Received: 1 April, 2009. Accepted: 8 June, 2009. Original Research Paper Dynamic Soil, Dynamic Plant ©2009 Global Science Books Characterization of Compost-Based Growing Media. A Chemical, Thermal, Spectroscopic and Isotopic Approach Camilla Giovannini * • Daniela Montecchio • Paola Gioacchini • Ornella Francioso • Claudio Ciavatta Department of Agro-Environmental Sciences and Technologies, Alma Mater Studiorum University of Bologna, viale G. Fanin n. 40 – I-40127 Bologna, Italy Corresponding author: * camilla.giovannini4@unibo.it ABSTRACT Peat is the best substrate in growing media, but its rapid depletion needs at least a partial substitution. Composting of urban green wastes and agro-industrial by-products can produce good compost (Cm) for this purpose. Cm-based substrates are increasing in the market. However, physical and chemical characterization of Cm-based growing media is the first step for their proper use. This paper reports the results of the fractionation of the bulk organic matter (OM) of four growing media, containing increasing amount (v/v) of Cm [100% Peat; 80% Peat + 20% Cm (CP20); 40% Peat + 60% Cm (CP60); 100% Cm) into well-defined fractions: humic acid-like (HAL), non-humic fraction (NH) and dissolved OM (DOM) and their characterization by a combination of chemical, thermogravimetric (TG), differential thermal analysis (DTA), spectroscopic (Diffuse Reflectance Infrared Fourier Transform-DRIFT) and isotopic ( 15 N) techniques. TG-DTA of bulk growing media, HAL, NH and DOM showed significant differences among increasing Cm in Cm-based growing media. DRIFT spectra confirmed these differences. The N isotope ratio ( 15 N) of all the substrates increased with the amount of Cm in the substrate. The combination of chemical, TG-DTA, DRIFT and  15 N techniques can be successfully applied for the chemical characterization of Cm- based growing media with a good identification of different matrices. _____________________________________________________________________________________________________________ Keywords: compost,  15 N, differential thermal analysis, diffuse reflectance infrared Fourier transform, dissolved organic matter, humic substances, non-humic fraction, peat, thermogravimetric analysis Abbreviations: C, carbon; Cm, compost; DOM, dissolved organic matter; FA, fulvic acid; FAL, fulvic acid-like fraction, HA, humic acid; HAL, humic acid-like fraction; HS, humic substances; HU, humin; N, nitrogen; NH, non-humic fraction; OM, organic matter; TOC, total organic carbon INTRODUCTION Peat was considered for many years for its high physical and chemical stability and low degradation rate the best substrate for growing seedlings, plant propagation, vege- table production and ornamental plants growth in pots. Unfortunately, peat is obtained from wetlands, which are being rapidly depleted, causing environmental concerns that have led to many individual countries to limit the extent of peat mining, and prices are increasing as a result. This con- text stimulated the search for alternative materials (Raviv et al. 1986; Abad et al. 2001; Garcia-Gómez et al. 2002; Guerrero et al. 2002; Chong 2005; Pérez-Murcia et al. 2006; Grigatti et al. 2007a; Herrera et al. 2008; Ostos et al. 2008). Developing alternative substrates to peat is necessary for three different reasons: limited peat resources, increased pressure for using wastes/by-products coming from human or agro-industrial activities and the economic necessity to use locally produced waste products. A lot of organic wastes (green wastes), sewage sludge, sawdust, town refuse, etc. have been investigated as peat total or partial substi- tutes in container media after proper composting (He et al. 1995; Burger et al. 1997; Raviv 1998; Riberio et al. 2000; Benito et al. 2005; Hernández-Apaolaza et al. 2005). Com- posting is defined as a biological treatment in which aerobic microorganisms utilize organic matter (OM) as a substrate. The final product (compost) consists of stable OM, water, minerals and ash. The agronomic value of compost (Cm) and its benefi- cial or detrimental effects on soil and the environment are closely linked to the nature and dynamics of its constituent OM (He et al. 1995). The OM is the most important compo- nent of the solid phase directly involved into transformation processes (mineralization/humification). OM improves moisture, nutritional and biological properties of the sub- strate conditioning plant growth (Chen et al. 2002). The soil OM was fractionated into well-defined fractions charac- terized by different physical and biochemical properties: the humic substances (HS); the non-humic (NH) substances and the dissolved OM (DOM). The HS represent the most stable reservoir of organic C in soils due to their recalcitrancy to microbial degradation and can be divided into three main fractions: humic acids (HA), fulvic acids (FA) and humin (HU) (http://ihss.gatech.edu/ihss2). HS are complex and heterogeneous mixtures of polydispersed materials formed by biochemical and chemical reactions during the decay and transformation of plant and microbial remains: a pro- cess called humification (Stevenson 1994; Senesi and Bru- netti 1996; Brunetti et al. 2007). They have remarkable ad- sorption and buffer capacity; hence, they can adsorb dif- ferent nutrients and regulate their uptake, thus contributing to the balanced nutrient supply to the plants and protecting them from the damaging salt-effect. The amount and quality of HS and humic-like substances (HLS) in organic amend- ments or Cm (HLS are chemically similar to soil native HS but they are formed in an environment different from soil) are, therefore, considered important indicators of the OM maturity and stability for a successful application in agricul- ture and safe environmental impact of the amendment (Senesi and Brunetti 1996). DOM is considered the active and more mobile OM fraction that plays a key role in a wide variety of chemical physical and biological processes. It is related to organic C and N availability, metal complexation and enhanced plant growth. The DOM composition is a mixture of low mole- ®