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Biodegradation of 2,4-D in a Noncontaminated Grassland Soil Profile

Research School of Biosciences, Univ. of Kent, Canterbury CT2 7NJ, UK.

^* Corresponding author (r.g.burns{at}ukc.ac.uk).

ABSTRACT

Prediction of groundwater contamination and the development of rational bioremediation strategies require a greater understanding of xenobiotic biodegradation at the soil profile scale. We studied 2,4-dichlorophenoxyacetic acid (2,4-D) biodegradation for up to 83 d in soil samples collected in 10-cm depth increments from a noncontaminated soil profile (0–50 cm), and examined factors that influenced biodegradation in the subsoil (40–50 cm). For the four samples collected from 0 to 40 cm, total evolution of ¹⁴CO₂ from ¹⁴C-ring-UL-2,4-D (20 mg kg^–1, 0.25 MBq kg^–1) was 50.02 to 60.46% (maximum rate = 4.84% d^–1 at 10–20 cm) at the end of the mineralization phase. Subsoil (40–50 cm) mineralization was either 5.47% (maximum rate = 0.113% d^–1) or 56.1% (maximum rate = 2.67% d^–1) after 83 d. Enhanced mineralization of successive 2,4-D applications was recorded in all topsoil (0–10 cm) samples and in the subsoil replicates that initially mineralized > 50%. Measurements of 2,4-D disappearance and most probable number analysis corresponded well with the biodegradation data. Inoculation of subsoil with topsoil (10%) increased 2,4-D mineralization in the 5% soil to > 50%, whereas amendment with N, P, K, and/or organic C sources (acetate, catechol, vanillin, 3,5-dihydroxybenzoic acid) did not. We conclude that the subsoil contained a low number of unevenly distributed microorganisms capable of using 2,4-D as a source of C and energy such that not all the subsamples had competent microorganisms present.

L.J. Shaw, current address: Institute of Terrestrial Ecology, Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire PE17 2LS, UK.

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