TECHNICAL REPORTS 683 Mineralization of organic chemicals in soil is typically studied using large homogenized samples, but little is known about the small-scale spatial distribution of mineralization potential. We studied centimeter-scale spatial distribution of 2-methyl-4- chlorophenoxyacetic acid (MCPA) mineralization activity at diferent depths (8–115 cm) in a Danish agricultural soil profle using a 96-well microplate 14 C-radiorespirometric method for small-volume samples. Te heterotrophic microbial population and specifc MCPA degraders decreased 10- to 100-fold from the plow layer to a depth of 115 cm. MCPA was mineralized in all samples in the plow layer, but only about 60% in the transition zone immediately below the plow layer showed mineralization; at greater depth even fewer samples showed mineralization. A patchy spatial distribution of mineralization activity was observed from right below the plow layer and in the subsoil, with a few clearly defned active zones surrounded by areas devoid of mineralization activity. Due to the patchy distribution of mineralization activity at the centimeter scale just beneath the plow layer, MCPA and presumably other weakly sorbing pesticides might be at risk of leaching to the groundwater if transported from the plow layer into the subsoil. Centimeter-Scale Spatial Variability in 2-Methyl-4-Chlorophenoxyacetic Acid Mineralization Increases with Depth in Agricultural Soil Nora Badawi, Anders R. Johnsen, Jan Sørensen, and Jens Aamand* T he fate and degradation of the pesticides used to control weeds in crops have been widely studied, with degradation and transport of pesticides in the vadose zone having attracted particular attention. Many pesticides are easily degraded in the plow layer, but the rate and total extent of mineralization decrease sharply if they are transported below the plow layer into the subsoil (Di et al., 1998). Degradation of pesticides in the subsoil may be slow or in some cases nonexis- tent, with the consequent risk that they may leach to underly- ing groundwater aquifers (Larsen et al., 2000; Kristensen et al., 2001; Sørensen et al., 2006). Leaching of pesticides is of great concern in Europe, where many countries exploit groundwater for their drinking water supply. In Denmark, more than 99% of the drinking water comes from groundwater (GEUS, 2011). Mineralization of pesticides is usually measured using soil microcosms consisting of relatively large homogenized soil samples, neglecting small-scale spatial heterogeneity. Te homogenization process per se destroys soil structure and microhabitats, including the spatial distribution of mineralization potential (Tomson et al., 2010). Several studies have examined the spatial distribution of pesticide mineralization potential, ofen in relation to soil physicochemical and microbial parameters, at levels ranging from the millimeter to the feld scale in agricultural soil (Grundmann and Debouzie, 2000; Nunan et al., 2002; Gonod et al., 2006; El Sebai et al., 2007; Fredslund et al., 2008). At the feld scale, the distribution of bacterial number and activity may be homogeneous, whereas marked spatial heterogeneity may occur at the centimeter or millimeter scales, as in the study by Gonod et al. (2006), who demonstrated increasing spatial variability of 2,4-D mineralization when down- scaling from the meter to the millimeter scale. Grundmann and Debouzie (2000) showed that the presence of NO 2 − and NH 4 + oxidizers was spatially structured at the millimeter scale. Consistent with this, the general microbial biomass in forest soil has been shown to be spatially structured on the centimeter scale (Morris, 1999; Morris and Boerner, 1999). Few studies have been performed on pesticide degradation at the small scale in which the observed heterogeneity has Abbreviations: CFU, colony-forming units; LP 10 , the time period (lag phase) needed to achieve 10% 2-methyl-4-chlorophenoxyacetic acid mineralization; MCPA, 2-methyl-4-chlorophenoxyacetic acid; PCR, polymerase chain reaction. N. Badawi, A.R. Johnsen, and J. Aamand, Dep. of Geochemistry, Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark; N. Badawi, and J. Sørensen, Dep. of Plant and Environmental Sciences, Section for Genetics and Microbiology, Univ. of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark. Assigned to Associate Editor Garey Fox. Copyright © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. J. Environ. Qual. 42:683–689 (2013) doi:10.2134/jeq2012.0397 Supplemental data fle is available online for this article. Received 15 Oct. 2012. *Corresponding author (jeaa@geus.dk). Journal of Environmental Quality BIODEGRADATION AND BIOREMEDIATION TECHNICAL REPORTS Published April 2, 2013