COS 25-2 - Biochar-composting is a climate-negative solution for dairy manure management that can help meet global methane goals

Background/Question/Methods

The leading source of anthropogenic methane (CH₄) emissions is livestock. In intensive dairy systems, up to half of CH₄ emissions come from manure management. Recent policies targeting CH₄ emissions, such as California's short-lived climate pollutant reduction law (SB 1383) and the Global Methane Pledge, will require a significant reduction in livestock CH₄ by 2030. However, strategies aimed at reducing CH₄ from dairy manure typically focus on the liquid portion of manure, whereas stockpiled solids can remain a large source of CH_4. To address this problem, we asked the question: what role can biochar-composting of dairy manure solids, a novel methane (CH₄) mitigation strategy, play in meeting California's short-lived climate pollutant reduction law (SB 1383) and the Global Methane Pledge? We answered this question by measuring daily greenhouse gas (GHG) fluxes during a field-scale biochar-composting experiment, incorporating experimental flux data into the first life-cycle assessment (LCA) of a biochar-composting dairy manure management system, and using LCA results to estimate the role that biochar-composting can play in meeting CH₄ goals in California and globally.

Results/Conclusions

We find that adding 6% biochar to composting dairy manure reduces CH₄ by 81% compared to compost without biochar. Using experimental data, we conduct the first life-cycle assessment (LCA) of a biochar-composting solid dairy manure management system and show that biochar-composting has a net global warming potential of -535 kg CO₂e Mg^-1 manure compared to -194 kg CO₂e Mg^-1 for composting and 102 kg CO₂e Mg^-1 for stockpiling. Using our LCA results, we estimate that when biochar-composting is scaled up in intensive dairy systems globally, the technical CH₄ mitigation potential from dairy manure increases from 4.54 Tg CH₄ yr^-1 to 6.13 Tg CH₄ yr^-1, and the technical net greenhouse gas mitigation potential nearly doubles, increasing from 154 Tg CO₂e yr^-1 to 297 Tg CO₂e yr^-1. This timely study comes as governments and farmers begin ramping up efforts to cut CH₄ emissions by the end of the decade. We believe this work can aid policymakers in this mission by demonstrating the potential of a promising, novel strategy to reduce emissions from livestock, the largest anthropogenic source of CH₄.