Research: Grass type, how you mow affect soil’s carbon content

Purdue University researchers are documenting the effects of grass mowing on soil’s carbon and nitrogen content, noting that both the type of grass and whether grass clippings are left on the lawn after mowing can have a significant impact on levels of both chemicals in the soil.

Quincy Law of Purdue University collects greenhouse gas samples from turf plots using a vented flux chamber. Photo: Jon Trappe.Quincy Law of Purdue University collects greenhouse gas samples from turf plots using a vented flux chamber.
Photo: Jon Trappe.

According to a news release from the American Society of Agronomy, Purdue turfgrass scientist Quincy Law studies many aspects of lawn care and how those practices affect the soil. At a time when lawns’ existence is being questioned by those who believe grass requires too much water – not to mention fertilizer and herbicides – and is thus environmentally unfriendly, Law suggests the opposite is true.

“My research demonstrates one of the many environmental benefits of natural grass lawns, golf courses, athletic fields, cemeteries, and similar areas,” he explains. “Grasses are able to remove carbon dioxide, a greenhouse gas, from the atmosphere and deposit it into the soil as organic matter.”

Law and his research team at Purdue found that the amount of carbon deposited into the soil was affected by the species of grass and how it was mown. Specifically, tall fescue resulted in more soil carbon than Kentucky bluegrass. However, tall fescue required more frequent mowing. Both are common grasses used in lawns in the United States.

“Soil is a nonrenewable resource that must be protected, which soil carbon helps to do,” he says. “Soil carbon helps to bind soil particles and build soil structure. It also decreases the risk of runoff and erosion, and improves soil-water relations.”

Leaving grass clippings on the lawn after mowing increased both soil carbon and nitrogen compared with when clippings were collected. In the soil, the grass clippings affect the carbon and nitrogen levels in two ways: directly, by simply being present, but also indirectly, by serving as fertilizer for the grass.

Labile soil carbon was determined using a colorimetric analysis. Lighter colors indicate a higher amount of labile soil carbon. Photo: Quincy Law.Labile soil carbon was determined using a colorimetric analysis. Lighter colors indicate a higher amount of labile soil carbon.
Photo: Quincy Law.

Law stops short of making recommendations about particular grass species. To obtain more grass clippings, one has to mow more often, Law says, and that can cause problems.

Lawn mowers, like cars and trucks, produce their own carbon emissions and can offset some of the benefits of increased grass clippings. Improvements in mower efficiency and alternative fuel sources may help offset concerns about mower emissions in the future, Law says.

“By carefully selecting a grass and properly managing grass clippings, homeowners can increase the carbon sequestered in the soil,” he says. “While this will not single-handedly significantly reduce carbon dioxide levels in the atmosphere, it is still helpful.”

For example, Law’s research shows that the carbon deposited by the grass more than offsets the carbon emitted from managing the lawn and helps to justify those emissions. What’s more, Law says, lawns do more than look good. They provide recreational areas as well as environmental and societal benefits.

For more information about turfgrass science at Purdue, visit this website. You can learn more about the American Society of Agronomy here. Funding for the research was provided by the United States Golf Association and the Midwest Regional Turf Foundation.

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