Much of the research data over the past 20 years has suggested that switching from conventional tillage to no-till can sequester more soil-organic carbon. Yet there’s a growing concern among scientists that no-tilling corn and soybeans without cover crops, small grains and forages in the rotation may not be increasing soil organic-carbon stocks to earlier predicted levels.

“Some studies have shown both moldboard and no-till systems are actually losing soil organic-carbon over time,” says University of Illinois soil scientist Ken Olson, who led a carbon sequestration review conducted by university researchers in four Corn Belt states. They looked at 120 published soil science and tillage research papers related to soil organic-carbon sequestration, storage, retention and loss.

No-Till Benefits

Olson says no-till systems on sloping and eroding sites retain more soil-organic carbon in the top 6 inches when compared to moldboard plowing. This is due to less soil disturbance, reduced soil erosion and decreased movement of soil organic-carbon-rich sediment off the plots.

However, Olson says the no-till subsurface layers also need to be sampled and tested to the depth of rooting, or roughly 3 to 6 feet, because the no-till subsurface layer is often losing more soil organic-carbon over time than is gained from the surface layer.

During the extensive analysis, Olson says it became apparent that there were a number of reasons for the conflicting findings. The team cited dozens of factors that need to be considered and projected it would require a highly impractical a 20- to 50-year-long study to come up with all the answers.

In the meantime, they found a study that fit the need for providing data on evaluating long-term carbon sequestration results at the Dixon Springs Agricultural Center in Simpson, Ill. This is where agronomist George McKibben conducted much of the early-day no-till research in the 1960s and 1970s. This study offered subsurface soil property data collected in the root zone before the tillage treatments were applied and then during the study.

Estimates Too High

Based on the overall review, Olson says the paired-method of comparing moldboard plowing with no-till overestimates soil organic-carbon sequestration as compared to the pre-treatment method.

“Another flaw in the paired-comparison method is that the results could not be validated where no pre-treatment baseline is available,” he says. “Because these long-term studies are used for crop-yield determinations, they need to be restarted without interruption and soil sampling can be done during the nongrowing season. Only then can the long-term experiments be used to measure soil organic-carbon sequestration rates.”