"There are many challenges or difficulties when it comes to accurately measuring carbon in the soil," says Soil Scientist Katie Lewis, Texas A&M AgriLife Research and Extension.
Lewis discussed three potential issues at the recent Texas Alliance for Water Conservation Field Day in Lubbock.
The first challenge is the depth at which carbon is measured, she says. "If we're taking a shallow sample, say zero to 4 or 6 inches, that is the most microbially active depth within the soil, so there's going to be such a high turnover that it's hard to get a good estimate of how much carbon is going to be stored.
"The best recommendation would be to take a deeper soil core to estimate carbon stocks, say to a 36-inch depth in the profile – 3 to 4 feet would be the best recommendation. Deeper in the profile, you're going to have greater potential to store long-term, more recalcitrant carbon because it's protected from disturbances and microbial activity."
The second issue is settling on a method to determine carbon concentrations. "There are different labs across the U.S. that use different methods that work best for their operation. Labs may use a dry combustion technique to measure the total carbon in the soil. But we have to account for the inorganic fraction so that we can determine the organic fraction," Lewis explains. "We don't want to measure calcium carbonates in the soil as part of organic carbon."
Other labs prefer to measure organic matter and use estimates to calculate organic carbon, she says. "As a scientific community, we would have to decide what method is going to be the most appropriate and is going to do the best job of determining carbon across the U.S., and we would have to settle on one method."
Consistency across labs is vital. If each lab is doing something different then, "your numbers are going to be underestimated in some cases and possibly overestimated in others."
The time of year carbon samples are gathered also is a concern. "Depending on when that sample is collected, you can get very different numbers throughout a growing season."
For example, if the soil is sampled while a crop is actively growing, Lewis says a lot of the carbon that's being photosynthesized, captured from the atmosphere by the plants, will be released through the roots as carbon exudates. "If you collected a sample at that point, you're going to have this massive overestimation of the long-term carbon stored in the soil."
How often samples are taken has yet to be determined as well, Lewis says.
"As a soil scientist, I think it needs to happen once a year at the same time in between crops. So, if you have a crop rotation of cotton and sorghum, you need to come out between that transition, between harvest and planting the following crop. It's the same with cover crops. You need to wait until the cover crops are terminated and before you plant your cash crop."
"Something that weighs heavily on me is that if we look at the carbon numbers in the High Plains of Texas, it might appear we have a small potential to make a large impact. But I'm a cup-half-full person. Anything we can do to keep carbon in our soil and out of the atmosphere is going to have a positive impact. I would keep that in mind, that it might be a small increase in carbon you see from a year-to-year basis, but you're doing something positive for the environment.”
"Every bit helps."
To view Part 1 of Lewis's interview with Farm Press, click here.
To learn more about what Economist Darren Hudson has to say about the carbon credit market, click the links below:
Economist Darren Hudson talks about the carbon credit market, the unknowns and what producers should consider before signing a contract.