During the 20 year span of global warming policy debates, climate scientists have used an estimate of soil organic carbon sequestration rates suggesting that soil organic carbon can be sequestered by simply switching from moldboard or conventional tillage systems to no-till systems.

More accurate analyses find that is not so; no-till systems in corn and soybean rotations without cover crops, small grains, and forages are not increasing soil organic carbon stocks as much as estimates. Worse, they may be an environmental negative. The review was conducted by a team of researchers who studied the published soil science and tillage literature related to soil organic carbon sequestration, storage, retention, and loss. They also propose a more accurate definition of soil sequestration.

After examining hundreds of original research and summary papers, 120 papers on all sides of the soil organic carbon sequestration, storage, retention, and loss issue were selected for review and analysis. 

Corn in Dixon Springs Agricultural Center, Illinois. Credit: University of Illinois

"Some studies have shown that both moldboard and no-till systems are actually losing soil organic carbon stocks over time," said review leader Ken Olson from  the University of Illinois, who explained that the difference between the no-till and moldboard plots at the end of a long-term study is only a measure of net soil organic carbon storage difference between treatments and does not support soil organic carbon sequestration claims.

No-till systems on sloping and eroding sites retain more soil organic carbon in the surface from 0 to 15 centimeters when compared to moldboard as a result of less disturbance and less soil erosion and transport of soil organic carbon-rich sediment off the plots.

"The subsurface layers also need to be sampled and tested to the depth of rooting or 1 or 2 meters," Olson said. "That no-till subsurface layer is often losing more soil organic carbon stock over time than is gained in the surface layer."

During the analysis, Olson said that it became apparent that there were a number of reasons for the conflicting findings, including the definition of soil organic carbon sequestration used by different researchers. 

The team proposed the definition of soil sequestration be: the process of transferring CO₂ from the atmosphere into the soil of a land unit through unit plants, plant residues, and other organic solids, which are stored or retained in the unit as part of the soil organic matter (humus). To claim soil organic carbon sequestration, management practices must lead to an increase in the net soil organic carbon from a previous pre-treatment baseline measurement and result in a net reduction in the CO₂ levels in the atmosphere. Carbon not directly originated from the atmosphere (from outside the land unit) cannot be counted as sequestered soil organic carbon. These external inputs may include organic fertilizers, manure, plant residues, topsoil, or natural input processes such as erosion of a sloping soil and sediment-rich carbon deposition on a soil located on a lower landscape position or in a waterway. The land unit could be a plot, plot area, parcel, tract, field, farm, landscape position, landscape, wetland, forest, or prairie with defined and identified boundaries.

The team identified a number of other methodological factors that could lead to errors in reported soil organic carbon sequestration rates, including: using inappropriate experimental methods; not collecting and testing the deeper surface layers; lack of soil bulk density measurements; not accounting for carbon in amendments being loaded on the plots from external sources; use of different soil organic carbon laboratory methods over the long-term study; effects of soil erosion; transport and deposition on the experimental tillage plots; lack of sloping and eroding sites included in summary studies; natural variability that was not captured by the sampling scheme; only sampling the plot areas once when trying to determine rate of change; insufficient frequency of sampling; and relying on an assumption that after 100 years of cultivation and before the tillage treatment was applied that the soil organic carbon had dropped 20 to 50 percent but was now at a steady state.

There remain a lot of assumptions in environmentalism, and they are invariably inclined to be weighted against conventional agriculture.

Olson said that aeration, drainage, tillage, disturbance, more intensive crop rotations, use of synthetic fertilizers, erosion and lack of cover crops can all result in reduced soil organic carbon stocks.

Because it would take 20 to 50 more years to design and run such a study, the team found a long-term study that had all the required soil property data collected for the root zone or to a depth of 1 or 2 meters from before the tillage treatments were applied and sampled frequently during and at the end of the study. This study is located at U of I's Dixon Springs Agricultural Center with tillage plots that are part of a North Central Region Soil Erosion and Productivity Committee study and located on a Grantsburg soil with a fragipan at 75 centimeters, moderately eroded, on 6 percent slopes and with six replications.

Olson said that the accuracy of determining soil organic carbon sequestration depends on the method used. "In this review, both the paired comparison method and the pre-treatment soil organic carbon method were tested using the same plots and experiment," he said.

The results of this comparison showed that the paired-method (no-till compared to moldboard) overestimated soil organic carbon sequestration as compared to pre-treatment method, where both no-till and moldboard compared to the same pre-treatment baseline. "Another flaw in the paired comparison method is that the results could not be validated where no pre-treatment baseline is available," Olson said.

The team of researchers recommend: (1) that researchers trying to determine and measure soil organic carbon sequestration rates no longer use the comparison method and adopt the pre-treatment soil organic carbon method, and (2) that existing long-term studies that researchers want to use to determine soil organic carbon sequestration rates be stopped temporarily and sampled following the soil organic carbon sequestration protocol outlined in their article.

"Because these long-term studies are used for crop-yield determinations they need to be re-started without interruption, and soil sampling can be done during the non-growing season," Olson said. "Then the long-term experiments can be used to measure soil organic carbon sequestration rates."