‘The best management systems for soil carbon are the ones that we don’t touch.’
By Emily Croft
Farmers are sometimes criticized in environmental sustainability conversations, but soil carbon sequestration is one opportunity to turn that around.
Carbon sequestration is the removal of carbon, in the form of carbon dioxide, from the atmosphere, where it will then be stored in another form, often in soil or plants.
Some producers are now seeing that in addition to improving public perception of the industry, managing farmland to increase soil carbon also benefits crop and pasture productivity, drought resilience, and erosion.
How does soil store carbon and how can farmers reap the benefits?
What is carbon sequestration?
There are a few ways that carbon is stored. It can be stored in soil or plant material, and can be organic or inorganic, and living or dead.
Cedric MacLeod, executive director of the Canadian Forage & Grassland Association, says that the process of carbon storage involves the interaction of a number of factors.
“It’s a complex interaction between plants, soil, and the sun,” explains MacLeod.
“Photosynthesis generates sugars, and plants use carbon and put it into their roots. When a plant dies, the roots stay behind, and that becomes organic matter through an interaction with many microscopic living creatures within the soil structure.”
MacLeod also notes that there is an equivalent amount of plant below the ground as what can be seen above-ground.
“A corn plant grows up to maybe 14 feet tall. There’s an equivalent amount of plant below-ground as there is above,” says MacLeod.
“If you consider a 22-tonne corn silage crop and all that biomass that’s generated and harvested and put into a bunker silo, that same amount of plant material is still left behind under the soil surface and that carbon drives microbial function. They chew on the roots and mineralize the material and turn it into organic matter, which will go into the next crop – and whatever is left behind contributes to carbon sequestration long-term.”
Dr. Rafael Santos, associate professor in Environmental Engineering at the University of Guelph, and Dr. Emily Chiang, an associate professor in the School of Engineering at the U of G, explain that carbon can also be sequestered as organic or inorganic carbon.
“There are different types of carbon in soil and different ways to store them,” says Santos.
“We work with inorganic, but there’s also organic carbon. They both occur in soils, but some soils will have more organic carbon and less inorganic carbon, and vice versa. There are ways that you can interfere to store carbon or ways you can manage your land to store more carbon. They tend to be complementary, so it can be all of the above, and you can use multiple ways to store carbon at the same time.”
MacLeod’s description of sequestration refers to organic carbon, but Chiang’s and Santos’s research focuses on soil amendments that sequester inorganic carbon. MacLeod also categorizes carbon in “living” and “decomposing” fractions.
He explains that lignin, a fibrous component of the plant, is more challenging for microbes to break down and remains in the soil longer term as a recalcitrant source of carbon. Instead, the microbes target the “juicy” part of the plant, which can then be mineralized and made available, or is more susceptible to re-release.
“It’s a very dynamic system that changes quickly. Within a wet year in Ontario soils, you may have more carbon sequestration, but might also have more microbial activity that releases more,” says MacLeod.
How can producers store more carbon in their soil?
Managing the soil
Soils that are undisturbed and remain covered by plant matter tend to sequester more carbon.
“The best management systems for soil carbon are the ones that we don’t touch. If you think about an alfalfa stand on a dairy or beef operation, you plant alfalfa and leave it for four years,” says MacLeod.
“For four years, the plant grows two feet above the ground and the roots grow two feet into the ground. Then it gets cut, and then it grows again, and the roots grow again. That may be 12 times that the plant has grown and pushed that carbon into the soil.
“Now you are moving to a corn silage crop. Option 1 is to terminate the alfalfa crop and no-till corn directly into the sod. You open a narrow slot and drop corn seed in, and the corn roots grow through now-dead alfalfa roots, and that carbon stays intact.”
When intact, these root systems also allow for greater water-holding capacity during drought without waterlogging in wet years, allowing for crop resilience.
“What often happens is we have four years of alfalfa and all that carbon in the root mass, and we come in with tillage and break that sod down.”
MacLeod says that the introduction of oxygen to soil during tillage increases microbial activity, converting stored carbon back into carbon dioxide and releasing it into the atmosphere. He recommends leaving tillage equipment in storage as often as possible, and if tillage is necessary, using a minimum tillage approach.
His three recommendations for maximizing soil carbon are:
- Minimize tillage wherever possible to avoid disturbing stored carbon.
- Include perennial forages in the rotation. This is an opportunity to work with livestock producers or take advantage of the forage export industry.
- Balance soil fertility to maximize biomass production.
John Cross of a7 Ranche, near Nanton, Alta., uses a planned grazing program to maximize soil carbon and pasture productivity.
“Carbon sequestration is an outcome of healthy plants. If you have healthy plants, you’re likely going to have more carbon in the soil,” says Cross.
“So it’s more like working at having a healthy landscape, and a byproduct of that is more than likely more carbon in the soil.”
