Accurate soil carbon measurement means digging for details

September 2018

Among the major variables supporting the science around climate change is the amount of carbon that is stored in the world’s soil. Predictions about the state of our environment regularly hinge on how much of this crucial element is locked away or may be released by future warming.

But while measurements of soil carbon may be widely cited in elaborate climate models, the techniques used to obtain these figures could well require a more critical assessment. According to Cole Gross, a doctoral student in the Department of Renewable Resources at the University of Alberta, the amount of information being yielded by samples does not merit the value that scientists want to assign to it.

 Dauren Kaliaskar

University of Alberta doctoral student Cole Gross uses an elemental analyzer to measure soil carbon. Photo credit: Dauren Kaliaskar

“We’re dealing with small numbers, both for bulk densities and the nutrient concentrations, and scaling them up to cover hectare-size plots of land,” he explains. “The biggest problem is that we’re not looking at soil deep enough. Many studies are just looking at the dynamics in the topsoil. Most trees root deeper than three metres, and yet the bulk of our knowledge of soil deals with the top 20 cm.”

That 20 cm limitation is common, notes Gross, who refers to the guidelines of agencies such as the US Forest Service, which fund much of the work that goes into collecting soil samples for carbon assessment. Going beyond this depth is more expensive, but it may be the only way of gaining a more complete understanding of just how much carbon there is in soil. 

And while the process of collecting samples may look like little more than digging in the dirt, the steps in obtaining accurate numbers can be far from straightforward. The simplest method is taking a core, which provides a known volume of soil. Once it is returned to a lab, each core is often air dried, to avoid heating any of the elements it contains, then ground up into a fine powder that is placed in an elemental analyzer. Such instruments use thermal conductivity or spectroscopic methods to determine the adsorption of combustion gases, which then yields mass fractions of carbon, hydrogen, nitrogen, and other elements.

When coring is not possible, Gross notes, sampling becomes more challenging. In soils that form clods, irregularly shaped clods of soil can be selected and carefully sealed with plastic wrap to prevent any moisture from escaping. Their volume is determined by dipping them in paraffin wax, then inserting them in a graduated cylinder of water. Finally, the clods are oven-dried to determine the moisture content of the soil. Because this process is destructive, subsamples of each clod are taken prior to these analyses and are air dried for elemental analysis.

Apart from the potential errors that could enter into such measurements, Gross insists there must also be an awareness of what else is happening to the soil. If sampling is taking place on agricultural land, it may well have a been recently tilled. If so, it will be less dense than deeper soil that was not affected by plowing.

“Your data might show that your stocks of soil organic carbon decreased,” Gross argues, “when in fact you just measured less soil.”

The most reliable way of gauging chemistry is likely the most ambitious — excavating large amounts of soil using shovels or heavy machinery. This ensures both depth and sufficient volume to provide reliable readings. Unfortunately, it is also the most expensive approach, and may not lend itself to working in more remote areas or areas where limiting soil disturbance is desirable.

As coring is still the most used and easiest of the soil sampling methods, Gross suggests using either the clod or excavation method to assess the potential of the core method to underestimate soil mass in a given soil and then adjusting to account for any discrepancies. He further stresses that if the soil is only sampled shallowly, the results will most likely underestimate the soil carbon. Nevertheless, he appreciates just how much digging deeper would demand.

“It’s a real challenge, not just physically to get down deeper,” he concludes. “It’s more soil, it’s more samples, it’s more labour, and it’s more money.”