Greener gas? Impact of biosolids on carbon intensity of switchgrass ethanol

Synthetic fertilizers make up a significant fraction of the energy required to grow switchgrass (Panicum virgatum L.) for ethanol production. A field study compared biosolids and synthetic fertilizers on biomass yield, ethanol production, and nitrous oxide (N₂ O) emissions of switchgrass to determine if using an alternative source of nutrient would lower the energy density of the fuel. Minimal N₂ O emissions were observed the first year of the study (0.99 ± 1.5 g N₂ O ha^-1 d^-1 for biosolids), with no difference between treatments. Biosolids were added in excess of agronomic rates, and gas samples were collected immediately after irrigation for the subsequent years to examine maximum N₂ O emissions. Mean Year 2 emissions increased for fertilizers to 1.8 ± 8 g N₂ O ha^-1 d^-1 (n = 131) and to 3.73 ± 10.2 g N₂ O ha^-1 d^-1 (n = 130) for biosolids-amended soils. Emissions in Year 3 were similar to Year 2. Yield was similar and ranged from 3.7 ± 5 to 11 ± 1.1 and from 5.0 ± 0.2 to 13.4 ± 1.7 Mg ha^-1 for biosolids and fertilizer, respectively. The potential ethanol yield was 365 ± 28 L Mg^-1 and 374 ± 34 L Mg^-1 for the biosolids- and fertilizer-grown grass, respectively. Greenhouse gas emissions associated with fertilizer production were considered for N, P, and K and totaled 1,653 kg carbon dioxide equivalent (CO₂ e) ha^-1 . The equivalent credits for substitution of biosolids (18 Mg ha^-1 ) were -2,492 kg CO₂ e ha^-1 . Nitrous oxide emissions were calculated based on 1% of total N applied for agronomic applications and were 8,600 and 3,500 g N₂ O ha^-1 for the biosolids and fertilizer treatments, respectively. Total carbon costs associated with fertilization were 2,700 kg CO₂ e ha^-1 for fertilizer and 60 kg CO₂ e ha^-1 for biosolids. Using measured N₂ O data would have resulted in lower emissions for both treatments.