Potential of carbon storage in soils

Soil organic carbon (SOC), the main element (55–60%) in soil organic matter, is crucially important for soil function. The richer a soil is in organic matter, the better it retains water and the less vulnerable it is to erosion. Organic matter also serves as the trophic resource for soil organisms, and its biotransformation and mineralization by micro-organisms release nutrients (nitrogen and phosphorus) that are vital for plant growth. SOC also plays an important role in climate regulation. Soils contain about three times more carbon than the atmosphere (2,400 GtC versus 890 GtC), and any variation in SOC stocks, either up or down, influences the concentration of CO ₂ in the air and therefore the climate.

Given its importance for the quality of soils and the climate, soil carbon has drawn much political attention in recent years. One outcome of this interest was the launch, at the 2015 UN Climate Change Conference (COP21, of the French “4 per 1,000” initiative ( http://www.4p1000.org/ ). This program is designed to promote concrete actions to increase organic carbon sequestration in soils and thereby improve food security while also mitigating global warming.

The launch of this initiative prompted intense scientific debate on the potential of carbon sequestration in soils and on the various technical and socioeconomic breakthroughs that would be needed to exploit this potential effectively. In this article, we first describe SOC and its dynamics. We then look at how SOC stocks have evolved in the past and how they are expected to evolve in the future. Lastly, we assess whether intentionally increasing SOC stocks to offset greenhouse gas emissions from human activity is realistic or merely a pipe dream.