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

Abstract

Livestock are the largest source of anthropogenic methane (CH₄) emissions, and in intensive dairy systems, manure management can contribute half of livestock CH₄. Recent policies such as California's short-lived climate pollutant reduction law (SB 1383) and the Global Methane Pledge call for cuts to livestock CH₄ by 2030. However, investments in CH₄ reduction strategies are primarily aimed at liquid dairy manure, whereas stockpiled solids remain a large source of CH₄. Here, we measure the CH₄ 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 CH₄ by 79%, compared to composting without biochar. In addition to reducing CH₄ 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 CO₂e Mg^-1 manure compared to -194 kg CO₂e Mg^-1 for composting and 102 kg CO₂e 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 CH₄ yr^-1 while doubling the climate change mitigation potential from dairy manure management.

Keywords: biochar; climate change mitigation; composting; livestock; methane; natural climate solutions.

Figure 1

Cumulative CH₄ (a) and CO₂ (b) emissions over the 35-day composting experiment. CH₄ emissions are expressed in units of g CH₄ kg^–1 dry feedstock. CO₂ emissions are expressed in units of g CO₂ kg^–1 dry feedstock. The black curve shows cumulative emissions from biochar-composting, and the yellow curve shows cumulative emissions from composting. The shaded region for each curve shows the 95% confidence interval for each pile’s gas flux measurements.

Figure 2

Life-cycle assessment of management strategies for separated solid dairy manure using 100-year GWPs. The number above each strategy is the net GWP in kg CO₂e Mg^–1 manure. Each color represents a different life-cycle stage and is referenced in the legend above. The transportation stages are removed from the figure due to their minuscule contribution to the total GWP of each strategy.

Figure 3

Number of anaerobic digesters needed to meet California’s 40% dairy CH₄ reduction goal mandated by SB 1383 under different scenarios. The digester + stockpiling scenario assumes that dairies with anaerobic digesters stockpile their separated solid manure. The digester + biochar-composting scenario assumes that dairies with anaerobic digesters biochar-compost their separated solid manure. The enhanced population reduction scenario assumes that dairies with anaerobic digesters biochar-compost their separated solid manure and that the statewide herd population declines at an annual rate of 1% instead of the California Air Resources Board’s (CARB) projected annual reduction of 0.5%, which is used in the other scenarios. In addition to assumed population reduction rates, each scenario assumes CARB projected rates for implementing new alternative manure management projects at dairies not large enough for anaerobic digesters.

Figure 4

Technical GHG and CH₄ global mitigation potential from dairy manure management. (a) Net life-cycle GHG mitigation from dairy manure management consists of anaerobic digestion of dairy manure and varying degrees of biochar-composting of separated solid manure. (b) CH₄ mitigation from dairy manure management consists of anaerobic digestion of dairy manure and varying degrees of biochar-composting of separated solid manure. For each figure, the x-axis shows the hypothetical number of dairy cows (in million heads) managed in systems with anaerobic digesters. We limit our analysis to the number of dairy cows kept in intensive systems globally. The y-axis shows the percent of solid manure separated from digesters that is managed through biochar-composting. Solid manure that is not biochar-composted is assumed to be stockpiled.