This paper reviews the evidence base around using soil organic carbon as a climate change mitigation measure. It notes that such climate solutions encompass both increasing soil carbon in soils that have not reached their maximum possible carbon content, and conserving carbon in soils that already have a high carbon content (thus avoiding losses that might otherwise have taken place).
The paper estimates that, by 2030, increasing and conserving soil carbon could provide 5.5 GtCO2 eq. yr−1 of climate mitigation potential. The figure below shows the solutions that contribute to this total. Around 40% of the total comes from avoiding soil carbon losses, e.g. from converting forests or grasslands. The authors note that avoiding soil carbon loss is an important climate mitigation strategy because soil carbon can (in many cases) be lost rapidly, but not be rebuilt quickly enough to mitigate climate change.
Image: Figure 1, Bossio et al. Additional SOC storage potential for 12 natural pathways to climate mitigation.
Only around half of the total mitigation potential is cost-effective when a carbon price of US$100/tonne CO2 is assumed (dark grey in the figure above); around one quarter of the mitigation potential is still cost-effective if a carbon price of US$10/tonne CO2 is assumed (light grey in the figure above). The measures that are cost-effective at US$100/tonne CO2 can mitigate around 7% of recent annual anthropogenic CO2 emissions.
The grazing mitigation pathways rely on (a) adjusting grazing intensity to optimise forage removal and increase soil carbon levels, and (b) sowing legumes on planted pastures to increase soil organic carbon through a fertilising effect (the reported mitigation potential accounts for increase in nitrous oxide emissions). For more on the carbon sequestration potential of grazing livestock, see the FCRN report Grazed and Confused?
The paper notes that further research is required on several issues, including:
- Better prediction of increased methane emissions when soil carbon is restored in wetlands
- Using deep-rooted grasses to build soil carbon in deeper soil layers
- Using organic waste and sewage from cities to build soil health and sequester carbon.
Mitigating climate change requires clean energy and the removal of atmospheric carbon. Building soil carbon is an appealing way to increase carbon sinks and reduce emissions owing to the associated benefits to agriculture. However, the practical implementation of soil carbon climate strategies lags behind the potential, partly because we lack clarity around the magnitude of opportunity and how to capitalise on it. Here we quantify the role of soil carbon in natural (land-based) climate solutions and review some of the project design mechanisms available to tap into the potential. We show that soil carbon represents 25% of the potential of natural climate solutions (total potential, 23.8 Gt of CO2-equivalent per year), of which 40% is protection of existing soil carbon and 60% is rebuilding depleted stocks. Soil carbon comprises 9% of the mitigation potential of forests, 72% for wetlands and 47% for agriculture and grasslands. Soil carbon is important to land-based efforts to prevent carbon emissions, remove atmospheric carbon dioxide and deliver ecosystem services in addition to climate mitigation.
Bossio, D.A., Cook-Patton, S.C., Ellis, P.W., Fargione, J., Sanderman, J., Smith, P., Wood, S., Zomer, R.J., von Unger, M., Emmer, I.M. and Griscom, B.W., 2020. The role of soil carbon in natural climate solutions. Nature Sustainability, pp.1-8.
Read the full paper here. See also the Foodsource resource How far could changes in production practices reduce GHG emissions? and the FCRN report Grazed and Confused?