Is no-till the best way to capture and keep carbon in the soil?

Let’s jump into a new analysis of 1061 pairs of published experimental data sets that compare how no-till (NT) and conventional tillage (CT) affect soil organic carbon (SOC) storage in terms of soil depth and over time.

The analysis found that SOC sequestration under NT was limited to the surface soil and only happens in the early years. When you include SOC in the deeper soil layers, it seems that NT causes SOC storage to decrease in the entire soil profile compared with CT, but these decreases dissipate over time. And in the end, analysis of the data sets show that after 14 years, SOC changes in the entire soil profile under NT were minimal, from nothing to only 1.07 Mg/ha being added.

This means that studies of NT and SOC going forward should look at the long term and down the soil profile – and that SOC models that only account for the top 15 or 20cm of soil are misleading.

What’s happening deeper down

Decreases in SOC storage at depth following the adoption of NT are not necessarily surprising, say these scientists, due to slower incorporation of crop residue into the deeper soil layers under NT. Of course, tilling mixes crop debris (carbon) into the soil, and not tilling mostly leaves it on top of the field.

But it’s a little more complex than that. Because it’s not constantly disturbed with plowing, NT soils develop more stratification, more distinct layering, and this might limit root growth. Less root growth means less carbon from decomposed roots is present in the soil.

But by itself, “stratification severely hinders the transfer of dissolved organic carbon from topsoil to move into the lower layers,” says study leader Dr. Andong Cai, of the Key Laboratory of Agricultural Environment at the Chinese Academy of Agricultural Sciences.

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Degree of stratification depends on farm location

However, Dr. Mario Tenuta, professor of Soil Ecology at the University of Manitoba in Canada, notes what whilst soil stratification is a natural process that results from not tilling, degree of stratification depends on farm location – the soil types that are present and how much natural mixing occurs such as from earthworm activity.

He explains that “there are more macropores in the soil when you don’t till, and carbon-rich crop residue can be brought down into these areas by earthworms if they are present.”

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Rate of decomposition

There is also a difference in the rate of decomposition of what carbon there is in NT soil profiles – from crop residue being pulled down by worms, from decomposing plant roots, from microbe activity near the soil surface – compared to the rate of decomposition from the larger amount of crop debris in tilled soils.

In short, the rate is much slower in NT soils, explains Dr. Jonathan Sanderman, one of Cai’s colleagues on the study and a senior scientist at the Woodwell Climate Research Center based in Massachusetts, US.

NT soils are generally colder, adds Tenuta, and so microbe activity to mineralize (consume and release organic matter as CO2) carbon is less. The crop debris holds snow into the spring, and those wetter soils don’t heat up as fast. Tilled fields also tend to be black from turning up of the topsoil to the surface. Black absorbs more heat compared to the surface of NT fields, which are generally brown in colour.

However, Tenuta notes that on the surface of NT soils, there can be a lot of carbon concentrated in the crop debris there. This can slow the rate of decomposition of the debris so that less carbon from that process is released into the air and more can be sequestered by microbes just under the soil surface, pulled down into the soil due to earthworm action, or fine particles of residue, roots and dead organisms trapped inside aggregates.

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Accounting for mass equivalency

As described in another SOC study, adjustments for mass equivalency of soil are needed to ensure that changes in SOC are not due to sampling differences.

That is, as these scientists note, the mechanism that contributes to changes in SOC in deeper soils with CT is that tillage reduces the bulk density of soil. Tillage “essentially raises the soil surface and increases the depth of the soil profile. This leads to a larger mass of soil in the surface topsoil with NT management, and therefore a higher SOC stock when calculated on a volumetric basis.”

Tenuta notes that adjusting for mass equivalency is now standard practice in SOC studies.

On this topic, Sanderman says “you really need to account for adjustments for mass equivalency but we didn’t have the data to do this for many of the studies. In the case of NT, failing to account for equivalent mass is likely leading to an overestimation of the carbon benefits in the topsoil.”

The bottom line

It’s certain that scientists will continue to look at various aspects of how much SOC is contained in CT and NT soils.

No matter, however, how much SOC is added to the soil each year from no till – from mineralization/sequestration of carbon by microbes at the soil surface, from decomposition of plant roots, from earthworms moving bits of crop debris down into the soil – we must remember that carbon release into the atmosphere from tilling is avoided.

That is, tilling releases SOC into the air every year, where not tilling keeps it in the soil. Indeed, researchers such as this team from Brazil found a few years ago that “converting an extra 8 million hectares of cropland from CT to NT represents an estimated soil carbon storage of about 8 Tg C/year during 10 to 15 years.”

For his part, Cai notes that “long-term NT does not significantly increase SOC, but it provides many other benefits.”

Lots of agronomic benefits and soil health reasons to do NT

Sanderman echoes this thought. “Overall, there are lots of agronomic benefits and soil health reasons to do NT, we just need to understand that it’s not as significant in terms of continual sequestration as we might have believed. That is, NT is not a carbon reduction strategy on its own and there are many other things, in addition to NT, that you would do on your farm to make a bigger difference in emissions.”

Tenuta sums the SOC situation up with a quick expression: “easy to lose, tough to gain.”

Whatever SOC gains we have made with NT, we don’t want to return to frequent tilling

“It’s easy and fast to lose SOC to the air with tillage, and to build up carbon in the soil with NT or CT is slow,” he explains. “This is the thing. You can build your SOC for five or ten years, and then if the field is heavily tilled, you will lose a portion of that carbon gain to the air. Not just one year’s carbon gain, but more. That’s been documented extremely well. We have to remember this. So, whatever SOC gains we have made with NT, we don’t want to return to frequent tilling.”

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