February 27, 2024

In the Australian outback, certain grasses grow in eerie rings, with dust-green walls at the edge of broad circles of bare red dirt. Often referred to as “fairy circles”, these rings made from Spinifex grass resemble structures first discovered in the Namibian desert. Both create enormous honeycomb patterns in the landscape that really show up in aerial photographs. In Namibia, scientists have used cameras on fishing rods, observed colonies of termites and even used mathematical models to explain how this phenomenon occurs.

A small study published last month in the Australian Journal of Botany suggests that soil-dwelling microbes may contribute to the formation of the rings in Australia, making the dirt in the ring hospitable to new seedlings and the dirt beyond the ring.

Spinifex grasses start out as little round mounds, said Angela Moles, an ecologist at the University of New South Wales and author of the new paper. Then, when new seedlings sprout outwards, the plants in the middle will die, resulting in the ring shape. Researchers have investigated whether the bare inner soil becomes poor in nutrients; whether it is too dry or compacted for new growth; and whether insects could destroy the Spinifex. However, a consensus on what drives the formation of rings has not yet been reached.

Dr. Moles had heard of a small European marsh grass growing in a ring pattern that was due to an accumulation of soil pathogens in the center. She and Neil Ross, a graduate student in their lab, were curious if sterilizing the soil from inner rings and killing microbial organisms there would make plants grow easier. If so, it would mean microbes were involved.

Mr. Ross carefully shoveled earth from within rings in the desert of Australia’s Northern Territory and from outside the rings as well. Back in the university greenhouses, he sterilized some of each. He then planted some Spinifex seeds in pots with microbe-free soil and some in unchanged soil.

The researchers found that the seeds germinated more easily in containers with a sterilized inner bottom. About the same number of seeds germinated from outside the unsterilized rings in the soil, suggesting that both the outer soil and the sterilized inner soil encouraged new growth. Sterilizing the inner soil seemed to remove anything that was preventing the plants from germinating.

This fits in with previous research, which suggests that as plants grow, pathogens that attack them settle in the soil around their roots. These pathogens can be tolerable for adult plants, but unpleasant for sensitive new seedlings. This may explain why trees of the same species are rarely found in close proximity to each other in rainforests.

“Sprouting is a really scary thing for a seed,” said Dr. Moles. “They don’t have a lot of resources there for a while.”

If new Spinifex grasses can’t handle the pathogens in the soil in the center of their clump, they can sprout just outside the clump instead, resulting in this distinctive ring pattern.

However, the researchers also found that sterilized soil from outside the ring is just as bad for new sprouts as untreated inner soil. Dr. Moles speculates that some other soil microbes may also help seedlings. Killing potential pathogens from inside the ring will make this soil liveable again, she says, but killing anything in the soil outside the ring could also be detrimental.

The researchers have not tried to cultivate bacteria or other microorganisms from the soil, so they are not sure what tiny ecologies have arisen around these rings in the desert and how exactly they change over time. But in the ongoing scientific discussion about these fairy rings, microbes may just have joined the party.