A soil organic carbon primer — Part One

With the federal government releasing details on its planned carbon offset system soon, soil organic carbon will be a key measurement, and one to pay close attention to going forward.

So far, Minister of Agriculture and Agri-Food Marie-Claude Bibeau sees the planned carbon offset system creating an opportunity for farmers to receive credit — and income — for reducing emissions and sequestering carbon.

One of the first offset protocols the government is considering is focused on organic carbon in soils. But what exactly is soil organic carbon, how is it measured, and why will that measurement matter? Those are the questions I’ll work to answer in this series.

Types of carbon in soil

Carbon in soils exists in both organic and inorganic forms. The inorganic component is made up of carbonate, in its own variety of forms. On the Prairies, inorganic C is usually found as calcium carbonate. A range of organic components make up the organic part — in essence, organic carbon is the total of all organic materials existing within and on soil particles.

Soil organic matter can be used as a proxy measurement for organic carbon, but organic matter isn’t always an appropriate measure of soil organic carbon. That’s because soil organic matter also includes living organisms that are still turning over plant residues and either using or respiring carbon dioxide. There’s an entirely different measurement for the microbial biomass and their outputs — the organisms in soil — both dead and alive.

Carbon cycling

When conditions are more favourable for plant production than for microbial decay, large quantities of atmospheric carbon dioxide used by plants in photosynthesis are sequestered into plant tissues, which eventually become part of the soil organic matter. Carbon dioxide is also lost to the atmosphere, naturally, when microbial decay conditions are more favourable.

The consistent turnover of plant, animal, and microorganism dead tissue and the synthesizing of those remains by other microorganisms is what makes humus, which is the most resistant form of organic matter to decay. The darker a soil, the more humus it contains, which is why you’ll find much higher organic matter percentages in the Black soil zone than in the Brown. Typical, well-drained surface soils contain one to six per cent organic matter.

For a measurement of the carbon in humus, the part that is resistant to decay, soil organic carbon itself must be measured.

Soil organic carbon is hard to measure, and at best, only an estimate can be obtained, but when it comes to a carbon-offset program and the potential dollar figure, an estimate is better than nothing. In part two of this series, I’ll cover how soil organic carbon is measured specifically and what variables can affect that measurement.

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