dioxide.

In the supporting study, reported by Oliver et al. (1982), the degradation of the test material was examined in soils that had been kept in an anaerobic state for 60 days prior to the start of the experiment. The stability of the test material and the formation of metabolites were also investigated.

Under the conditions of the study, the test material degraded rapidly under anaerobic soil conditions with less than 50% remaining in the soil and water phases after approximately three hours. Test material was completely degraded in the water phase within three days. The degradation of test material resulted in the formation of the intermediate metabolites: DCMP, 2-C-6-CMP, and the end product 6-CPA.

 

Degradation Rate in Topsoil

The laboratory DT50s for the substance were taken from Unger et al (1981) and Regoli et al (1976) and are given in the following Table.

Laboratory DT50s for the substance normalised for temperature and moisture

Study	Soil	DT50 (days)	Temp	T-Corr	Moist. Cont. (%)	Moist.Corr	DT50 normalised to 20°C & pF2
Unger et al (1981)	Commerce	19.69	25˚C	1.57	15.75	0.72	22.36
	Catline	25.87	25˚C	1.57	20.25	0.76	30.83
	Fargo	134.78	25˚C	1.57	32.25	0.76	160.12
	Keith	8.3	25˚C	1.57	18.75	0.76	9.84
	Walla Walla	23.27	25˚C	1.57	15.75	0.70	25.71
	Cecil	27.1	25˚C	1.57	11.25	0.69	29.47
Regoli et al (1976)	Davis	21.97	25˚C	1.57	21.75	0.91	31.28
	Wayside	28.46	35˚C	3.70	17.84	0.79	83.13
	Geometric mean						34.78