With the holistic planned grazing approach to land management, Cross says it’s essential to commit to it.
“We really do it. Lots of people say they do it, but they really don’t,” says Cross.
“We have three herds in about 120 pastures with a 60-day recovery period and an average grazing period of a day and a half. What we try to do is have a long rest period and short grazing period. Appropriate utilization is also important. You can have long rest periods and short grazing periods, but if utilization isn’t pushing land enough, you can actually go backwards.”
Cross says they have a land description for what they want their system to look like, and then they use planned grazing to achieve that and meet the needs of livestock. It is important to plan, monitor, control and replan as the needs of the land and cattle evolve.
They have started working with the Food Water Wellness Foundation in Alberta to measure the carbon in their soil to compare to other management styles and also create baseline measurements of soil carbon in the Prairies.
As his properties are resampled over time, Cross believes improving sampling systems will offer producers more opportunities to get paid for the improvements they make to carbon sequestration.
“If you can keep sampling and the carbon is still there, and you can sample lots of acres, this whole process is now a lot more robust, repeatable, and affordable,” says Cross.
Another way that a7 Ranche has measured improvements to the soil is by calculating animal grazing days per acre. The work that the family has put into improved soil management has yielded increases in productivity.
“I took over the ranch in ’86. At that time, the ranch was at about 30 animal days per acre. After using the planned grazing strategy, we are getting up to 90 to 120 animal days per acre,” says Cross.
With multiple categories of soil carbon and different ways to sequester them, research is occurring across Canada to improve and monitor carbon sequestration in agricultural soils.
Innovative approaches
Traditionally, approaches to increasing soil carbon have focused on management of crops, forages, and pastures.
Chiang’s and Santos’s research at the U of G is taking another approach to improving carbon sequestration in Ontario. Their enhanced rock weathering project uses minerals applied to crop land, similarly to other minerals or hard fertilizers, to increase the carbon removed from the atmosphere.
“Rock weathering is a natural process and is one of the ways that the planet has been balancing carbon dioxide throughout geological time,” says Chiang.
“It’s a very slow reaction, so it takes a very long time. That’s why we call our process enhanced. We would like to advance the rate of this process because human activity has been too busy releasing carbon.”
Wollastonite is a mineral that is currently mined in Southern Ontario. In their research, Chiang and Santos have found that it is ideal for the enhanced rock weathering process.
“Basically, we spread these minerals in the field and then the minerals dissolve because of plant activity and the interaction with soil and microbe. This interaction generates acid and dissolves the rock,” explains Chiang.
“When the rock is dissolved, you have calcium and magnesium, and when they react with carbon dioxide, they capture it as a solid, rather than a gas.”
Santos adds that rainwater also contributes to this process.
“Once the mineral is dissolved in water it can do a few things. It can precipitate into inorganic solids in the soil or go down through the soil to an aquifer,” says Santos.
He says the overall goal is to use the mineral to essentially develop new soil that can store carbon.
Chiang says they have begun discussions with companies and farmers to determine how this could become available in the industry.
“When you talk to growers who have used these minerals, they are very happy with crop growth. In some of our experiments we have seen more organic carbon increase as well, which leads us to believe it could improve both inorganic and organic carbon sequestration,” says Chiang.
“We want to see carbon sequestration but want to make sure it benefits growers applying the technology as well because they see the benefit in their crops.”
In addition to the observed benefits to crop quality, soil health, and carbon sequestration, Chiang says this could eventually be an opportunity for carbon credits, offering producers more payoff for their work.
“What we need now is for the policy makers to make this real and accredit this technology in the field,” says Chiang.
Another project at the University of Alberta hopes to recognize farmers for their efforts in increasing soil carbon.
Cameron Carlyle, associate professor in the department of Agricultural, Food & Nutritional Science at the University of Alberta, has been mapping soil carbon in grasslands across the Prairies.
“We had two objectives,” says Carlyle.
“One was to do a better job of quantifying the amount of carbon held in soils across anywhere used for perennial forage production for livestock. That means going out to areas that vary based on climate and location, but also capturing different types of forages to capture variation across environments and types of forage in terms of how much carbon is stored.
“The second was to identify forage management practices that increase carbon stored in soils to reduce greenhouse gas in the atmosphere.”
They are also using this mapping to model how different climate-change scenarios might affect soil carbon and find management strategies for drought resistance based on which practices store carbon best.
Improved soil carbon mapping may present further opportunities for innovation in soil management, as well as methodology for soil carbon documentation in the case of monetization of carbon storage.
In managing for better soil carbon sequestration, both crop and livestock farmers can play a role in the protection of the environment and their land. A focus on building soil carbon may also generate financial opportunities in the future, in addition to the benefits for crops, pasture, resilience, and soil health which are already seen by many farmers. BF
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