No-till soil organic carbon sequestration rates questioned
For the past 20 years, researchers have published soil organic carbon sequestration rates. Many of these findings have suggested that soil organic carbon (SOC) can be sequestered in soil, or stored long-term, simply by switching from conventional tillage to no-till systems.
But a growing body of research indicates that no-till systems in corn and soybean rotations without cover crops, small grains, and forages may not be increasing SOC stocks at published rates.
"Some studies have shown that both conventional and no-till systems are actually losing soil organic carbon stocks over time," says University of Illinois soil scientist Ken Olson. Factors other than tillage that can cause losses include aeration, drainage, more intensive crop rotations, use of synthetic fertilizers, and lack of cover crops.
Olson and a team of senior researchers from universities in Illinois, Wisconsin, Iowa, and Ohio recently reviewed the soil science and tillage literature related to SOC sequestration, storage, retention, and loss. After examining hundreds of original research and summary papers, the scientists selected 120 papers for review and analysis.
Their review uncovered many conflicting results. For example, no-till systems on sloping and eroding sites retain more SOC in the top 0 to 15 centimeters of soil when compared to conventional systems, because the soil is disturbed less and thus erodes less. But deeper soil layers can tell a different story.
"The subsurface layers also need to be sampled and tested to the depth of rooting, or 1 or 2 meters," Olson says. "That no-till subsurface layer is often losing more soil organic carbon stock over time than is gained in the surface layer."
Another reason for inconsistent results among studies, the review found, is that different scientists use different definitions of SOC sequestration. Olson’s team proposes its definition as: the process of transferring carbon dioxide from the atmosphere into the soil of a land unit through 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 SOC is truly being sequestered, the researchers also state that management practices must cause an increase in net SOC from a previous pre-treatment baseline, as well result in a net reduction in atmospheric carbon dioxide levels. In other words, carbon that doesn’t come directly from the atmosphere but from elsewhere outside the land unit cannot be counted as sequestered SOC. These external inputs may include organic fertilizers, manure, topsoil, or natural inputs such as sediments in floodplain and depressional soils.
The team also identified a number of other study factors that could lead to errors in reported SOC sequestration rates such as not including eroding and sloping sites in summary studies; lack of soil bulk density measurements; use of different SOC lab methods over a long-term study; natural variability not captured by the sampling scheme; only sampling plot areas once when trying to determine rates of change; and several others.
A final key finding of the team’s study relates to the method used to measure SOC rates. “In this review, both the 'paired comparison' and the 'pre-treatment' SOC methods were tested using the same plots and experiment," Olson says.
The results of this work showed that the paired method (i.e., no-till versus conventional) overestimated SOC sequestration as compared with the pre-treatment method, where both no-till and conventional are compared to the same pre-treatment baseline. And "another flaw in the paired comparison method is that the results cannot be validated where no pre-treatment baseline is available," Olson adds.
The team therefore recommends: (1) that researchers who are trying to measure SOC sequestration rates no longer use the paired comparison method, and adopt the pre-treatment method instead, and (2) that existing long-term studies of SOC sequestration rates be stopped temporarily and sampled following the SOC sequestration protocol outlined by Olson's team.
Read more in the two open access articles:
Experimental Consideration, Treatments, and Methods in Determining Soil Organic Carbon Sequestration Rates," authored by Kenneth R. Olson, Mahdi M. Al-Kaisi, Rattan Lal, and Birl Lowery, in the Soil Science Society of America Journal and is available at: https://dl.sciencesocieties.org/publications/sssaj/articles/78/2/348.
"Soil organic carbon sequestration, storage, retention and loss in U.S. croplands: Issues paper for protocol development," authored by Kenneth R. Olson, in Geoderma: http://www.sciencedirect.com/science/article/pii/S0016706112004211.
Source: University of Illinois press release