A farming practice that has been used for generations may have removed far more CO₂ from the atmosphere than previously thought.
In the Mississippi Basin, farmers have long spread ground limestone on their fields. This operation, known as liming, reduces soil acidity. It therefore helps maintain conditions that are more favorable for crops. Until now, however, its climate balance was generally counted in the wrong direction: limestone contains carbon and can release CO₂.
Liming activity.
Image Wikimedia
A team led by Yale University revisited the issue by studying more than 120 years of data. Its finding changes everything. Over the long term, liming may have removed more CO₂ from the atmosphere than it released in this vast agricultural region.
To understand this result, we need to follow what happens to limestone in the soil. By reacting with water and CO₂, the rock can form bicarbonates. These dissolved substances then circulate through the soil, groundwater and rivers. Some eventually reach the ocean, where the associated carbon can remain stored for a long time.
The calculation becomes significantly different when we ask what would have happened without limestone. Fertilizers and certain types of pollution make soils more acidic. Without liming, this acidity would have triggered reactions capable of releasing CO₂. The researchers therefore compared the two situations.
The result is far from negligible. Since 1900, liming agricultural land in the Mississippi Basin may have removed around 300 to 400 million tonnes of CO₂. This estimate covers an immense region that contains a large share of the United States' cultivated land.
The balance depends on the soil's acidity, its composition and the movement of water. Some CO₂ may also be released at first, before the balance becomes favorable over time.
This discovery could change how agricultural emissions are accounted for. Common methods generally consider liming a source of CO₂. The researchers now propose taking into account the reactions that would occur even without liming, as well as carbon transported in dissolved form.
The next step will be to determine where this practice actually offers a climate benefit. The results obtained in the Mississippi Basin cannot be directly applied to all agricultural regions. Local measurements of soil and water will be necessary before estimating the potential elsewhere.