The list of very knowledgeable folk who still are pushing no-till farming as a greenhouse-gas mitigation strategy — even though science passed them by a while ago — includes:
- Sen. John McCain
- Princeton University* [PDF]
- The Chicago Climate Exchange [PDF]
- The Nicholas Institute for Environmental Policy Solutions [PDF]
I buried the science in the McCain post, but it deserves higher visibility. As a major review article [PDF] from Agriculture, Ecosystems and Environment, “Tillage and soil carbon sequestration — What do we really know?” concluded:
In essentially all cases where conservation tillage was found to sequester C[arbon], soils were only sampled to a depth of 30 cm or less, even though crop roots often extend much deeper. In the few studies where sampling extended deeper than 30 cm, conservation tillage has shown no consistent accrual of SOC [soil organic carbon], instead showing a difference in the distribution of SOC, with higher concentrations near the surface in conservation tillage and higher concentrations in deeper layers under conventional tillage … Long-term, continuous gas exchange measurements have also been unable to detect C gain due to reduced tillage. Though there are other good reasons to use conservation tillage, evidence that it promotes C sequestration is not compelling.
(Conservation tillage is “broadly defined as any tillage method that leaves sufficient crop residue in place to cover at least 30 percent of the soil surface after planting.)
This is actually not especially new research. The review article went online in June 2006, and, of course, as a review article, it was based on even earlier research — including a 1981 (!) study that came to the same exact conclusion.
That study compared SOC and microbial biomass in long-term plowed and no-till cereal plots. They found no differences in either parameter between the two treatments when they sampled to 40 cm on an equivalent depth basis (equal mass per unit area), and concluded that no-till
has little effect on soil organic matter, other than altering its distribution in the profile.
Even worse, the review article notes this:
Studies that have involved deeper sampling generally show no C sequestration advantage for conservation tillage, and in fact often show more C in conventionally tilled systems.
Time to scrap no-till farming as a carbon offset or greenhouse-gas mitigation strategy.
*I confess that I relied on the Princeton “stabilization wedges” analysis myself for including this as one of the 14 or so needed to stabilize carbon dioxide concentrations below 450 ppm (see here). It was only at this April American Meteorological Society seminar, “Biofuels, Land Conversion & Climate Change,” where I learned of this review article. It just goes to show you that you should always check things yourself as much as possible with primary sources. That’s why I am trying to go through all of the major climate solutions (and non-solutions) this year as thoroughly as possible with numerous links to primary sources.