Assessing changes in physical and biological properties in a soil contaminated by oil sludges under semiarid Mediterranean conditions F. Caravaca * , A. Rolda ´n CSIC-Centro de Edafologı ´a y Biologı ´a Aplicada del Segura, Department of Soil and Water Conservation, P.O. Box 4195, Campus de Espinardo 30100-Murcia, Spain Received 16 July 2002; accepted 6 March 2003 Abstract We have studied the changes in biological and physical properties of a soil, in a semiarid area, contaminated by hydrocarbons in order to predict the potential for bioremediation of this soil. The microbial biomass C, the basal respiration and the metabolic quotient ( qCO 2 ) of contaminated soil were significantly higher than that of the control soil, which points to a markedly reduced efficiency of substrate use and to a possible toxic effect of hydrocarbons in the soil. The low global rate of mineralisation in the contaminated soil also indicates the presence of hydrocarbons resistant to biodegradation. The dehydrogenase activity and the activities of hydrolases involved in the N and P cycles (urease, protease and phosphatase) were stimulated by the contamination with hydrocarbons. Soil total porosity was increased by the contamination, about 15-fold in comparison with the control soil. The contamination by hydrocarbons led to an increase in cracks in the 100 – 200 Am size classes, which can be regarded as a reservoir to hold water for plants and microorganisms. The biodegradation of hydrocarbons in this soil would require long periods of time, and it would be advisable to apply methods of bioremediation to this contaminated soil. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Enzyme activities; Soil microbiological biomass; Metabolic quotient; Porosity; Shrinkage; Hydrocarbon 1. Introduction Hydrocarbons are common contaminants found in soil and groundwater as a result of past and current industrial activity. The greatest concern regarding the contamination by hydrocarbons lies in the mutagenic, carcinogenic and toxic characteristics of such contam- inants. The extent of environmental contamination depends on the chemical composition and concentra- tion of the contaminant and the properties of the soil (Fine et al., 1997). In general, the presence of high molecular weight compounds characterised by very low solubility in water hinders the natural biodegra- dation for soils contaminated by hydrocarbons. For soils from semiarid Mediterranean regions of Spain, their low organic matter contents can directly affect sorption of hydrocarbons by the soil and indirectly affect biodegradation of hydrocarbons (Mohn and Stewart, 2000). 0016-7061/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0016-7061(03)00118-6 * Corresponding author. Tel.: +34-968-396337; fax: +34-968- 396213. E-mail address: fcb@cebas.csic.es (F. Caravaca). www.elsevier.com/locate/geoderma Geoderma 117 (2003) 53 – 61