Scientists look to grazing to aid carbon retention in soil

The symbiotic relationship of plants, soil and animals has long been understood among those living off the land.

Soil with high carbon content has increased fertility, water retention and productivity.

Now scientists are assessing the value of grazing on grasslands and that system’s ability to sequester more carbon in the soil and stave off greenhouse gas emissions.

A carbon benchmarking study to quantify the carbon pool associated with grasslands across Alberta was recently completed.

“We are building on that by looking at trying to understand the mechanisms as to where, when and how grazing specifically leads to greater carbon,” said Ed Bork, director of the Rangeland Research Institute at the University of Alberta.

Carbon sequestration is the process of storing carbon in the soil through crop residue and other plant material. It depends on vegetation respiration and photosynthesis. Carbon enters the soil in the form of litter, harvest residue and manure. These are stored in the soil as organic matter and are typically found in the top 10 centimetres.

The research began by using rangeland reference exclosures run by Alberta Environment — more than 100 small, enclosed parcels of land used to monitor grassland composition and productivity over the long term. They have never been cultivated but in some cases there are some additional species like clover, bluegrass and brome seeded into them.

The university researchers extracted soil cores to look at the carbon profile. They also compared similar results from native grasslands, tame pasture and adjacent cropland.

Past studies did not have as many sites to compare the wide variety of soil types and different agri-climatic conditions on parkland, fescue grasslands and dry, mixed prairie.

They are also interested in comparing carbon sequestration with land that has been farmed. Depending on the type of cultivation, losses of carbon are significant and take up to 50 years to come back.

“If we take a native grassland and flip it into an alternate land use, particularly annual cultivation, there is a big penalty to pay in terms of the amount of carbon that is lost from that system,” Bork said.

“Our data is suggesting it is typically 40 to 45 percent but other studies worldwide suggest it is even 50 to 60 percent lost,” he said.

When land is broken up the soil temperature increases and there is more oxygen present. That encourages more microbial activity that breaks down material and releases more carbon into the atmosphere.

Eventually, the soil will reach to a new equilibrium state.

“If you vegetate it back to perennial cover, it is going to take a long time to rebuild that carbon in the system. It means the potential opportunity costs of lost carbon through land-use conversion is not a short-term thing, it is a potentially long-term thing,” he said.

“The ultimate sequesterer of carbon is plant productivity so the more plant productivity you have, the more carbon you are going to be putting into the system,” he said.

The next round of research is evaluating the effects of adaptive multi-paddock grazing (AMP) or management intensive grazing using high animal densities on a tight rotation. This form of intensive management is supposed to stimulate plant growth and so more carbon should be added.

Many experienced livestock grazers say they have improved soil health and fertility, as well as water filtration but they do not have empirical data to prove it.

“Our push is really to drive the development of a database that will create a defendable policy change in support of valuing and rewarding producers for not only maintaining but building carbon in the soil,” he said.

Policy makers are reluctant to put a price on existing carbon without that information.

Researchers like Bork want to show the value of the forage supply to livestock, as well as the true value of existing environmental goods and services from existing native grasslands.

Often overlooked are services like water storage and purification and biodiversity, as well as long-term carbon storage and greenhouse gas storage like methane and nitrous oxide.

Research in the montaine grasslands of British Columbia is producing similar results.

Current research is based around climate change and its mitigation through grasslands management and grazing, said researcher Lauchlin Fraser of Thompson Rivers University in Kamloops. He has studied soil carbon sequestration in grasslands for 10 years.

“Carbon sequestration is one of the potential ways we can mitigate climate change by pulling carbon dioxide out of the atmosphere. If we can do that through good range management practices and if there are markets for that, ranchers should receive some funding to support and encourage those management decisions,” he said.

“We need to make sure that first of all we can measure soil carbon effectively so we can monitor it and demonstrate that where, when and why those changes occur in soil carbon,” he said.

“A big driving factor is looking for those positive benefits of cattle on the land and how we can maximize that,” he said.

Research sites in B.C. included about 300 exclosures, which are called range reference sites run by the province. These were located in the Kamloops area, as well as the regions of the Kootenays, Cariboo, Chilcotin and Peace River District offering different soil types and precipitation levels.

“We think soil carbon storage is very context dependent on soil type and the climate,” he said.

For example, less carbon is stored in the semi arid grasslands of some parts of the province compared to wetter areas.

In addition the researchers are working with seven co-operating ranchers who practice intensive management grazing, as well as rest rotation on their own land.

Many of the ranch sites are former hayfields and some were irrigated. The producers have moved away from baling those sites and focus more on grazing.

“Rather than putting your hay up there is planned managed grazing practices and over winter you buy your hay and you bring that hay onto your own site. You are actually bringing fertilizer onto your own land,” Fraser said.

Year round grazing is not feasible because of cold and deep snow.

“We are linking grazing practices with soil carbon but some of the issues that we find is because we are following operational practices by ranchers, we don’t have the control,” he said.

Science needs some control sites so they can collect data and create patterns.

“When we are all using the same methodology at the same time, then we can make much more powerful results and make more powerful conclusions from that,” he said.

In addition, the B.C. project used a remote sensing system called normalized difference vegetation index to predict soil carbon across B.C. grasslands.

It measures the greenness of plants, which is tightly correlated with productivity.

By measuring it along with mean annual precipitation and elevation, these three properties can explain with up to 70 percent accuracy how much carbon is in the soil on a particular landscape.

A lot of grazing in B.C. takes place in forested areas under a canopy of trees. Fraser’s team also wants to learn if there is a relation to carbon sequestration as well as productivity in those areas.

“Forested range is huge in B.C. and so we are also looking at agro-forestry practices and we’ve submitted another grant to the BCRC to determine whether silvapasture and forest thinning can increase forage productivity,” he said.

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