Biodegradation-transport interactions of pulse applied 2,4-D in repacked soil column microcosms

To predict leaching to groundwater, reliable data are needed regarding the in situ biodegradation of pesticides. Repacked soil column microcosms were used to examine the transport and biodegradation of 2,4- dichlorophenoxyacetic acid (2,4-D) in a noncontaminated soil profile. For columns spiked with 2,4-D (2.4 mg) and leached with 1.81 cm d-1, maximum leachate concentrations were 4.65 mg L-1. After 30 d, 41.0% was recovered in the leachate and <0.04% by soil extraction. 2,4-D biodegradation and the distribution and abundance of degrading microorganisms was determined by the leaching rate and the number of herbicide applications. For example, biodegradation of repeat 2,4-D applications was enhanced: the first and second applications were degraded by 44.5 and 72.6%, respectively. This corresponded to a >103-fold increase in numbers of 2,4-D-degraders at all depths following the first application. A twofold reduction in the irrigation rate also increased biodegradation from 44.5 to 77.2%. Columns of different lengths were used to quantify in situ biodegradation for discrete soil profile sections (10 cm). Subsoil biodegradation was higher (43.0%) in columns than it was in batch ex situ experiments (22.3%) after 30 d. The detection of 2,4-D-degraders in column subsoil and leachate suggests that downward transport of degrading microorganisms through the soil profile occurs, and could account for discrepancy between batch and column data