The composting of organic waste has been defined as a controlled microbial aerobic decomposition process and therefore information on the microbial component of the composting substrates could be expected to give valuable information on the factors influencing the process, on its optimization and on the quality of the end product (Jedidi et al., 2004; Mondini et al., 1997; 2002).
These methods are based on the concept of soil microbial biomass as introduced by Jenkinson (Powlson, 1994). Determination of microbial biomass in soil is currently based on the so-called fumigation-extraction method (Vance et al., 1987b). It is possible to determine the soil microbial Carbon (C), Nitrogen (N), Phosphorus (P), Sulphur (S) (Vance et al., 1987b) and ninhydrinreactive N (Amato and Ladd, 1988; Joergensen and Brooks, 1990; Joergensen, 1996; Mergulhao et al., 2010) in the extracts. The microbial biomass plays a crucial role in the carbon cycle via the organic matter humification-mineralization processes. It also presents a determinant role in the nitrogen cycle with transformation of the organic mater into mineral elements. In fact, chloroform fumigation lyses the cells of micro-organisms and makes part of the cytoplasm extractable with various reagents ([K.sub.2]S[O.sub.4]).
The evaluation of microbial biomass content could be investigated by direct methods, such as the counting of micro-organisms (Paul and Johnson, 1977), or by several indirect methods, such as the measurement of cellular compounds or the fumigation-extraction approach (Vance et al., 1987a). Many authors have investigated the carbon and nitrogen content of the microbial biomass with the later approach.
Several authors have applied the fumigation-extraction technique with compost substrates (De Nobili et al., 1996; Hellmann et al., 1997; Horwath and Elliot, 1996; Mondini et al., 1997). It is important to note that the present study was quite different from theirs in several respects. Firstly in the fumigation-extraction technique, we used a Kjeldahl distillation to measure the nitrogen ratio which is relatively simple, easy and more reproducible than the ninhydrin reactive nitrogen method, as used by De Nobili et al. (1996) or Mondini et al. (1997). Moreover, the study represents a first report of an evaluation of the microbial biomass in Tunisian compost under the local climate conditions and specific waste content. These two parameters make the present study an original contribution to compost microbial biomass evaluation.
However, the application of the microbial biomass technique to composting substrates requires special care to obtain realistic and reproducible results, because of the particular properties of compost substrates, such as the fast changes in physicochemical and microbiological properties, the high spatial heterogeneity, the coloration of the extracts and the problems related to representative sampling and storage of samples.
The aim of this study was mainly focused on the evolution of microbial biomasses C and N during the composting of municipal solid wastes.
MATERIALS AND METHODS
Preparation of windrows: The study was performed in the composting plant of Beja located 100 km from the north of Tunis, using two types of windrows: (i) the first windrow W1 constituted with 100% of municipal solid wastes and (ii) the second windrow W2 composed by weight of 60% of municipal solid wastes and 40% of dried sewage sludge.
Municipal solid wastes from the Erriadh city of Beja were pre-selected at source (household preselection with average physical-chemical characteristics, humidity = 60%, organic matter = 30% dry weight and C/N = 32) and subjected to an accurately manual sorting in the plant, weighted and put on the composting platform, in the form of windrows of 7.5*3*1.5 m (length X width X height, respectively). Stabilized sewage sludge from anaerobic digestion of the urban wastewater treatment plant of Beja was integrated in the process at the dry state (with average physical-chemical characteristics, humidity = 30%, organic matter = 65% dry weight and C/N = 12.5) and primarily for the cover of windrow W2.
Temperature and humidity were controlled daily. When the mean temperature recorded in the different depths (depths 20, 40 and 60 cm) of the pile averaged 55[degrees]C (using a thermo-couple iron-constantan type J), the windrow was turned and watered. These operations of turning and watering with tap water were performed almost twice monthly in considering ambient temperature.
Microbial Biomass determination: Solid waste microbial C and N biomasses were evaluated using the fumigation-extraction method (Sparling and West, 1988). Duplicate samples (10 g) of the treated and non-treated compost (control) were fumigated with ethanol-free CHC[l.sub.3] for 24 h at 25[degrees]C in a dessicator. The fumigated and non-fumigated compost samples were extracted...