Determining the climate change mitigation potential of grassland soils

Background/Question/Methods

The fields of ecology and biogeochemistry hold tremendous potential to contribute to our understanding of climate change. The majority of research to date has focused on describing the problem – estimating rates of carbon (C) losses and greenhouse gas 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 ecological and biogeochemical potential of grasslands to help mitigate climate change. We explored direct effects through enhanced net primary production (NPP) and soil carbon (C) sequestration, and indirect effects through diversion of high emitting sources to lower emitting organic matter dynamics. Our research incorporated long-term field experiments in California annual grasslands, short-term experiments, modeling, laboratory assays, biogeochemical modeling, life cycle assessment (LCA), and meta-analyses and was designed in consultation with a diverse group of stakeholders from both the private and public sectors.

Results/Conclusions

Organic matter amendments increased soil C storage by 0.5-1.0 Mg C ha^-1 y^-1 in surface soils over 5 y, increased NPP, and led to gains of 1.0 Mg of new C ha^-1 y^-1 over 3 y. Long-term amendments 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 grasslands, 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 greenhouse gas savings. In collaboration with our partners, our research was used to develop 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 ecological community by highlighting a new and much needed arena for rigorous ecological research.