A recent paper (Olson, 2013) finds a number of long-term studies were wrong about no-till practices building soil organic matter and thus sequestering carbon. The problem, says Kenneth Olson, soil scientist at the University of Illinois, is how the studies in question measured the gains or losses in soil organic carbon (SOC; organic carbon is about 50% of soil organic matter by weight). According to Olson, these long-term studies made soil carbon measurements during or at the end of the experiments which compared the results of no-till (NT1 in figure 1) to moldboard plowing (MP). They then concluded that carbon was sequestered in the soil under no-till but not in tillage systems. The figure below represents what Olson says these studies measured.
The problem pointed out by Olson is that this scenario compares everything to the carbon levels in the moldboard plow system (MP) which is assumed to be at a steady state. As Olson states, “without…pre-treatment SOC data for the baseline treatment (MP), the SOC sequestration magnitude and rate…cannot be verified.” Olson’s point is that the snapshot measurement of SOC does not tell the whole story.
He then argues convincingly that a baseline measurement of SOC is needed in all cases to determine both the sequestration rate and magnitude of both the no-till and moldboard plow systems. When this baseline measurement (A on the bottom axis) is included, as shown in the second figure below, the conclusions can be quite different.
Starting before the treatments have been applied (point A at the bottom of the graph) the SOC levels are the same (the plot averages are not significantly different). The treatments are then applied for 10 to 20 years and then the SOC levels are measured again, point B on the graph. Olson argues that because SOC is rarely steady, even over long time periods, SOC levels will have changed in both treatments. He points out that SOC levels in the moldboard plow treatment (MP) will often be lower at B than at the A, showing that carbon is still being lost in this system. Olson found in his own research, and suspects the same in other studies, that the SOC levels in the no-till also decreased, but at a slower rate than the MP soil (see the NT1 line). Therefore, carbon sequestration, as Olson defines it, “the process of transferring CO2 from the atmosphere into the soil of a land unit through unit plants [plants growing on that land unit], plant residues and other organic solids, which are stored or retained in the unit as part of the soil organic matter,” did not occur. The no-till system is losing organic carbon, but at a slower rate than the moldboard plow system. Only if the carbon levels in the NT system increased between A and B (NT2 on the chart) could it be said that carbon was sequestered.
Olson’s conclusions, if they stand up under further scrutiny (it is a peer-reviewed paper) bring up several important points. First, they highlight the fact that agricultural systems, even those that disturb the soil the least, are still degrading compared to native conditions, at least in the eastern half of the U.S. To this, I say, of course they are. The prairie did not allow for much export of food, so why is it the target for agriculture? Let’s move beyond comparing agriculture to untouched prairie and aim for something that works for us. If no-till protects the soil from erosion, slows the loss of organic matter and still produces food, then it is the best option we have.
Second, at least where soil organic matter levels were high before agriculture was introduced, the ability of agriculture to sequester carbon to mitigate greenhouse gas emissions seems to be limited. This may affect the ability of agriculture to be a player in any future carbon sequestration market.
Finally, the situation in the arid West is different. Here, where native soils are very low in organic matter, adding irrigation and high yielding crops has the potential to increase soil organic matter. However, high value vegetable production (potatoes, onions, carrots) which at present require tillage, and the complex, dynamic rotations make it unlikely that continuous no-till, the focus of many of these Midwest long term studies, will be widely adopted in these irrigated regions.
The scientific community should review Olson’s revised definition of carbon sequestration, and if they help us get a better view of reality, adopt them and adjust our course accordingly.
Kenneth R. Olson. Soil organic carbon sequestration, storage, retention and loss in U.S. croplands: Issues paper for protocol development. Geoderma, 2013; 195-196: 201 DOI: 10.1016/j.geoderma.2012.12.004
April 2014 update: an expanded and more detailed paper (press release) on this topic has been published:
Olson, K. R., Al-Kaisi, M. M., Lal, R., & Lowery, B. (2014). Experimental Consideration, Treatments, and Methods in Determining Soil Organic Carbon Sequestration Rates. Soil Science Society of America Journal, 78(2), 348. doi:10.2136/sssaj2013.09.0412