Edward Bork’s research surrounding how livestock grazing affects soil carbon has made him a believer in the beneficial role cattle can potentially play in a changing climate.
“Because their grazing contributes to the concentration of carbon in the soil – a helpful process – livestock can be a tool to help reduce atmospheric carbon and thus mitigate climate change,” says Bork, director of the Rangeland Research Institute, University of Alberta.
Cattle critics say otherwise, calling for decreases in numbers or even elimination of ruminants as a means of reducing the greenhouse gases contributing to climate change. They point to the methane cattle emit as a key polluter of the atmosphere. Methane is a potent greenhouse gas that ruminants put out as part of their digestive process.
Bork calls for a balanced view, one that weighs the drawbacks against the benefits.
“Pointing the finger at methane emissions of livestock is a convenient excuse people use,” he says. “It’s a red herring to claim that cattle are destroying the planet and ignores the fact that these grasslands evolved with grazing – and even depend on it to exist. Discussions of cattle and climate change should not talk solely about methane but should also include a full accounting of the role livestock play in storing carbon in the soil as well as in enhancing biodiversity.”
GRAZING INCREASES CARBON
The natural carbon cycle in soil begins as plants take in carbon dioxide (CO2) from the air. Plants use the carbon to make roots, shoots, and leaves. With the help of soil microbes, plants then transfer carbon to the soil through roots and decomposing residue. The stable storage of this carbon below ground builds soil organic matter, and it reduces levels of atmospheric carbon. The process is called carbon sequestration.
While the carbon cycle occurs naturally with varying degrees of efficiency in croplands as well as in grasslands – with or without grazing – grazing at moderate levels tends to increase the amount of carbon sequestered in grassland soils.
Drawing from her research and the research of other scientists, North Dakota grassland ecologist and science application educator Rebecca Phillips says, “We are finding plants put more carbon below ground as a result of the grazing of livestock. A well-managed grazing system stores more carbon in the soil than grasslands that are not grazed.
“The key is the activity in the plant roots,” she says. “Their response to grazing is to produce more roots and more exudates through the roots. Exudates feed the microbial population in the soil. In turn, microbes process the root materials, transforming them into forms used by microbes stored in soil – effectively improving the health of the soil to support plant growth. Grazing is one key to supporting healthy microbial communities in soil.”
Phillips cites results of a study published in Global Change Biology, where the average mass of plant roots harvested from grazed grasslands was more than 2,400 pounds per acre. In comparable ungrazed grasslands, the average mass of plant roots was 740 pounds per acre.
The greater root mass produced in grazed grassland partially explains why grazing tends to concentrate more carbon in the soil. Bork’s research bears this out. He and his team measured soil carbon at more than 100 grassland sites spanning six distinct climate subregions in Alberta. Each site included an area of long-term grazing as well as one that was not grazed.
“We found moderate grazing enhanced soil organic carbon concentration by 12% in the upper 15 centimeters of soil,” Bork says. “Total carbon stocks – most of which was in the soil – within the grazed areas weighed from 20 tons per acre to 80 tons per acre.”
The wide, site-specific range in the levels of carbon potentially stored in soil as a result of grazing causes ranchers to wonder what level of carbon their systems are storing. Phillips says, “Increasing organic carbon from 3% to 4% in the top 4 inches of the soil profile is approximately equivalent to storing an additional 11 tons of carbon per acre.”
This gives producers a seat-of-the-pants yardstick of how much carbon regenerative grazing systems potentially draw out of the atmosphere. This, set against the backdrop of Bork’s measurement of vast stores of carbon stocks in grasslands, suggests the stakes are high in the debate over cattle’s role in climate change.
Movement of this large mass of sequestered carbon back into the atmosphere, as occurs under cultivation, could result from a swing in consumer demand away from beef and toward plant-based replacements. Economic-driven changes in land use could result, and the upshot could be significant new emissions of CO2 from the soil back into the atmosphere. This CO2 would contribute to global warming.
Ongoing land-use changes could intensify as ranchers are forced to convert grassland to cropland in an effort to replace income lost from displaced beef sales. Conversion typically involves using chemicals to kill grass and may also include tillage to break up sod, in the process reducing biodiversity.
Much of North America’s “mixed-grass prairie should never have been plowed in the first place due to poor agronomic conditions,” Bork says. Original plowing of the prairie resulted in an explosion of stored CO2 into the atmosphere. Some researchers estimate soils of the Great Plains region have lost as much 50% of their carbon-loaded topsoils after decades of farming.
“There’s a significant environmental penalty to pay,” Bork says. “Yet grassland is still being converted to cropland in western Canada and many other regions of the globe. This conversion results in a 30% to 50% loss in soil carbon, released as CO2 into the atmosphere. Restoring soil carbon takes years.
“Everything is at stake when grassland is converted,” he says. “A broad suite of goods and services provided by grasslands to society at little cost is lost, and it’s much more than carbon sequestration. Grasslands provide water purification, flood mitigation through reduced runoff, and habitat for birds and wildlife.”
Public criticism of livestock because of methane emissions picks away at one piece of a much larger holistic puzzle. “It’s taking a very limited view of the carbon balance,” Phillips says. “There needs to be a greater understanding of the whole picture.”
She estimates, based on a study by Francis Kelliher and Harry Clark in 2010, enteric methane produced today on the Great Plains by 40 million cattle amounts to “13 pounds of carbon as methane per acre per year.” This amount is dwarfed by Bork’s and others’ measurements of total carbon stored in grassland soils – carbon that would be at risk of release back into the atmosphere if the grassland became cropland to produce plant-based protein.
The amount of methane emitted by cattle also needs to be balanced against the natural processes built into grassland ecosystems that scrub methane from the atmosphere. “Studies are showing grasslands can be a net sink for methane because there are microbes in the soil that break down methane,” Bork says. “Because of these little scrubbers, healthy grassland soils are able to offset a portion of the methane produced by cattle.”
The holistic circle of benefits grazers provide includes consumers. “Plants, grazers, and microbes all work together to build healthy soils while converting grass to a healthy source of protein for people,” Phillips says.
（January 28, 2021. From “Successful Farming”)
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Post time: Jan-28-2021