Slow-release nitrogen fertilizers are used to increase nitrogen use efficiency (NUE) and extend N availability over a growing plant season. Benefits from using these N fertilizers include increasing NUE by plants, reducing N losses, prolonging N availability and reducing potential risk of N pollution to water resources. Formulations of these fertilizers are available in the market. They range from slightly soluble, low solubility, high molecular complexity, to organic-based condensed materials such as methyleneureas (MU) and triazone. After applied to the soil N release is subject to biodegradation (mainly bacteria). Bacterial degradation permits N release from MU as ammonia and urea (using the enzyme MUase). Once urea is released, it is hydrolyzed in the presence of the enzyme urease to ammonia/ammonium. Because microbial activity controls the rate of bioavailable N in the soil, an efficient N-release is dependent on soil humidity, temperature, chemistry, and the ecological status of the soil. Only recently more specific information on the organisms capable of hydrolyzing MU has been published. Jahns et. al, isolated and identified two MU-degrading organisms in German soils: Ochrobactrum anthropi and Ralstonia paucula. Recently a plant pathogenic strain of Rhizobium radiobacter was isolated and related to MU bacteria presence. These results might have a practical significance, demonstrating the potential to select for this plant pathogen in soils fertilized with MU. It is still do not know the long term effects of the application of MU's in the soil microbial population and its effects on soil quality. Preliminary data in tomato plants have suggested that MU caused additional effects than those attributed to an NUE increase. Tomato plants treated with a single application of MU outperformed those treated with weekly N applications in a Georgia soil. The objective of this study is to asses the soil degradation processes involved between MU, bacteria, soil and tomato plants and its consequences in soil quality. A field experiment with two different soil management histories (manure and no manure) was tested. Tomato (Lycopersicon esculentum) was selected as indicator crop. In both soils was considered the effect of urea and MU on the microbial population. Soil samples were taken before and after an equivalent commercial N application of the fertilizers was made and was getermined: 1) Differences in in the microbiological population diversity and the isolation of the bacteria responsible of the MU degradation and a phenotypic characterization; 2) Soil Chemical and fertility analysis; N-forms concentration, pH., electrical conductivity, and presence of other nutrients; 3) the genetic sequence of one of the codifying genes for MUase; 4) Plant growth parameters, N plant uptake. The experimental unit was a 15 Kg soil-pot planted with tomato. Initially growing under field conditions and later transported to greenhouse conditions. The treatments consist of urea and triazone applications, replicated four times in a randomized block design. Results indicated a direct effect between N source and type of microbial population present.