Microbial aspects of the interaction between soil depth and biodegradation of the herbicide isoproturon Gary D. Bending * , M. Sonia Rodriguez-Cruz 1 Warwick HRI, University of Warwick, Wellesbourne, Warwick CV35 9EF, UK Received 22 June 2006; received in revised form 28 July 2006; accepted 28 July 2006 Available online 22 September 2006 Abstract Factors controlling change in biodegradation rate of the pesticide isoproturon with soil depth were investigated in a field with sandy- loam soil. Soil was sampled at five depths between 0–10 and 70–80 cm. Degradation rate declined progressively down the soil profile, with degradation slower, and relative differences in degradation rate between soil depths greater, in intact cores relative to sieved soil. Neither the maximum rate of degradation, or sorption, changed with soil depth, indicating that there was no variation in bioavailability. Differences in degradation rate between soil depths were not associated with the starting population size of catabolic organisms or the number of catabolic organisms proliferating following 100% degradation. Decreasing degradation rates with soil depth were associated with an increase in the length of the lag phase prior to exponential degradation, suggesting the time required for adaptation within com- munities controlled degradation rates. 16S rRNA PCR denaturing gradient gel electrophoresis showed that degradation in sub-soil between 40–50 and 70–80 cm depths was associated with proliferation of the same strains of Sphingomonas spp. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Pesticide; Leaching; Biodegradation; Bioavailability; Sub-soil; Sphingomonas spp. 1. Introduction The major environmental concern arising from pesticide use is the capacity of pesticides to leach from soil and con- taminate water resources (Kookana et al., 1998). The amount of pesticides leaching through soil reflects the inter- action of degradation and sorption processes in both top- soil and sub-soil (Fomsgaard, 1995). Many pesticides are degraded by cometabolism in which degradation follows first order kinetics, with the organisms responsible appar- ently showing no capacity to proliferate following degrada- tion of the compound. Other pesticides are degraded by growth-linked metabolism, in which organisms responsible for biodegradation have adapted to use the pesticide as an energy and nutrient source, resulting in cell proliferation and an increase in degradation rate over time (Aislabie and Lloyd-Jones, 1995). Degradation rates of pesticides are usually assumed to decrease down the soil profile (Fomsgaard, 1995). However, in some instances degradation rates of pesticides susceptible to both cometabolic and growth-linked degradation can be greater in sub-soil than in top-soil. The precise relationship between top- and sub-soil degradation rate can vary between different compounds at single sites, and at different sites for individual compounds (Di et al., 1998; Karpouzas et al., 2001; Mills et al., 2001). The reasons for contrasting patterns of degradation rates in sub- and top-soil are unclear. A number of counteracting biotic and abiotic factors could be important for determining differences in pesticide degradation rate through the soil profile. The size of the microbial community, which decreases with soil depth (Fomsgaard, 1995) could determine both the availability of suitable microbial strains and genetic elements for adap- tation to enable pesticide degradation, the survival of 0045-6535/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2006.07.099 * Corresponding author. Tel./fax: +44 24 76575057. E-mail address: gary.bending@warwick.ac.uk (G.D. Bending). 1 Present address: Institute of Natural Resources and Agrobiology (CSIC), Department of Environmental Chemistry and Geochemistry, Salamanca 37008, Spain. www.elsevier.com/locate/chemosphere Chemosphere 66 (2007) 664–671