Switchgrass Deep Roots Store Carbon in Soil

Corn roots usually extend only to around 0.6 metres. Pull up a stalk after harvest and its root ball is fairly compact, with just a few inches of tangled growth sitting in the topsoil.

Switchgrass is unlike this. This prairie grass, long viewed as a candidate for biofuel production, grows roots much farther down than most annual crops - in some cases, ten times deeper.

Scientists had long thought that such extra depth could affect the climate, but it had not been demonstrated at scale before a recent study.

Coast-to-coast tests

Professor Eric Slessarev of Yale University (Yale) headed the research alongside Erin Nuccio, a researcher at Lawrence Livermore National Laboratory (LLNL) in California.

Their researchers set switchgrass against the shallow-rooted annual crops planted immediately beside it.

The 12 locations ran from the southern plains to the Great Lakes region. At every site, a switchgrass stand aged between eight and 30 years grew next to corn, soy, wheat or comparable row crops.

Slessarev personally drove a pickup and sampling trailer to most of these locations.

The group collected more than 700 soil cores, including samples from well beneath the topsoil. In the laboratory, they manually separated the roots from every core.

Roots that run deep

Annual crops such as corn and wheat typically grow roots down 30–60 centimetres before redirecting their energy towards flowering and producing seed.

Switchgrass follows the reverse pattern. Its roots continue downwards, threading through soil layers that conventional farm crops do not reach. Scientists have considered the importance of this depth for decades.

One paper calculated that about 12,000 years of farming have removed a vast proportion of carbon from the world’s soils, including carbon formerly held beyond the reach of most crop roots.

Perennial grasses including switchgrass retain their roots throughout the year, supplying organic matter to these overlooked depths.

Previous research indicated that this might restore part of what agriculture had removed. This study was the first to examine the idea on a large scale.

Carbon under the floor

Laboratory findings gave a straightforward result. Switchgrass roots contained around 0.22 tons more carbon per acre (0.5 metric tons per hectare) than the roots of adjacent annual crops.

The pattern remained consistent. In markedly different fields - from sandy plots in the south to denser northern clays - the grass built up approximately the same additional quantity of root carbon.

“It’s a night and day difference,” said Slessarev. An earlier study had found that living roots transfer more carbon into soil than fallen leaves.

Carbon levels in upper soil layers also tended to be greater at most sites, although the evidence was not conclusive.

No hidden cost

The researchers initially had a concern. Fresh plant matter entering deep, old soil can activate microbes that have remained inactive for centuries.

These microbes may then begin consuming the older stored carbon that they had previously left alone.

To establish whether switchgrass was causing this response, the team tested radiocarbon in soil cores taken from every plot.

New carbon from living roots has a distinct chemical signature from carbon that has remained below ground for centuries. Scientists use this difference to distinguish between them.

The laboratory results delivered what many researchers had been hoping to see. Fresh carbon was accumulating, while the older carbon remained in place.

Earlier studies had suggested this outcome could occur. It has now been verified across 12 working farms.

From a biofuel angle

Ecologists are still examining switchgrass as a possible biofuel crop. Corn ethanol exhausts soil at every harvest.

Perennial grasses such as switchgrass can be harvested for fuel annually while leaving the soil undisturbed.

The study provides one clear part of the answer. Even when switchgrass is cut above ground for biomass, it continues moving root carbon into deep soil, keeping it out of the atmosphere.

A broader assessment of human land use concluded that cropland now has shallower root systems than the prairies and forests it replaced.

Undoing part of this shift, even across only a portion of agricultural land, could provide a climate benefit.

The path ahead

The results give farmers, landscapers and conservation planners greater certainty than was previously available.

Growing deep-rooted perennials on unused or marginal land stores additional carbon below ground, with the effect persisting across many different conditions.

The work also creates questions that researchers can now explore more precisely. Switchgrass grows in almost every type of soil.

The next issue is identifying which other perennials act in the same manner, and which combinations could draw down still more carbon without removing food-producing fields from use.

For climate planners, the implication is straightforward. Restoring roots to depleted soil offers a gradual, unobtrusive method of carbon removal that also supports healthier soil.