By Karen Hills

Biochar has the potential to sequester carbon and improve the properties of soils when used as an agricultural amendment. However, biochar will only be a viable option for carbon sequestration if there are uses and viable markets for this biochar. In recent years, there has been interest in adding biochar to agricultural soils in conjunction with compost, and in some cases, “co-composting” biochar—putting the biochar in with the feedstock before the composting altogether. Read on to learn about a study led by Dr. David Gang, a professor at Washington State University’s Institute of Biological Chemistry, indicating that co-composting can provide additional benefits, both during the composting process and to the crops grown in soil amended with the resulting co-composted biochar.

Figure 1. Mark Fuchs (left), John Cleary (right) (both of the Washington Department of Ecology) and Nathan Stacey (middle, WSU) use equipment to measure gas emissions from a commercial scale co-composting experiment. Photo: Doug Collins, WSU.

The co-composted biochar used in this study was made using a set proportion of screened dairy manure solids and bedding straw, woody yard waste, and food scraps. Some of the compost piles also contained 2.5% or 5% (by volume) biochar. Even before adding the compost to the soil there were benefits: the addition of biochar to the feedstocks led to significant reductions in the volatile organic compounds measured during the composting process, which can make compost smell bad (Figure 1).

While sweet basil is not considered one of the Pacific Northwest’s major commodity crops, it is a high value crop that is frequently grown under organic conditions, making it well suited to receive high-value organic amendments, such as compost and biochar. Gang and collaborators tested the co-compost by blending it as part of a soil mixture and using it to grow two different cultivars of sweet basil (Eleanora and TSQ) in pots in a greenhouse.

Interestingly, neither compost alone, nor compost with biochar added when applied to the soil, made a difference to the growth of the sweet basil plants. Co-composting the biochar (at 2.5% or 5%), however, caused a significant increase in plant fresh weight relative to treatments receiving a combination of biochar and compost (Figure 2).  This result suggests that something occurred during the co-composting process that affected the co-compost’s ability to promote plant growth.

Figure 2. Impact of biochar co-composting on biomass/yield of sweet basil cultivar Eleanora (a Genovese type of basil). Different letters indicate significant differences between treatments. (Source: Gang et al. 2018).

Sometimes getting bigger plants can be counter-productive, because the quality can be diluted. Gang and colleagues also measured the effects of the biochar and co-compost treatments on levels of the antioxidants and volatile compounds that create the characteristic flavor of basil. They found very little impact on either antioxidant levels or the production of flavor compounds in sweet basil, per gram fresh weight. This is a very positive result, showing that the higher yields did not result in decreased quality.

The mechanism by which co-composted biochar increased plant growth has yet to be fully understood, but the study authors suggest that these effects may be due to positive impacts on soil health, particularly composition and activity levels of the microbial community. That is, qualities of the co-composted biochar may have helped create a better environment in the soil for microbes that provided benefits to the basil plants.

The only potential downside to using biochar in co-composting is the potential for additional cost associated with the energy for producing biochar. However, biochar cost can be minimized if the energy for its production can be derived from the source materials, and if it and can be produced relatively locally, minimizing transportation costs. Even if there is some cost associated with biochar for co-composting, Gang is optimistic about the potential to offset it by the downstream benefits on crop yield.

Figure 3. Co-composted biochar spread on experimental field plots before tillage and planting of potatoes. Photo: Doug Collins, WSU.

While positive effects of soil amendments such as biochar and co-composted biochar are dependent on the specific combination of biochar, soil conditions, and crop cultivar, this study raises some interesting questions and the potential for a win-win situation, with benefits seen both during the composting process and for crops grown using the resulting product. If these play out as hoped and the costs of adding biochar are indeed outweighed by the benefits, compost facilities could be in the market for biochar, leading to greater capturing of carbon in this product, in agricultural soils. Gang’s team, working with a number of other collaborators, are currently following up on these intriguing results to test the impacts of biochar co-compost on gas emissions, co-compost quality, and crop yield and quality of sweet basil, strawberries, and potatoes, both in the greenhouse and in the field (Figure 3).

For more information on this project and others funded through the Waste to Fuels Partnership, please see the Waste to Fuels Technology Partnership 2015-2017 Biennium Report.

This work was funded through the Waste to Fuels Technology Partnership between the Center for Sustaining Agriculture and Natural Resources at Washington State University and the Washington Department of Ecology’s Solid Waste Management Program (previously Waste 2 Resources Program). This partnership advances targeted applied research and extension on emerging technologies for managing residual organic matter.

Reference:

Gang, D.R., A. Berim, R. Long, J. Cleary, M. Fuchs, R.W. Finch, M. Garcia-Perez, and B.T. Jobson.  2018.Evaluation of Impact of Biochar-Amended Compost on Organic Herb Yield and Quality. Chapter 10 in Chen, S. et. al. 2018. Advancing Organics Management in Washington State: The Waste to Fuels Technology Partnership. Waste 2 Resources, Washington State Department of Ecology Publication No. 18-07-010. Olympia, Washington. https://fortress.wa.gov/ecy/publications/SummaryPages/1807010.html