From green waste to horticultural resources: A novel approach to quantifying greenhouse gas emission factors for decision-making.

A substantial amount of green waste is incinerated, while the horticultural sector faces a severe shortage of resources. To promote the use of materials derived from green waste in horticulture, more concise and robust evidence is essential to guide policy decisions in waste management and horticulture sectors. This study focused on greenhouse gas (GHG) budgets and aimed to clarify GHG emission factors through disposal or recycling methods for mowed levee weeds. In addition, the quality and cultivability of the biochar and compost were evaluated. GHG budgets for carbonization and composting of mowed levee weeds were quantified using two novel approaches. First, we verified whether the residual carbon content in the compost could be interpreted as a CO2 sequestration effect. Second, we introduced a time-integrated carbon sequestration model and quantified the CO2 sequestration effect of biochar and compost "over" a 100-year period. GHG emission factors of incineration, carbonization, and composting were calculated at 0.245 kg-CO2e/kg-FW, -0.395 kg-CO2e/kg-FW, and -0.033 kg-CO2e/kg-FW, respectively, which enabled regional GHG budget projections. Biochar and compost produced from levee weeds with quality and cultivation properties equivalent to those of 21 commercially available horticultural materials. Actual potting cultivation tests demonstrated that replacing these materials with peat and perlite can dramatically reduce the GHG footprint of potting production, bringing it close to neutral. These GHG emission factors and footprints help us elucidate the value of recycling green waste more intuitively.