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 (N2O) emissions of switchgrass to determine if using an alternative source of nutrient would lower the energy density of the fuel. Minimal N2O emissions were observed the first year of the study (0.99 ± 1.5 g N2O 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 N2O emissions. Mean Year 2 emissions increased for fertilizers to 1.8 ± 8 g N2O ha−1 d−1 (n = 131) and to 3.73 ± 10.2 g N2O 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 (CO2e) ha−1. The equivalent credits for substitution of biosolids (18 Mg ha−1) were −2,492 kg CO2e 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 N2O ha−1 for the biosolids and fertilizer treatments, respectively. Total carbon costs associated with fertilization were 2,700 kg CO2e ha−1 for fertilizer and 60 kg CO2e ha−1 for biosolids. Using measured N2O data would have resulted in lower emissions for both treatments.