120 Years of Data Recast Farm Liming as a Net Carbon Sink

A Yale led study published in Nature has reached a counterintuitive conclusion about one of agriculture's oldest soil management practices. The researchers estimate that applying agricultural limestone across the Mississippi River Basin removed a net 300 to 400 million metric tonnes of carbon dioxide from the atmosphere between 1900 and the present.

The scale of the study is unusually large. The Mississippi River Basin covers about 41 percent of the contiguous United States and contains roughly 65 percent of U.S. cropland. Farmers have used crushed limestone there for generations to raise soil pH, reduce acidity and support crop yields.

The climate mechanism is tied to weathering chemistry. Carbonate rock reacts in soil and water and can convert carbon dioxide into bicarbonate ions that move through groundwater and rivers and can ultimately reach the ocean, where the associated carbon may remain stored for long periods.

The key methodological change was the study's comparison with a world in which farmers did not apply lime. The researchers accounted for acidity created by fertilizer use and air pollution, which would still drive chemical reactions and carbon dioxide release without liming. Against that baseline, the long term effect of lime became net carbon removal.

That result challenges conventional greenhouse gas accounting for agricultural lime. The Intergovernmental Panel on Climate Change default methodology treats the carbon contained in applied lime as emitted carbon dioxide. The Yale team argues that a more complete system should account for both emissions and the subsequent removal processes.

The authors also stress that the Mississippi Basin result should not be copied automatically to every farming region. Soil acidity, buffering capacity, hydrology and the timing of emissions and removals can materially change the climate outcome in another location.

For agribusiness, the finding creates a potential link between a familiar yield and soil health practice and carbon finance. The researchers say many farms cannot afford optimal pH management, so better measurement of verified carbon removal could eventually affect how liming is valued and financed.