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 4 ) emissions, and in intensive dairy systems, manure management can contribute half of livestock CH 4 . 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 4 by 2030. However, investments in CH 4 reduction strategies are primarily aimed at liquid dairy manure, whereas stockpiled solids remain a large source of CH 4 . Here, we measure the CH 4 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 4 by 79%, compared to composting without biochar. In addition to reducing CH 4 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 CO 2 e Mg -1 manure compared to -194 kg CO 2 e Mg -1 for composting and 102 kg CO 2 e Mg -1 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 4 yr -1 while doubling the climate change mitigation potential from dairy manure management.