Can Stability and Maturity Be Evaluated in Finished Composts from Different Sources?

A combination of physical, chemical, spectroscopic and biochemical parameters, along with different plant assays, was used to assess the maturity and stability of nine finished composts obtained from several raw materials (biodegradable fraction of municipal solid waste, green waste, sewage sludge, manure, and grape marc), and by different procedures (aerobic and anaerobic processing, industrial-scale and laboratory scale). Measures of total, alkali-soluble and water-soluble organic matter, N forms, colour, absorbance, respiration, dehydrogenase activity, and phytotoxicity were performed. The selection of a single parameter for stability and maturity evaluation for all the composts was not possible. A significant positive correlation (P<0.01) was found between microbial respiration and dehydrogenase activity. Also, significant correlations were found between dehydrogenase activity and water-soluble C (P<0.05) and water-soluble N (P<0.05), which are presumably the main forms of energy used by microorganisms. Nevertheless, common maturity/stability indicators, such as the C/N ratio in the solid and the aqueous phase, were not related either to microbial activity or phytotoxicity. In fact, the results of the plant growth tests were not correlated to any other parameter. A principal component analysis was performed to differentiate those parameters giving the maximum information about the status of each compost. A combination of a measure of humification and a measure of microbial activity or water-soluble organic carbon could be used to explain the differences in the stability of the composts, whereas for maturity evaluation plant tests are necessary.