Spatial variability of glyphosate mineralization and soil microbial characteristics in two Norwegian sandy loam soils as affected by surface topographical features Marianne Stenrød a,b, * , Marie-Paule Charnay c , Pierre Benoit c , Ole Martin Eklo a,b a Department of Herbology, Plant Protection Centre, Norwegian Crop Research Institute, Høgskoleveien 7, N-1432 A ˚ s, Norway b Department of Plant and Environmental Sciences, Norwegian University of Life Sciences, P.O. Box 5003, N-1432 A ˚ s, Norway c Unite ´ Mixte de Recherche Institut National de la Recherche Agronomique et Institut National Agronomique Paris-Grignon (UMR INRA INA-PG), Environnement et Grandes Cultures, F-78850 Thiverval-Grignon, France Received 27 October 2004; received in revised form 15 August 2005; accepted 17 August 2005 Available online 26 September 2005 Abstract The aim of this study was to investigate the possible influence of surface topographical features on the spatial variability of glyphosate degradation and some microbial characteristics in sandy loam soil. Soil samples were taken from the ploughed layer across an agricultural field after seedbed preparation for grain (Grue site), and down to 1 m depth under a ridge tilled field (Ma ˚lselv site), both sites having similar soil textural characteristics (sandy loam soil). Laboratory experiments were performed looking at glyphosate mineralization and soil microbial activity at the Grue site, as well as microbial biomass, activity and substrate utilization patterns at the Ma ˚lselv site. Microbial biomass and activity decreased, and substrate utilization patterns changed with increasing soil depth, reflecting naturally occurring changes in quantity and quality of soil organic carbon. Further, our results show that considerable spatial heterogeneity in the degradation rate of glyphosate and general carbon utilization exists even across small areas within a single agricultural field. This horizontal variability was observed over several spatial scales, and could not be clearly explained. It evidently arose from differences in environmental factors affecting microbial activity and growth, and topographical features controlling redistribution of water and matter flow patterns were correlated to the investigated soil microbial variables. q 2005 Elsevier Ltd. All rights reserved. Keywords: Soil microbial biomass; Biolog EcoPlate; Organic C mineralization; Glyphosate degradation; Surface topography 1. Introduction Soil is a heterogeneous medium due to the variability in its physical, chemical and biological constituents. The physico- chemical and structural characteristics of soil provide many microhabitats in which complex bacterial populations can evolve, and the variability in soil microbial factors is found to be higher than the variability in physical and chemical parameters (Ro ¨ver and Kaiser, 1999). As a prerequisite for studies examining the effects of agricultural practices and other perturbations on soil communities, we need more information on the natural variability within the agricultural system. Spatial variability—both horizontally and vertically within an agricultural field—has been found for different soil microbial properties including microbial biomass size (Fierer et al., 2003a), general activity (Fierer et al., 2003b), community structure (Fierer et al., 2003a), and pesticide degradation activity (Fomsgaard and Kristensen, 1999; Walker et al., 2002). These properties have been shown to be largely governed by the environmental variables temperature (Mikan et al., 2002), soil moisture (Griffiths et al., 2003), and substrate availability (Fierer et al., 2003a), that give rise to temporally variable, but spatially defined, niches within the soil profile. To find operational mean values of a specific soil property for an agricultural field, the methods of soil sampling generally involve bulking and homogenizing of samples. As this might lead to over- or underestimation of the actual field values due to the variability observed within agricultural fields, microsam- pling (Grundmann and Gourbie `re, 1999) and sampling on several scales (Franklin and Mills, 2003) have been proposed. The topic of different scales of spatial variation and the factors driving the variability has been reviewed by Parkin (1993). Surface topography is closely linked to water transport Soil Biology & Biochemistry 38 (2006) 962–971 www.elsevier.com/locate/soilbio 0038-0717/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2005.08.014 * Corresponding author. Address: Department of Herbology, Plant Protection Centre, Norwegian Crop Research Institute, Høgskoleveien 7, N-1432 A ˚ s, Norway. Tel.: C47 64 949 343; fax: C47 64 949 226. E-mail address: marianne.stenrod@planteforsk.no (M. Stenrød).