A number of research scientists believe that the potential of our soils to sequester carbon has been seriously overestimated.
Some recent studies, estimating the potential of farmland to store more carbon through innovative soil management, are presenting an overoptimistic picture of what can be achieved and the analyses need to undergo a ‘reality check’, according to a group of leading researchers.
In a letter to the journal Global Change Biology, the group pointed out that estimates for the potential magnitude of soil carbon sequestration (SCS) vary dramatically, from very modest to very substantial.
Estimates on the high end are “unrealistic” say the team and a more rigorous approach is needed.
“When organic material is added to fields, only about one third of carbon is incorporated into the soil itself in the first year; the rest is decomposed by soil microbes and ends up back in the atmosphere,” Stephan Haefele, a soil scientist at Rothamsted Research explained.
He is one of the aforementioned letter’s authors.
Haefele added: “To achieve a specified soil carbon increase that persists for 30 years, it’s necessary to add about 10 times that much.
“So, you need to add many tons of organic matter per hectare to increase soil carbon by one tonne per hectare.”
While boosting soil organic matter has many long-term benefits for soil structure and sustainability of cropping systems, according to the letter’s authors, it is unlikely to have a substantial effect on mitigating climate change in the short-term.
Soils and carbon
The team looked in particular at a recent study which estimated the carbon storage gains that could be achieved from various practices including adding biochar and compost to soils.
Not only are substantial amounts of carbon lost during composting or biochar production, but there is also considerable uncertainty regarding availability of these organic resources.
Given the logistics and production costs at global scales, it seems overly optimistic to promote these practices as major contributors to climate change mitigation in the short-term.
Addition of compost, and in some situations biochar, can be beneficial for soil health and functioning.
But in the context of mitigating climate change, they simply represent a redistribution of organic carbon already removed from the atmosphere by photosynthesis, not additional removal, according to the researchers.
A new approach to carbon sequestration, termed ‘enhanced rock weathering’, is also being promoted.
“Some early results look promising, but much remains to be understood. It seems premature to propose it to policymakers as a practical approach that can be applied immediately,” Haefele continued.
Overall, however, the authors were supportive of efforts to better understand the potential of SCS.
Gabriel Moinet, lead author of the letter and a soil expert at Wageningen University in the Netherlands commented:
“It is critical to quantify any potential trade-offs and to provide realistic evaluations of the practical, infrastructural, social, or financial limitations to the uptake of such practices.
“However overly optimistic estimates for current technical potential can be highly misleading for policymakers and may hamper rather than aid the fight against global warming.”