Dairy Manure Co-composting with Wood Biochar Plays a Critical Role in Meeting Global Methane Goals.

Livestock are the largest source of anthropogenic methane (CH) emissions, and in intensive dairy systems, manure management can contribute half of livestock CH. Recent policies such as California's short-lived climate pollutant reduction law (SB 1383) and the Global Methane Pledge call for cuts to livestock CH by 2030. However, investments in CH reduction strategies are primarily aimed at liquid dairy manure, whereas stockpiled solids remain a large source of CH. Here, we measure the CH and net greenhouse gas reduction potential of dairy manure biochar-composting, a novel manure management strategy, through a composting experiment and life-cycle analysis. We found that biochar-composting reduces CH by 79%, compared to composting without biochar. In addition to reducing CH during composting, we show that the added climate benefit from biochar production and application contributes to a substantially reduced life-cycle global warming potential for biochar-composting: -535 kg COe Mg manure compared to -194 kg COe Mg for composting and 102 kg COe Mg for stockpiling. If biochar-composting replaces manure stockpiling and complements anaerobic digestion, California could meet SB 1383 with 132 less digesters. When scaled up globally, biochar-composting could mitigate 1.59 Tg CH yr while doubling the climate change mitigation potential from dairy manure management.