A soil organic carbon primer — Part Two


The federal government announced its draft regulations for a national carbon offset framework, and while the kinks still need to be worked out, the first protocol that is intended to proceed is for soil organic carbon. As such, I’ve been working to lay out what this may entail and why carbon sequestration may not be the slam-dunk that agriculture believes it to be.

In Part One, I covered what forms of carbon exist in soil and what soil organic carbon (SOC) is, exactly. But, how it is measured and the variables that can affect that measurement create a rather complicated, imperfect system. Let’s explore.

There are a number of approaches for measuring total carbon and organic carbon in soils, which are broadly based on either the chemical or thermal oxidation of SOC.

Chemical or wet oxidation is followed by the measurement of expelled carbon dioxide or the consumption of oxidant required to quantitatively oxidize the organic carbon. Under acidic conditions, any chemical or wet oxidation methods that measure expelled carbon dioxide will also include carbonate carbon and will be a measure of total carbon.

In dry combustion methods, samples are heated to high temperatures, usually exceeding 850 to 1000 degrees C in the presence of excess dioxide. Under these conditions, all carbon present in SOC and carbonate is quantitatively converted to carbon dioxide.

For both methods where carbon dioxide is measured, a correction for carbonate can be made from a separate carbonate measurement or the carbonate may be removed with acid before carbon analysis.

A LECO C632 dry combustion furnace

Previously, in my own work, I have experience using the second method to measure SOC by removing carbonates from soil samples before measuring carbon dioxide during dry combustion. If not enough of the acid is applied to remove all of the carbonates, you can end up with erroneous measurements. That’s one point of entry for error, while another is the temperature at which the furnace is set.

Another source for error comes inherently with the nature of soil variability within a field. SOC can change from one square foot to the next, so remember that even this type of measurement is still an estimate.

Monitoring SOC can be labour intensive because of the number of samples needed, and analysis intensive in the lab. The final estimate of SOC is based on the equivalent soil mass and these final numbers based on soil density are another error-prone, highly variable measurement. This is one of the weaknesses of a current method to measure SOC, as identified by the Soil Carbon Balance project from the Saskatchewan Soil Conservation Association.

Although there are challenges within the standard methods to measure SOC, there are ways to change the protocols to eliminate some of these errors. Some combustion furnaces can measure the total inorganic carbon, switch to a different gas, and then measure the organic carbon component, which would eliminate the acid treatment step. It would level the playing field for any type of soil analyzed, which would be more optimal for an across-Canada analysis program.

In a briefing published on March 5, the federal government stated that the Enhanced Soil Organic Carbon protocol “aligns with increased interest in potential climate benefits from the adoption of regenerative agriculture land-management practices that go above and beyond business as usual.”

As part of the development process, Environment and Climate Change Canada says that members of the public will have opportunity to comment on draft protocols until May 5.

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