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 (CO2)-C-14 from C-14.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 rm) 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 add) 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 nut all the subsamples had competent mircoorganisms present
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