information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Soil Chemistry S oil quality is a key element sustaining the productivity of agroecosystems, and there is increasing interest in its relationship to other environmental issues such as land protection and soil C sequestration (Lal, 2001). Intensive agricultural systems deteriorate soil quality and fertility by diminishing the levels of soil or-ganic matter (SOM). Th e loss of soil quality is aggravated by the negative eff ects of climate change such as decreased rainfall and enhanced soil erosion, which are especially important in semiarid areas such as the Mediterranean region (Spaccini et al., 2002; Bastida et al., 2007). Under these conditions, where semiarid soils are characterized by low levels of organic matter (OM), exogenous organic C inputs represent an eff ective strategy to improve soil quality and maintain ecosystem sus-tainability. Th e increasing world population is leading to the production of large amounts of organic wastes from urban activities, which could be used as exogenous OM inputs into the soil (Albaladejo et al., 2008; Singh and Agrawal, 2008). Senesi et al. (2007) reviewed the availability of diff erent organic materials generated from urban activities, such as sewage sludge (SS), municipal solid waste (MSW) and other agro-industrial wastes. Th ey highlighted the importance of these wastes as soil organic amendments and as a valuable source of humic material for the soil. Humic substances, mainly composed of humic acid (HA) and fulvic acid (FA), are an essential part of SOM because they are closely related to soil C and