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. Kenneth Olson, soil scientist at the University of Illinois, says the problem is how the studies measured changes in soil organic carbon (SOC, which is about 50% of soil organic matter by weight). According to Olson, these long-term studies compared the soil carbon measurements of no-till (NT1 in figure 1) to moldboard plowing (MP). They concluded that carbon was sequestered in the soil under no-till but not in tillage systems. Figure 1 shows 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. 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 a snapshot measurement of SOC does not tell the whole story.
He 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 (in science-speak: the plot averages are not significantly different). The treatments are applied for 10 to 20 years, then SOC levels are measured again (point B on the graph). Olson argues that SOC levels will change in both treatments because SOC is rarely steady, even over long time periods.
He points out that SOC levels in the moldboard plow treatment (MP) will often be lower at time B than A, showing that carbon is lost in this system. Olson found in his own research, and suspects the same in other studies, that SOC levels in no-till (NT1) fields also decreased, but at slower rates than the MP soil. 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.
In other words, 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 complex rotations, make it unlikely that continuous no-till 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.
Reference
Olson K.R. (2013). Soil organic carbon sequestration, storage, retention and loss in U.S. croplands: Issues paper for protocol development, Geoderma, 195-196 201-206. DOI: 10.1016/j.geoderma.2012.12.004
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Boy does this turn things on their head! It still shows that no-till (which in a lot of cases means glyphosate tolerant crops) is better for SOC levels than plowing (at least MP*) but it means that it is incorrect to say that no-till increases SOC. I hope agronomists take note and adjust their study design so we can have better information moving forward.
*how comparable is moldboard plow to other plow types? Are there plows that result in levels of SOC loss between no-till and MP?
Andrew,
I have never seen NT implemented solely as a carbon sequestering scheme. Instead, it has been primarily promoted as an erosion control measure and, more recently, as a means of reducing fuel use and compaction through fewer equipment passes. I don’t see how any agricultural system can result in a net sequestration gain, but they may, as you state, minimize carbon loss. Is this your experience too? Is there really a push out there to use NT as a sequestration method?
I question that comes to mind when considering no till, soil health, etc. Is the goal for maximum soil health or for maximum continuing yield?
Is organic carbon a really good metric of soil health?
Plants get carbon from the atmosphere anyway… Would organic nitrogen be a better characteristic of soil fertility?
That was the impression I had as well when no-till is being discussed (reducing erosion, pollution and dead zones in waterways) One still needed to plant cover crops or fertilize to return depleted carbon and other minerals.
I can only view the abstract and do not know what treatments (i.e. cover crops, imported manures, etc), if any, along with the tillage methods were compared in the study. It would also be interesting to compare tillage methods to a third no-farming at all treatment whereby native plant communities are seeded and allowed to exist undisturbed, escept for replicating grazing and manure deposit, over the study period. There would be some advantage in that there is not a depletion of soil nutrients via the harvesting of crops and transporting nutrients out of the system. Even then, is is reasonable to anticipate that the rate of SOC increase in the restoration treatment will be very slow and the rate would slow as the sequestered soil carbon trended toward an equilibrium? After all, pristine native soils had thousands of years to reach their soil carbon levels prior to disturbing the soil for cultivation. If the only method to achieve increased soil carbon is to abandon farming the land, lower harvestable expectations, import composts from offsite (i.e. steal potential soil carbon from other sites), pair tillage with carbon sequestering cover crops, or a combination of these, what is the practical value of the knowlege that no-till alone does not necessarily improve soil carbon over time, or at least is not as good at restoring soil carbon than abandoning rowcrop farming altogether,outside of carbon sequestration markets.
I would imagine the author is not arguing that no-till is pointless, he is only interested in measuring whether any induced soil carbon storage is achieved. I would assume there is near absolute consensus among soil scientists and agronomists that reduced tillage has many economic and ecological benefits whether or not there is a net increase in carbon storage. Additionally, although I anticipate that this study will be latched on to by organic proponents and anti-biotech activists, I am not sure that the study in and of itself would vindicate organic farming methodology as superior vis-a-vis conventional ag utilizing no-till, particularly if any net benefit in carbon sequestration comes as the cost of less yield or if any net increase is because of a net reduction in soil carbon storage elsewhere.
I would guess that a chisel plow, which does not invert the soil like a moldboard plow, would not result in as much SOC loss. The moldboard plow is the worst case scenario as far as soil disturbance.
Bill, yes, using NT as a sequestration method is talked about and some farmers here in the Pacific Northwest have even received payments based on their direct seeding (as they call NT here).
In irrigated ag systems in the West, where native soil carbon is very low (<1% OM), agriculture can increase soil organic carbon, so sequestration is possible, even without going to NT. Farmers here in the irrigated part of Washington state are just starting to adopt no-till and strip-till, so we don't really know how much sequestration is possible. The production of crops like onions and potatoes however, will be a limiting factor as current production requires tillage for these, at minimum for harvest.
Very good question, one that we here in the irrigated West are asking. We have high yields, and do not have the water erosion issues, in general, that the corn belt has. So, if we can control wind erosion, do we need to worry about building soil organic matter?
Lots of money has been spent trying to find the best metric of soil health. Soil organic matter/carbon is always one of the best, probably because much of the other metrics are dependent on organic matter levels. However, researchers have realized that there are differing pools within soil organic matter, and measuring these pools can tell us different things about soil health. As always with the soil, it is much more complicated than we would like it to be.
Organic nitrogen would be a percentage of organic matter, but here the size of the pools would make a difference, with younger OM having more nitrogen than older OM.
Your right, the author is not arguing that NT is pointless, just that it is not a good way to sequester carbon. It’s still a useful practice to control erosion, reduce fuel use, AND slow the loss of carbon from the soil. Cover crops would help, imported manures really help but at the cost of other land that grew the crops for the feed that eventually became manure, but did not get manure back.
Thanks Andrew. I was not aware of that aspect of direct payments.
So in irrigated western soils, you are saying the crops can produce more C than they/we are extracting? I wouldn’t have guessed that (obviously 🙂 ).
I was also surprised that neglecting pre experimental SOC measurements would be wide spread. To a statistician, taking pre experiment measurements would be intuitive 🙂 Admittedly, however, I have seen people attempt it over the years, but I’ve always tried to convince them otherwise.
Thanks. That seems to have gotten lost. I guess the soil was at it’s healthiest in the midwest when it was covered with wild prairie grasses, etc. But we can’t eat those prairie grasses.