Biochar-composting substantially reduces methane and air pollutant emissions from dairy manure

Abstract Dairy manure is one of the largest sources of methane (CH 4 ) emissions and air pollution from agriculture. In a previous study, we showed that composting dairy manure with biochar substantially reduces CH 4 emissions and could help the dairy industry meet climate goals. However, it remained unclear whether biochar could also mitigate the emission of air pollutants and odor during composting. Here, we conducted a full-scale composting study at a dairy farm and monitored the emission of greenhouse gases (CO 2 , CH 4 , N 2 O) and air pollutants (H 2 S, VOCs, NO x , NH 3 ) from compost piles amended with or without biochar. We found that amending compost with biochar significantly reduced total CH 4 emissions by 58% (± 22%) and cut H 2 S, VOCs, and NO x emissions 67% (± 24%), 61% (± 19%) and 70% (± 22%), respectively. We attribute this reduction in emissions to improved oxygen diffusion from the porous biochar and the adsorption of gas precursors to the biochar surface. Interestingly, NO x fluxes from the composting dairy manure were much higher than the few values reported in the literature, suggesting that dairy manure could also be a significant source of NO x emissions. We estimate that biochar-composting of dairy manure would reduce the social cost of manure emissions from this farm by over $66,000 annually. Results from this study show that in addition to being a powerful CH 4 mitigation strategy, biochar-composting could play an important role in improving air quality and the health and wellbeing of rural communities.