Rotational grazing’s effect on carbon storage studied

University of Alberta researchers say interpreting grazing comparison results in the field is tricky because greenhouse gas emissions are a result of both plant and soil factors. | File photo

Researchers want to know if the use of multiple paddocks has an impact on how pastures manage greenhouse gases

Learning how different grazing systems affect greenhouse gas uptake and carbon storage is important as carbon offsets are developed.

A wide ranging five-year study examined how adaptive multi-paddock grazing (AMP), also known as intensive rotational grazing, compared to neighbouring conventional grazing in terms of carbon storage and greenhouse gas fluxes.

Dr. Ed Bork from the University of Alberta said AMP grazing can increase soil carbon but how this occurs in terms of greenhouse gas dynamics isn’t yet clear.

The study took place on sites representing a range of climatic conditions, vegetation and soils.

Bork said the team wanted to know how animal management influences all types of things, including carbon, water and soil microbial composition.

He told a webinar hosted by the Prairie Conservation Action Plan that grasslands represent the largest portion of the Earth’s surface and about a third of total terrestrial carbon stock is found in them.

How ranchers manage their cattle could affect how carbon is maintained or stored.

He said a predictive framework to link land managers’ actions to specific outcomes, in this case carbon storage and greenhouse gas fluxes, is necessary.

Previous studies compared continuous grazing with specialized grazing, but binary comparisons defeat the purpose, Bork said.

“They do not fully document the full range of variability among producers in terms of their behaviour,” he said.

This study involved identifying AMP producers in the three prairie provinces who were interested in participating and then randomly selecting smaller cohorts. For each, a non-AMP grazer in the same area, with the same soils and climatic conditions, was also selected. Data was gathered from 60 ranches.

About 80 percent of the AMP ranches and 70 percent of the non-AMP ranches had been cultivated at one time, Bork said, with an average 19 years since planting to forage.

The AMP producers were more likely to have seeded native species. About 20 percent of them had at least one native plant in their mix and they were twice as likely to use a mix of at least five species.

“AMP operators were much, much larger in size,” Bork said. “They supported a larger total herd.”

The AMP ranches averaged 60 pastures per operation, compared to less than five for non-AMP ranches. Not surprisingly, the AMP pastures were much smaller at about 50 acres compared to 300 acres for the others.

“There is no difference in the mean stocking rate,” Bork said, although AMP ranches were slightly higher. “What is very different is the mean stock density.”

AMP ranches were able to maintain much higher stock density.

AMP grazing began earlier, around April 25, compared to May 17 for the non-AMP ranches, and the grazing seasons were about seven months and four-and-a-half months, respectively.

In terms of the length of time in a given pasture, AMP producers aimed for less than eight days, and often less than four days, Bork said, which provides a much longer rest period for the pasture.

“Most of them had 60 days of rest before coming back into that individual paddock to regraze,” he said. “The neighbouring producers were exactly the opposite. The majority had a minimum rest period that was quite short.”

Researchers developed a days-of-rest to days-of-grazing ratio as a possible measurement of grazing adaptation. Bork said each ranch is individual and specific metrics likely better represent grazing impacts.

To look specifically at carbon storage and greenhouse gas fluxes researchers used two approaches.

One was an incubation trial, in which soil was removed from the field and incubated in the lab, and the other was a field trial.

This work took place on only Alberta study sites from 2017-19.

In the field, the grazing systems themselves didn’t show differences in greenhouse gas fluxes, but management and environmental factors contributed.

Bork said greater stocking rates resulted in higher carbon emissions, regardless of grazing system.

In an email, he said that interpreting results in the field is tricky because greenhouse gas emissions are a result of both plant and soil factors.

“The net effect of no change in emissions for a given GHG could be due to offsetting results of soil and plant biological activity,” he said.

The lab incubation study showed that soil from the AMP grazed areas, particularly those that were warmer, took up more methane. Because there were no plants in the incubation, soil microbes were likely at work.

Dr. Bharat Shrestha, also from the University of Alberta, noted that methane uptake was greater in grasslands that had never been cultivated.

“AMP grazed grasslands had greater carbon sequestration in the topsoil,” Shrestha said.

“Carbon sequestration improved specifically from the combination of increasing cattle stocking rates plus longer rest after grazing.”

Bork said the results showing that soil carbon is higher in AMP grazed areas could be because that system accumulates more plant biomass. Over time, the effect could be a gradual building of soil carbon.

A recording of the full presentation is available at www.pcap-sk.org.

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