Land Quality and Management Practices Strongly Affect Greenhouse Gas Emissions of Bioenergy Feedstocks
We estimated emissions of greenhouse gases (GHGs) from development of a large bioenergy industry in New York State, a humid continental region of 12.6 million hectares of mixed urban, agricultural, and forest land uses. Six representative feedstocks were considered: maize grain, maize stover, warm season grass, short rotation willow, and hardwood and softwood residues from existing mixed-species forests. We analyzed how variation in land quality and management affects feedstock yields and related GHG emissions. Emissions from existing forest harvest are only due to equipment use and emit 0.01–0.02 Mg CO2e/Mg hardwood and softwood, respectively. Development of purpose-grown biomass crops will require the use of lower quality lands (the extensive margin), resulting in higher GHG emissions per metric ton of biomass harvested. For example, projected grass yields for 2020 range from 7–16 Mg/ha Wightman et al (Bioenerg Res doi: 10.1007/s12155-015-9618-x) with emissions ranging from 0.14 to 0.32 Mg CO2e/Mg grass. Average emissions from willow and grass across available land are 0.12 and 0.19 Mg CO2e/Mg dry biomass, respectively. Emissions from biomass crops are primarily due to nitrogen (61–75 %), lime (14–22 %), and equipment (6–11 %). Based on our analysis, production of these feedstocks emits 0.5 % (hardwood) to 37.8 % (maize grain) of the CO2-equivalents captured by photosynthesis in the biomass prior to conversion to usable energy. This analysis indicates that perennial systems (existing forest, woody crops, and grasses) provide the best yield on the lowest quality lands with lowest GHG emissions per metric ton of harvested biomass.