By optimizing the amount of carbon returned to the soil, forage production can play a major role in carbon sequestration while improving soil quality. Not only does this benefit yield, it has environmental advantages.
“Soils contain two to three times more carbon than the vegetation or atmosphere, and that’s why we talk about soil carbon when we talk about climate,” says Dr. Denis Angers, scientist in soil management and conservation with Agriculture and Agri-Food Canada (AAFC). Angers, who is based in Quebec City, spoke about this relationship at the 2019 Canadian Forage and Grassland Association conference in Moncton, New Brunswick.
“Twenty per cent of that soil carbon pool at the global scale is under grasslands and perennial forages,” he says. “So there’s a real importance of maintaining current stock.”
Angers defines carbon sequestration as “a net removal of atmospheric CO2 through its transfer into long-lived pools, such as soil organic matter.” In examining the change in soil carbon content across North America, beginning before European settlement, 20 to 30 per cent of Canada’s soil carbon was lost when land was first cultivated for agriculture.
“The contribution is about 20 to 30 per cent of the increase in atmospheric CO2 since the beginning of industrialization,” he says. “The idea of carbon sequestration is to try to gain some of that carbon back through change in management or land use in such a way that you decrease atmospheric CO2.”
Organic matter, the key indicator of soil health, is primarily comprised of carbon and is the main energy source for many organisms in soil. As well, organic matter allows for proper infiltration.
“A healthier soil will be better aggregated, rich in organic matter and allow water to infiltrate into the soil,” says Angers.
It also affects the amount of nitrogen available. “There’s usually a very strong relationship between soil carbon and productivity due to the effect… of organic matter on water retention, on nutrient cycling and on soil density.”
There is more carbon in soils under perennial forage than annual crops, due in part to the former’s ability to better transfer carbon to the soil. Forage rooting systems and their longer growing seasons allow for greater capture of solar energy each year and thus fix more carbon. While cereal crops in general return around 20 to 30 per cent of carbon fixed by photosynthesis to the soil, perennials can return about 30 to 50 per cent to the soil.
“There is about 15 to 20 per cent more carbon under perennial forage pasture than under annual crops in the eastern part of the country, and it’s basically the same in the West,” says Angers. For example, he explained that the soil carbon content in rural areas around Montreal, which has more annual crop production, is considerably lower than in Quebec’s Eastern Townships, where there is more forage and pasture to support dairy production in the area.
Soil carbon content has been increasing in Western Canada, which Angers attributed to the general reduction in summer fallow and increase in no-till systems in the past five decades.
“We’re still seeing a residual effect of that switch to no till,” he says.
In Eastern Canada, however, “our soil carbon content is going down in general, and that’s basically because of cultivation of former pasture and hay land,” he says. It’s estimated that Eastern Canada has lost between one and two million hectares of pasture and hay land in the last 50 years.
Increases in soil carbon content are highest when transitioning from annual cropping to a perennial forage system.
“What you see is an increase, which is sharp at the beginning and levels off after several decades,” says Angers. The rate of increase when transitioning from annual to perennial production is 500 to 1,000 kilograms of carbon per hectare per year before it reaches an equilibrium.
“You get as much carbon coming in as you get carbon coming out after a few decades of having established a perennial,” he says. “Maintaining perennial systems is critical to avoid further (carbon) losses to the atmosphere.”
A long-term study conducted by AAFC in southern Ontario, beginning in the 1950s, compared the carbon content of the soil in a 35-year continuous corn crop with that of 35-year continuous grass. They found a total difference of 37 tonnes of carbon per hectare between the two systems.
“If you rotate annuals and perennials, you’ll see an increase under perennials and a decrease under annuals,” says Angers.
In that same AAFC study, a third system — two years of annual crops followed by two years of alfalfa — was compared to the two continuous systems. It had half of the increase in soil carbon of the continuous grass system.
More research is required to better understand the effect that specific management practices in forage production have on soil carbon content. Although it’s proven that perennial forage will increase the amount of carbon in soil, there’s little data on the effect of different forage species on soil carbon content.
Research shows that applying manure on forage has a positive effect on soil carbon, particularly in grass stands, though it’s harder to determine the effects of mineral fertilizer. That varies based on the fertilizer’s effect on yield, says Angers.
“If you… increase yield through fertilizer, the odds are that it may increase soil carbon, but it takes quite a bit of a change in productivity to see a measurable change in soil carbon.”
The effect of grazing also requires more research under Canadian conditions. A study involving researchers at the AAFC Nappan, Nova Scotia location and Dalhousie University is exploring the role of rotational grazing and different stocking rates on carbon sequestration in an Eastern Canadian context.
“When you look at the international literature, it’s that decreasing grazing intensity can increase soil organic carbon of degraded or overgrazed pastures,” says Angers. “But most of the data that we have on that comes from arid or tropical climates.”