Building the Case for Compost and Carbon Sequestration

Authors: Katie Doonan, Georgine Yorgey, Kirsten Ball, and Michael Brady

How can you tell if something works?

In the spirit of “what gets measured gets managed”, there has been recent attention directed to how we can quantify potential benefits of compost as an agricultural soil amendment, and its potential to sequester carbon. Accounting for benefits in a defensible way is one key to creating channels for the most impactful action. The beauty of CSANR often lies in its ability to meet challenges like this where they are, to bring science to bear, and provide pathways forward to sustainable solutions.

In 2021, the Washington Legislature directed WSU to carry out some specific tasks to assess two areas related to compost and compost use: 1) model reliability and applicability of carbon accounting tools to compost applications within agriculture in Washington state and 2) municipal compost buy-back demand. This occurred at the same time that two major pieces of state legislation were passed: the Organics Management Law (HB 1799) and the Sustainable Farms and Fields Program established under RCW 89.08.615. The Organics Management Law requires that landfilling of organic materials be reduced by 75% by 2030. Meanwhile, the Sustainable Farms and Fields Program provides funding to support the implementation of climate friendly farming practices statewide, including support for the use of compost. CSANR led the response to this directive.

Under the modeling portion of the work, Kirsten Ball, a post-doctorate scholar with CSANR, evaluated four models in the context of two case studies. The first case study was based on a long-term field experiment at Wilke Farm led by Ian Burke, a CSANR Leadership Team member. The field experiment assessed a substantial one-time application of compost in a dryland system. In contrast, the second case study was based on yearly application of compost in conjunction with fertilizer in a westside irrigated vegetable production system. This field experiment was based out of WSU’s Puyallup Research and Extension Center, was started by former CSANR affiliated faculty and continued by CSANR’s Extension Specialist Doug Collins.

Key findings from this work include:

1. Carbon accounting tools run a spectrum from simple online tools that emphasize ease-of-use, to research grade tools that require a high level of user expertise and input data. For models that require low amounts of input data, increased bias and therefore uncertainty may be a tradeoff for ease-of-use.
2. There is a certain level of uncertainty inherent in carbon sequestration modeling. Comparing an accumulation of field data from long-term research to modeling results can help clarify the sources of uncertainty and improve these models over time.
3. Carbon accounting tools can help estimate carbon sequestration potentials of compost applications but are likely more relevant when applied over larger areas or groups of farms. Users should be cautious in relying on these tools for site-specific estimates.

The second component of the proviso aimed to establish a baseline assessment of municipal compost collection, usage, and subsequent demand of compost for municipal projects. Michael Brady, a CSANR Leadership Team member, led this effort. With the Organics Management Law (HB 1799) of reducing organic waste in landfills by 75% and added targets for municipal compost buy-back, an initial understanding of the current state of municipal compost is essential across the state. A survey was conducted of current composters, which helped establish a qualitative understanding of current composting and potential barriers in future collection and use of compost.

Key findings include:

1. Current administrative systems for tracking use and collection of compost need improvement. Historical documentation of compost collection and use is especially limited in smaller municipalities, which may impact assessment of improvement through future practices.
2. There is inconsistency of compost demand between municipalities, especially based on municipality size.
3. A major barrier to compost utilization is contamination of final compost product by non-compostable waste. Improved quality of final product will reduce the barrier to use.

The final culmination of these studies is available as the legislative report entitled “Evaluating Compost Application for Soil Carbon Sequestration on Agricultural Land and Compost Buy-Back Programs in Washington”. This effort not only helps inform future legislation, but also helps CSANR prioritize projects to address areas of need.

This article was originally published as a CSANR 2022 Annual Report story. For this story and more, check out the Annual Reports here.