Feature

 

 

The give and take of sequestering carbon
Oct.21, 2004
Carbon sequestration doesn’t happen continuously in forage systems, but productive grass stands are important to economics and the environment.
While producing and maintaining good quality forage stands is important to sequestering carbon in the soil, there is a natural ebb and flow to the process, says an Agriculture and Agri-Food Canada (AAFC) researcher.

Much like the average personal savings account, sequestering soil carbon is an ongoing process of deposits and withdrawals, says Dr. Vern Baron, a specialist in forage physiology and agronomy at the Lacombe Research Centre in central Alberta. The upper limit is influence by soil, climate and crop production factors.

“There are a lot of misconceptions about what happens with carbon,” says Baron. “Carbon is always in a state of flux. Sometimes it is being stored or sequestered and sometimes it is being released. Nature’s goal is to reach an equilibrium - strike a balance to match the amount of carbon being stored with the amount being released.” That would be a success.

A pristine stand of native grassland, for example, has likely reached that equilibrium, which means carbon credits and debits are about equal over the calendar year. At the very least over several years the carbon account will balance with a net loss in one year offset by net gains in the next.

Major greenhouse gas

Carbon sequestering is one of the often talked about pillars in reducing greenhouse gas emissions. Carbon dioxide in the atmosphere is captured by plants and stored as carbon in plant tissue and in the soil. If there is a net carbon gain over a year the crop-soil continuum is a carbon sink.

Healthy, vigorously growing forage stands, annual crops and land that is not cultivated, have greatest potential to store or sequester carbon, says Baron. Overgrazed pastures, for example, and traditional summerfallow will release more carbon to the atmosphere than is saved and are known as carbon sources.

“But even vigorously growing forages and crops will also release carbon during the year” he says. “There is a continual respiration process from plants and the soil throughout the growing and dormant seasons. What really counts in any given year is the net carbon sequestration.”

While plants potentially store carbon while they are actively growing, greatest carbon losses occur during the dormant period and even through winter. “Carbon is given off through the respiratory process of soil micro-organisms and plants themselves,” says Baron. “Respiration losses come from vegetation, roots, soil organic matter and from litter on the soil surface. It is just a natural process.”

Target equilibrium

The goal is to manage resources so carbon withdrawals (losses) don’t exceed deposits (sink), or at least balance out. “We have to keep in mind that nature is always working toward that equilibrium,” he says.

Several factors affect the amount of carbon returned to the atmosphere. Soil moisture and temperature, length of the dormant period and health of the plant stand are all part of the equation, he says.

Field cultivation, pastures that are continuously overgrazed, or forage and crop stands under drought conditions are prime for net carbon respiration. Under overgrazed and drought conditions a forage stand has reduced ability to store carbon, because photosynthesis is limited. “One of the greatest contributors of carbon sources is overgrazing,” says Baron. “If range and pastureland is overgrazed and has a poor crop canopy then it is predisposed to be an emitter of carbon dioxide in the same order as summerfallow.”

Research in Canada and the United States demonstrates the potential of a forage stand to store and release carbon. Studies measuring carbon dioxide show losses over winter range between 0 and 1.5 grams per metre square (m2 ) per day. During the dormant season average carbon dioxide losses range from 1 to 3 grams per m2 per day. While during the growing season the sequestering or storage of carbon dioxide averages 2.3 grams per m2 per day or more.

Improved forage and grazing management often increases the equilibrium point for carbon storage or the amount the carbon account can hold. Sequestration rates increase for a period of years until a steady state carbon-equilibrium is reached.

Work in the U.S. looking at pure carbon levels for rangeland under average management estimates the overall annual net rate of carbon sequestration can range from minus10 to plus10 grams of carbon per m2 over a year. Improved range management could increase the rate of C sequestration to about 50 to 150 grams of carbon per m2 per year; a rate that would last until a new equilibrium point is reached. Planting adapted and improved species on tame pastures could increase rate of C sequestration over average pastures by 100 to 300 grams carbon per m2 per year until a new equilibrium point is reached.

Dormant season

“The real issue is the length of the dormant season,” says Baron. “Whether you end the year with a net sink or net source depends on the year, the length of the dormant period, moisture and temperature. Losses during the dormant period, compared to gains during the growing season determine the net amount of carbon sequestered or lost.” In short growing regions, like western Canada, the number of effective growing days is less than the sum of winter and dormant periods.

The ebb and flow of carbon levels over the year, doesn’t discount the value of producers optimizing forage, crop and pasture growth. Properly managed forages and annual crops are more productive and more profitable and provide the best opportunity to sequester carbon, he says.

Prairie farming practices over the past 100 years, which have mismanaged pasture and rangeland through overgrazing, seen marginal land cultivated, all add to atmospheric carbon dioxide levels, says Baron. These are the areas where improved management for carbon sequestration should pay the greatest dividends.

Adopting new management practices that maintain healthy and productive forage stands on this land will reverse that trend over the next half century, he estimates.

“The role of good range management practices and improved forage productivity is to create a trajectory toward that improved carbon equilibrium,” he says. “We’re not sure how long it will take to reach that point. But improved management of these lands should result in a net carbon sink in future decades.

“We will see the biggest gains by moving away from fragile land that has been cropped or even grasslands that shouldn’t be grazed at all. Small improvements to carbon sequestration rates over the large area of pasture and rangeland in western Canada could have a large impact. We will still be gaining and losing carbon on an annual basis, but over the long haul we will reach that new, higher soil-carbon equilibrium.”

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(Article by Dr. Vern Baron, AAFC, Lacombe and Lee Hart, Meristem Information Resources, Calgary)

© Canadian Cattlemen's Association, 2004,

 

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