Dissolved organic carbon (DOC) as a parameter of compost maturity

Despite numerous investigations of the maturation process of composts, a simple and straightforward parameter which can predict plant response upon compost application has yet to be defined. In light of results accumulated over a decade, we examined simple, chemical parameters of three composts from three types of source materials (municipal solid waste (MSW), separated cow manure (CSM), biosolids (BS)). These materials were composted using different procedures and facilities. The chemical parameters were correlated to the growth response of cucumbers or ryegrass sown in potting media amended with the composts sampled at different stages of the process. The dissolved organic carbon (DOC) concentration of all composts decreased rapidly within the first month, then, towards the end of the process, stabilized at concentration below 4 g kg^-1. DOC correlated highly and significantly to the absorbance at 465 nm in all composts, and also to the C/N ratio. Nitrate evolution was similar in all composts, but the final concentrations differed among them. Plant biomass increased with composting time. For CSM and BS compost maximum biomass was reached when the DOC reached levels below 4 g kg^-1. DOC concentration is suggested for use as a simple method of determining maturity, with 4 g kg^-1 recommended as a threshold level indicating maturity. Absorbance at 465 nm can be used instead of DOC concentration after appropriate calibration.