UC Merced

Dairy manure is one of the largest emitters of greenhouse gases (GHG) and air pollutants from agriculture. In California, the nation's leader in dairy production, dairy manure accounts for 25% of total methane (CH4) emissions and contributes to poor air quality in agricultural regions. Recent policies such as the Global Methane Pledge and California's SB 1383 will require large reductions in CH4 emissions from dairies. While anaerobic digesters can mitigate emissions from liquid manure, very few management strategies exist to reduce emissions from solid manure. Recently, biochar-composting has been proposed as a strategy to cut emissions during the composting of organic materials. However, limited information exists on whether biochar could reduce emissions during the composting of dairy manure. Here, we investigated the potential of dairy manure biochar-composting to reduce both GHG and air pollutant emissions from dairies. In the first project, we measured GHG emissions during the full-scale composting of dairy manure with or without biochar, and we found that biochar-composting reduced CH4 emissions by 84%. Using life-cycle assessment (LCA), we also estimated the net climate impact of managing solid dairy manure through biochar-composting, composting, or stockpiling, and we found that biochar-composting was a climate-negative dairy manure management strategy. In the second project, we investigated whether biochar-composting could also reduce the emission of air pollutants from dairy manure. We found that biochar-composting cut the emission of hydrogen sulfide, volatile organic compounds, and nitrogen oxides by 70%, 61% and 67%, respectively. Using an integrated assessment model, we estimated that this reduction in air pollution would reduce health-related social costs by $66,000 annually per dairy farm. In the third project, we conducted a laboratory composting experiment to test the effect of biochar feedstock (walnut shells, almond shells, and almond clippings) and application rate (5% and 20%, by mass) on composting emissions. While there was no difference in emissions between biochar type, we found that high application rates increased CH4 emissions and reduced N2O emissions. We ascribe this pollution swapping to an increase in compost aggregation with biochar application, as aggregates likely acted as anoxic microsites for methanogenesis and complete denitrification. We recommend using lower biochar application rates, which reduced N2O emissions without increasing CH4 emissions in our study. This dissertation shows that biochar-composting of solid dairy manure is an untapped natural climate solution with a high potential to help meet climate goals and improve the health of rural, disadvantaged communities by reducing air pollution.