Insights into carbon loss reduction during aerobic composting of organic solid waste: A meta-analysis and comprehensive literature review.

Carbon neutrality is now receiving global concerns for the sustainable development of human societies, of which how to reduce greenhouse gases (GHGs) emissions and enhance carbon conservation and sequestration becomes increasingly critical. Therefore, this study conducted a meta-analysis and literature review to assess carbon loss and to explore the main factors that impact carbon loss during organic solid waste (OSW) composting. The results indicated that over 40 % of carbon was lost through composting, mainly as CO2-C and merely as CH4-C. Experimental scale, feedstock varieties, composting systems, etc., all impacted the carbon loss, and there was generally higher carbon loss under optimal conditions (i.e., C/N ratio (15-25), pH (6.5-7.5), moisture content (65-75 %)). Most mitigation strategies in conventional composting (CC) systems (e.g., additive supplementary, feedstock adjustment, and optimized aeration, etc.) barely mediated the TC and CO2-C loss but dramatically reduced the emission of CH4-C through composting. Among them, feedstock adjustment by elevating the feedstock C/N ratio effectively reduced the TC loss, and chemical additives facilitated the conservation of both carbon and nitrogen. By comparison, there was generally higher carbon loss in the novel composting systems (e.g. hyperthermophilic and electric field enhanced composting, etc.). However, the impacts of different mitigation strategies and novel composting systems on carbon loss reduction through composting were probably underestimated for the inappropriate evaluation methods (composting period-dependent instead of maturity originated). Therefore, further studies are needed to explore carbon transformation through composting, to establish methods and standards for carbon loss evaluation, and to develop novel techniques and systems for enhanced carbon conservation through composting. Overall, the results of this study could provide a reference for carbon-friendly composting for future OSW management under the background of global carbon neutrality.