Soils as a Solution: The Potential of Rangelands to Contribute to Climate Change Mitigation

Abstract:

The majority of soil-related climate change research has focused on describing the problem – estimating rates of carbon (C) losses and greenhouse gas (GHG) emissions from natural and managed ecosystems. More research is needed to explore potential solutions to climate change through mitigation and adaptation. Here we report on an integrated set of studies aimed at critically evaluating the biogeochemical potential of rangeland soils to help mitigate climate change, while improving the sustainability and productivity of food production systems. We explored direct effects through enhanced net primary production (NPP) and soil C sequestration, and indirect effects through diversion of high emitting sources to lower emitting organic matter dynamics. We used a combination of long- and short-term field experiments, modeling, laboratory assays, life cycle assessment (LCA), and meta-analyses in consultation with a diverse group of stakeholders from both the private and public sectors.

We found that organic matter amendments held particularly strong potential. Compost amendments increased soil C storage by 0.5-1.0 Mg C ha^-1 y^-1 in surface soils over 5 y, and increased NPP and water holding capacity. We measured 1.0 Mg of new C ha^-1 y^-1 over 3 y. Long-term amendment of cattle manure increased surface soil C by 19.0±7.3 Mg C ha^-1 relative to unmanured fields. However, field and modeling experiments suggested that manure amendments lead to large nitrous oxide emissions that eventually eliminated CO₂e benefits, whereas compost amendments continued to benefit climate for decades longer. An LCA identified a broader range of climate impacts. When scaled to an area of 25% of California’s rangelands, new C sequestered following compost amendments (21 million Mg CO₂e) exceeded emissions from cattle (15 million Mg CO₂e); diverting organics from waste streams to amendments led to additional GHG savings. In collaboration with our partners, our research contributed to the development of a protocol for compost amendments, which is being used by stakeholders in C markets and by government agencies in climate action planning. In summary, we hope that our research and related activities will serve as a “call to arms” to the scientific community by highlighting a new and much needed arena for rigorous scientific research.