CARBON SEQUESTRATION AND GREENHOUSE GAS MITIGATION POTENTIAL OF COMPOSTING AND SOIL AMENDMENTS ON CALIFORNIA’S RANGELANDS A Report for: California’s Fourth Climate Change Assessment Prepared By: Whendee L. Silver1, Sintana E. Vergara1, Allegra Mayer1 1 Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720. DISCLAIMER This report was prepared as the result of work sponsored by the California Natural Resources Agency. It does not necessarily represent the views of the Natural Resources Agency, its employees or the State of California. The Natural Resources Agency, the State of California, its employees, contractors and subcontractors make no warrant, express or implied, and assume no legal liability for the information in this report; nor does any party represent that the uses of this information will not infringe upon privately owned rights. This report has not been approved or disapproved by the Natural Resources Agency nor has the Natural Resources Agency passed upon the accuracy or adequacy of the information in this report. Edmund G. Brown, Jr., Governor August 2018 CCC4A-CNRA-2018-002 ACKNOWLEDGEMENTS We would like to thank several people whose input has been instrumental in this work. We are grateful to Jeff Borum and the Natural Resource Conservation Service (NRCS) for organizing the field campaign and assisting with sampling. Tom Hedt facilitated excellent discussions on the field design and preliminary sampling. Ken Oster helped in the field and also provided valuable input. Sam Grubinger, Summer Ahmed, Ken Marchus, Josh Schimel, and Alison Haddad helped in the field and the lab. This project gave many undergraduate students an opportunity to learn about soil processing (they were champions at sorting compost and roots out of the hundreds of soil samples). We are grateful for the information and land for the field experiment provided by ranchers John Wick and Peggy Rathmann, Kevin Muno, Jeff Romm, Russell Chamberlin, Hilary Dustin and Ben Munger, Jared Lewis and Jeff Creque, and Dustin Flavell among others. Dr. Melannie Hartman provided technical support with the DayCent Model, and Dr. Rebecca Ryals shared the initial parameterization of the DayCent Model for the Marin and Yuba County sites. Dr. Jeff Creque and Will Bakx provided invaluable assistance at the composting site, and Mr. Kevin Lunny generously provided access and helped with powering our instruments. We thank West Marin Composting Facility and employees for their labor and support. John Wick saved the day more than once – and helped with many of the logistical challenges for the project. He provided and drove the backhoe, purchased and delivered the compost, and was an inspiration throughout. Thanks, John! Crystal Reul-Chen from CalRecycle provided valuable input and helped with access to key data. Jamie Anderson from the Department of Water Resources provided excellent leadership throughout for the 4th California Climate Change Assessment effort. The coastal sites included were originally for a project funded by the California Coastal Conservancy. Matching funds were generously provided by the Rathmann Family Foundation, helping to move solution-based science forward. i PREFACE California’s Climate Change Assessments provide a scientific foundation for understanding climate-related vulnerability at the local scale and informing resilience actions. These Assessments contribute to the advancement of science-based policies, plans, and programs to promote effective climate leadership in California. In 2006, California released its First Climate Change Assessment, which shed light on the impacts of climate change on specific sectors in California and was instrumental in supporting the passage of the landmark legislation Assembly Bill 32 (Nez, Chapter 488, Statutes of 2006), California’s Global Warming Solutions Act. The Second Assessment concluded that adaptation is a crucial complement to reducing greenhouse gas emissions (2009), given that some changes to the climate are ongoing and inevitable, motivating and informing California’s first Climate Adaptation Strategy released the same year. In 2012, California’s Third Climate Change Assessment made substantial progress in projecting local impacts of climate change, investigating consequences to human and natural systems, and exploring barriers to adaptation. Under the leadership of Governor Edmund G. Brown, Jr., a trio of state agencies jointly managed and supported California’s Fourth Climate Change Assessment: California’s Natural Resources Agency (CNRA), the Governor’s Office of Planning and Research (OPR), and the California Energy Commission (Energy Commission). The Climate Action Team Research Working Group, through which more than 20 state agencies coordinate