Monitoring of greenhouse gas emissions and compost quality during olive mill waste co-composting at industrial scale: The effect of N and C sources.

Olive mill wastes (OMW) management by composting allows to obtain valuable fertilizing products, but also implies significant fluxes of greenhouse gases (GHG). For a proper OMW composting, high C- and N co-substrates are necessary, but little is known concerning their effect on GHG emissions in OMW-industrial scale composting. In this study, different co-composting agents (cattle manure (CM), poultry manure (PM), sheep manure (SM) and pig slurry solid fraction (PSSF) as N sources and olive leaves (OLW) and urban pruning residues (UPR) as bulking agents and C sources) were used for OMW composting at industrial scale. Physico-chemical and chemical properties in the composting samples, and GHG (CO, CH and NO) fluxes were monitored in 12 industrial-scale windrows. GHG emissions were firstly influenced by N source, with the highest accumulated global warming potential (GWP) associated with PM (512 kg COeq pile), since PM composts were associated with the greatest NO (0.33 kg pile) and CH emissions (15.67 kg pile). Meanwhile, PSSF was associated with the highest CO emissions (1113 kg pile). UPR as a bulking agent facilitated 10 % greater mineralization of the biomass than OLW, however this C-source was not associated with higher GHG emissions. The results showed that while mineralization dynamics may be impacted by C sources, GHG emissions were mainly conditioned by the characteristics of nutrient-heavy feedstocks (PM and SM). Moreover, manures as nitrogen-laden co-substrates had widely differing effects on total GWP, and that of individual gases, but further research is necessary to understand the mechanisms explaining such differences.