Characterization of organic matter from composting of different residues by physicochemical and spectroscopic methods

Chemical and spectroscopic methods were used to characterize organic matter transformations during the composting process. Four different residue mixtures were studied: P1 – garden trimmings (GT) only, P2 – GT plus fresh cattle manure, P3 – GT plus orange pomace and P4 – GT plus filter cake. The thermophilic phase was not reached in P1 compost, but the P2, P3 and P4 composts showed all three typical process phases. The thermophilic phase and CEC/C ratio stabilized after 90 days, while C/N ratio and the ash content stabilized after 60 days. The increasing E4/E6 ratio indicated oxidation reactions occurring during the process in the material from P2, P3 and P4. The 13C NMR and FTIR results suggested extraction of both pectin and lignin in the HA-like fraction. The CEC/C ratio, temperature and E4/E6 ratio showed that within 90 days P2, P3 and P4 composts were humified. However, material from P1 did not show characteristics of humified compost. From these data, it is apparent that C/N ratio and ash content are not reliable methods for monitoring the composting process.