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Vadose Zone Processes and Chemical Transport

Spatial Variation in 2-Methyl-4-chlorophenoxyacetic Acid Mineralization and Sorption in a Sandy Soil at Field Level

^a Dep. of Geochemistry, Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen, Denmark
^b Univ. of Aarhus, Faculty of Agricultural Sciences, Inst. of Agroecology and Environment, DK-8830 Tjele, Denmark
^c Biotech Research and Innovation Centre (BRIC), Fruebjergvej 3, DK-2100 Copenhagen, Denmark
^d Dep. of Natural Sciences, Faculty of Life Sciences, Univ. of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg, Denmark

^* Corresponding author (lf{at}geus.dk).

Received for publication May 30, 2006. The phenoxyacetic acid herbicide MCPA (2-methyl-4-chlorophenoxyacetic acid) is frequently detected in groundwater beneath Danish agricultural fields. We investigated spatial variation in microbial MCPA mineralization potential in a flat agricultural field of fine sandy soil (USDA classification: Humic Dystrudept) located on the Yoldia plains of Northern Jutland, Denmark. Samples for determination of MCPA mineralization and sorption were collected from the Ap and Bs horizons at 51 sampling sites located in a 200 x 220 m grid. Spatial variation in sorption was low in both horizons (distribution coefficient, 0.36–4.16 L kg^–1). Sorption correlated strongly with soil organic carbon content in both horizons (CV, 93 and 83%, respectively) and negatively with soil pH. [Ring-¹⁴C]-MCPA mineralized readily in the Ap horizon, with 49 to 62% of the ¹⁴C-MCPA being converted to ¹⁴CO₂ during the 67-d incubation period. With the subsoil, mineralization of ¹⁴C-MCPA varied considerably between samples (0.5–72.8%). At neither depth was there correlation between ¹⁴C-MCPA mineralization and sorption, soil pH, organic carbon content, clay content, number of colony-forming units (CFU), pseudomonad CFU, or any of the four microbial activity parameters measured. The presence of microbial genes encoding for the TfdA enzyme was quantified using real-time polymerase chain reaction. No correlation was found between MCPA mineralization potential and the natural background number of tfdA genes present in the soil samples. The degradation kinetics suggests that the high ¹⁴C-MCPA mineralization rate detected in soil samples was linked to growth of the MCPA-degrading soil microbial community.

Abbreviations: ASA, arylsulfatase activity • CFU, colony-forming units • Ct, cycle threshold • 2,4-D, 2,4-dichlorophenoxyacetic acid • FDA, fluorescein diacetate hydrolysis • ISR, in situ soil respiration • MCP, 2-methyl-4-chorophenol • MCPA, 2-methyl-4-chlorophenoxyacetic acid • C_org, organic carbon • PCR, polymerase chain reaction • SIR, substrate-induced respiration • TSA, tryptic soy broth agar