1. ECOSER PROJECT: TOWARD A BETTER UNDERSTANDING OF THE MECHANISMS OF CARBON STABILIZATION IN AGRICULTURAL SOILS AMENDED WITH MUNICIPAL ORGANIC WASTES OBJECTIVES The main objective of the ECOSER project is to develop a better understanding of the processes controlling the transformation and stabilization of organic matter in agricultural soils amended with municipal organic wastes, and to evaluate the agricultural and environmental implications related to plant nutrient supply and C sequestration. C. Plaza, R. Sanz, P. J. Hernáiz, J. C. García-Gil, P. A. Soler-Rovira, J. M. Fernández, D. Hernández, I. Haller, and A. Polo METHODOLOGY In October, 2008, we started a field plot experiment at “La Poveda” experimental farm (Center of Environmental Sciences, Spanish Research Council, Spanish acronym CCMA-CSIC) located in Arganda del Rey (Madrid, Spain). The experiment was set up as a randomized complete block design with four replicates in a split plot treatment arrangement. The main plot treatments are (1) an unamended control, (2) a mineral fertilization, and (3) municipal solid waste compost, (4) composted sewage sludge, and (5) thermally-dried sewage sludge applied at a rate of 30 t ha -1. The subplot treatments are (1) only one application of organic amendment made at the beginning of the experiment and (2) annually consecutive additions. Physical fractionation techniques based on density and size characteristics to divide organic matter according to location in soil (organic particles material freely located between and within soil macroaggregates, fine organic particles protected within stable microaggregates, and silt- and clay- associated organic matter) will be combined with advanced physicochemical, chemical, isotopic, microbiological, and modeling approaches. ACKNOWLEDGMENTS This project is being funded by the Directorate-General for Universities and Research of the Autonomous Community of Madrid (DGUI-CAM) and the Spanish Research Council (CSIC) through the program “Grants for Creation and Consolidation of CSIC Research Groups in the CAM” (CCG08-CSIC/AMB-3439). Instituto de Ciencias Agrarias, Centro de Ciencias Medioambientales, Consejo Superior de Investigaciones Científicas, Serrano 115 dpdo., 28006 Madrid, Spain STATE-OF-THE-ART Land application of municipal organic wastes, especially to organically-depleted agricultural soils, is believed to be an efficient practice for improving soil quality and crop production, while reducing the amount of waste to be disposed of in landfills or burnt in incineration plants, enhancing C sequestration, and thus helping head-off the global warming. According to current concepts, the biogeochemical pathways responsible for exogenous organic matter stabilization in soil include (a) selective preservation and formation of recalcitrant organic compounds (biochemical protection), (b) occlusion within soil aggregates (physical protection), and (c) association with mineral surfaces (chemical protection). To date, however, how these protective mechanisms act and interact in soils amended with municipal organic wastes remains largely unknown. THERMALLY-DRIED SEWAGE SLUDGE (TS) COMPOSTED SEWAGE SLUDGE (CS) MUNICIPAL SOLID WASTE COMPOST (MC)