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Soil Matrix and Macropore Biodegradation of 2,4-D

ManTech Environmental Res. Serv. Corp., Ada, OK,

T. S. Steenhuis^*

Dep. of Agric. and Biological Eng., Cornell Univ., Ithaca, NY 14853.

^* Corresponding author (tssl{at}cornell.edu).

ABSTRACT

Preferential flow of pesticides in macropores can lead to decreased travel times through the vadose zone and increased groundwater contamination. Macropores, however, may present a favorable environment for biodegradation because of greater oxygen, nutrient, and substrate supply, and higher microbial populations in earthworm burrows, compared to the soil matrix. The biodegradation of 2,4-dichlorophenoxyacetic acid (2,4-D) was measured in macropores and soil matrix of packed soil columns (7.0-cm diam., 10-cm length) and undisturbed soil cores (10-cm diam., 7.3 or 8.0-cm length). Each packed column contained a well-defined artificial macropore and the undisturbed cores contained earthworm-burrow macropores. A 50 µg/L 2,4-D solution was continuously applied to the unsaturated soil surface and breakthrough curves (BTCs) indicating pesticide loss in the effluent were obtained from the soil matrix and macropore flow paths. Biodegradation rates were calculated separately for each flow path by comparing the BTCs to BTCs representing abiotic conditions, and dividing the 2,4-D loss by the travel time through each flow path. The biodegradation rates increased with time in both flow paths, and the final biodegradation rate in the macropore region surpassed that of the matrix, presumably because of increased microbial populations in the macropore. Complete loss of the 2,4-D in both flow paths was observed after continuous application of 2,4-D for 400 h, with maximum column-averaged 2,4-D loss rates of 0.879 µg/(L h) in the matrix and 1.073 µg/(L h) in the macropore. Biodegradation of 2,4-D was also observed in the macropore and matrix regions of the undisturbed soil cores.

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