Purpose
Biochar is a carbon-rich product derived from pyrolysis and is commonly used as a soil amendment. When applied to soil, biochar has been shown to sequester carbon, enhance aggregate stability, and improve soil nutrient and water retention. Recently, several states have adopted the USDA Natural Resources Conservation Service (NRCS) Code 336, which addresses soil carbon amendments, including biochar, as a conservation practice. This has led to increased awareness of biochar in agricultural systems. While the application of biochar to soil systems has been extensively studied, there are other agricultural sectors where biochar could be incorporated to provide additional benefits. This study explores the potential for incorporating biochar into manure management systems, specifically anaerobic digestion and manure storage.
What Did We Do?
Two different studies were conducted as part of this research. The first study investigated how biochar could be implemented into manure storage systems. Manure storage is a common practice at livestock facilities; however, emissions of ammonia (NH₃), methane (CH₄), and nitrous oxide (N₂O) are released into the atmosphere during storage. Additionally, the increasing use of solid-liquid separation in mid- to large-scale farms has resulted in emissions occurring outside of land application due to the lack of crust formation on manure storage. This study assessed emissions from pilot manure storage units (5-gallon buckets) after applying a 2-inch layer of raw feedstock or biochar as a cover over dairy manure. Different feedstocks, including woodchips, corn stover, and manure solids, were evaluated, and emissions were measured weekly over four months to determine NH₃, CH₄, and N₂O emissions (Figure 1).
The second study examined the incorporation of biochar into dairy manure anaerobic digestion systems. Anaerobic digestion of livestock manure produces biogas, which contains significant concentrations of hydrogen sulfide (H₂S) that must be removed before energy utilization. This study evaluated how dosing biochar—produced from different feedstocks and at varying pyrolysis temperatures—impacted hydrogen sulfide reduction during anaerobic digestion. A bench-scale study was conducted using batch reactors dosed at 0.75% (w/w) (approximately 62 lbs per 1,000 gallons), and biogas was analyzed every 2–3 days for H₂S, CH₄, and CO₂ concentrations.
What Have We Learned?
In the manure storage study, both raw feedstocks and biochar reduced NH₃ emissions. The greatest reductions in NH₃ emissions were observed with woodchip biochar, which achieved an average reduction of 82–97% in cumulative emissions. The manure solids and corn stover biochar resulted in average reductions of 35% and 55%, respectively. However, while NH₃ emissions were reduced, an increase in greenhouse gas emissions—particularly N₂O—was observed in treatments with biochar covers.
In anaerobic digestion systems, the addition of biochar at 0.75% (w/w) reduced H₂S production. The degree of reduction was influenced by the biochar production temperature, with lower-temperature biochars being more effective at reducing H₂S. During the batch anaerobic digestion tests, no significant impact was observed on CH₄ or CO₂ concentrations in the biogas.
Future Plans
For the manure storage study, while the reductions in NH₃ emissions were promising, the observed increase in N2O emissions requires further investigation. The highest N₂O emissions were associated with large-particle woodchip biochar, likely due to the creation of an anoxic environment within the biochar cover. Future studies will examine whether reducing biochar particle size can mitigate these N₂O emissions. Additionally, further research will assess the long-term impacts of these treatments on soil health and crop production following land application.
For the anaerobic digestion study, additional work is needed to determine the specific biochar characteristics responsible for the greater H₂S reductions observed with lower-temperature biochars. Since the study was conducted at a batch scale, further evaluation in a continuous system is necessary. Lastly, full-scale digester trials are needed before widespread adoption of biochar in anaerobic digestion systems.
Authors
Presenting & corresponding author
Joseph R. Sanford, Assistant Professor, University of Wisconsin–Platteville, sanfordj@uwplatt.edu
Additional authors
Ben Raimonde, Undergraduate Research Assistant, University of Wisconsin–Platteville
John Rodwell, Undergraduate Research Assistant, University of Wisconsin–Platteville
Jeffery Smolinski, Undergraduate Research Assistant, University of Wisconsin–Platteville
Acknowledgements
This material is supported by the Wisconsin Dairy Innovation Hub and the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2022-70001-37309. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the U.S. Department of Agriculture or the Wisconsin Dairy Innovation Hub.
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