Carbon Sequestration and Greenhouse Gas Mitigation Potential of Composting and Soil Amendments on California’s Rangelands
Authors
Silver, Whendee, Sintana Vergara, Allegra Mayer, University of California, Berkeley
Publication
California’s Fourth Climate Change Assessment, California Natural Resources Agency. Publication number: CCCA4-CNRA2018-002 | 2018
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 longterm 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 were 0.6 and 0.7 g CH4 kg-1 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.
