Methane and nitrous oxide emissions during biochar-composting are driven by biochar application rate and aggregate formation

Authors: HARRISON, BRENDAN - University Of California; GAO, SI - California State University; Thao, Touyee; GONZALES, MELINDA - University Of California; WILLIAMS, KENNEDY - Howard University; Scott, Natalie; Hale, Lauren; GHEZZEHEI, TEAMRAT - University Of California; DIAZ, GERARDO - University Of California; RYALS, REBECCA - University Of California

Submitted to: Global Change Biology Bioenergy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/6/2023
Publication Date: 12/20/2023
Citation: Harrison, B.P., Gao, S., Thao, T., Gonzales, M., Williams, K.L., Scott, N.M., Hale, L.E., Ghezzehei, T., Diaz, G., Ryals, R.A. 2023. Methane and nitrous oxide emissions during biochar-composting are driven by biochar application rate and aggregate formation. Global Change Biology Bioenergy. Article e13121. https://doi.org/10.1111/gcbb.13121.
DOI: https://doi.org/10.1111/gcbb.13121

Interpretive Summary: Manure compost piles emit greenhouse warming and air pollution gasses, but co-composting with biochar can substantially reduce gas emissions. Manure co-composting with 20% biochar application reduced net global warming potential (GWP) due to reductions in nitrous oxide and ammonia emissions. However, methane (CH4) emissions were increased. Owing to this, we recommend lower application rates, as the 5% biochar treatments also reduced GWP but without increasing CH4 emissions.

Technical Abstract: Manure is a leading source of methane (CH4), nitrous oxide (N2O), and ammonia (NH3) emissions, and alternative manure management practices can help society meet climate goals and mitigate air pollution. Recent studies show that biochar-composting can substantially reduce emissions from manure. However, most studies test only one type of biochar applied at a single application rate, leading to high variation in emission reductions between studies. Here, we measured greenhouse gas and NH3 emissions during biochar-composting of dairy manure with biochar applied at 5% or 20%, by mass, and made from walnut shells, almond shells, or almond clippings. We found little difference in emissions between biochar type. However, we found that the 20% application rates increased CH4 emissions and decreased N2O and NH3 emissions, resulting in a net reduction in global warming potential (GWP). We attribute this result to biochar increasing the formation of compost aggregates, which likely acted as anaerobic reactors for methanogenesis and complete denitrification. Biochar may have further fueled CH4 production and N2O consumption by acting as an electron shuttle within aggregates. We recommend lower application rates, as we found that the 5% treatments in our study led to a similar reduction in GWP without increasing CH4 emissions.