One of the most practical ways to utilise municipal solid waste is composting, thereby producing materials that may be productively used to improve soil properties (Weber et al. 2007). Municipal solid waste (MSW) have variable composition, with the main ingredients being: comestible waste products (40-60 %); paper and cardboard (20 %); and glass, plastic and metal up to 40 % (Costa et al. 1991). During composting, some of these materials are biotransformed into more stable products that are rich in humic substances. According to numerous studies, the quality and quantity of these humic materials influences the stability and maturity of the final product (Garcia et al. 1992; Spaccini and Piccolo 2009). However, the

actual structure of the humic substances remains controversial (de Leeuw and Largeau 1993). Most studies suggest a predominance of aromatic units in matured humic substances, while other results indicate largely aliphatic structures in humic extracts. These dissimilarities may result from differing microbial activities, especially in resynthesis processes occurring during composting (Gea et al. 2007).