climate-related research, served as the steering committee, providing input for a multisector call for proposals, participating in selection of research teams, and offering technical guidance throughout the process. California’s Fourth Climate Change Assessment (Fourth Assessment) advances actionable science that serves the growing needs of state and local-level decision-makers from a variety of sectors. It includes research to develop rigorous, comprehensive climate change scenarios at a scale suitable for illuminating regional vulnerabilities and localized adaptation strategies in California; datasets and tools that improve integration of observed and projected knowledge about climate change into decision-making; and recommendations and information to directly inform vulnerability assessments and adaptation strategies for California’s energy sector, water resources and management, oceans and coasts, forests, wildfires, agriculture, biodiversity and habitat, and public health. The Fourth Assessment includes 44 technical reports to advance the scientific foundation for understanding climate-related risks and resilience options, nine regional reports plus an oceans and coast report to outline climate risks and adaptation options, reports on tribal and indigenous issues as well as climate justice, and a comprehensive statewide summary report. All research contributing to the Fourth Assessment was peer-reviewed to ensure scientific rigor and relevance to practitioners and stakeholders. For the full suite of Fourth Assessment research products, please visit www.climateassessment.ca.gov. This report advances the understanding of how rangeland management can contribute to climate change mitigation and adaptation by repurposing California’s organic waste-stream to compost for emissions reduction and carbon sequestration. ii ABSTRACT Land management offers significant potential to both help lower greenhouse gas emissions and reduce atmospheric carbon dioxide. The goals of this research were to determine the short- and long-term potential of compost amendments to sequester carbon (C) in rangeland soil, and to determine the effects of future climate change scenarios on C storage and loss. The project also explored the emissions from the composting process itself, a poorly quantified component of the waste-to-rangeland lifecycle. Finally, the lifecycle emissions from rangeland compost amendments were compared to those of other fates of waste to determine potential benefits or tradeoffs among a range of common practices. Compost amendments (0.25 inch) to 15 diverse rangelands led to a detectable increase in surface (0-10 cm) soil C stocks (2.1 ± 1.0 Mg C ha-1) over a single growing season. The DayCent biogeochemical model was used to explore long- term effects of compost application in a subset of these rangelands, and to determine interactions with future climate change scenarios. Results showed that the overall climate benefit of compost amendments peaked 15 years after application. The benefit decreased over time, and decreased more quickly in a high emissions scenario. Two 100-day experiments using micrometeorological approaches yielded the first whole-pile, continuous measurements of greenhouse gas emissions from windrow composting. The total methane (CH4) emission factors -1 were 0.6 and 0.7 g CH4 kg feedstock, and were more sensitive to pile management than initial feedstock chemistry. Nitrous oxide emissions were below the instrument detection limit (25 ppb + 0.05%, or 4.5E-5 g m-2) throughout the experiments. A lifecycle assessment model suggested that diverting organic waste to composted field amendments resulted in greater CO2e savings compared to anaerobic digestion or incineration for energy, due to the combination of new C sequestration and emission reductions. In sum, results showed considerable potential for repurposing California’s organic waste-stream to compost for emissions reduction and C sequestration. Rangeland compost application, where appropriate, can contribute to climate change mitigation, as well as improve ecosystem productivity and sustainability. Keywords: compost, climate change mitigation, rangelands, lifecycle assessment model, DayCent biogeochemical model, carbon, greenhouse gases Please use the following citation for this paper: Silver, Whendee, Sintana Vergara, Allegra Mayer. (University of California, Berkeley). 2018. Carbon Sequestration and Greenhouse Gas Mitigation Potential of Composting and Soil Amendments on California’s Rangelands. California’s Fourth Climate Change Assessment, California Natural Resources Agency. Publication number: CCCA4-CNRA- 2018-002. iii HIGHLIGHTS The addition of 0.25 inch of compost to a diverse set of California’s rangelands resulted