From the Midwest to the East Coast, cicadas from Brood X have emerged in full force, making a lot of racket while trying to find a mate and then actually doing the deed. Although their days are numbered, they’ll leave behind billions of eggs and millions of holes in the ground. These pits, created when the cicadas emerge from the soil, will stick around for the duration of the growing season, venting greenhouse gases. There are a number of effects from this, says Richard Phillips, an ecologist at Indiana University whose team is exploring the tunnels’ influence on water infiltration, gas exchange, and fertilization.
Ordinarily, when subterranean species tunnel through the soil, the holes collapse behind them. Phillips explains that cicadas produce chemicals that act as a glue, holding the tunnel’s shape for the entirety of the season. With as many as 400 holes per square meter going down about three meters underground, the soil is aerated in a deep and lasting way.
Using two types of infiltrometers—instruments used to monitor the rate at which precipitation permeates soil—Phillips’s team compares how the density of emergence holes affects the penetration of deep rainwater into the soil. Although it is fairly intuitive that the tunnels will bring water beneath the topsoil, he says, these measurements will be taken over the next several months to determine how the soil is affected over the course of the season.
His team is also looking at greenhouse gas emissions from the holes. To take the measurements, the team is using a portable infrared gas analyzer to measure the carbon dioxide (CO2) on site and gas chambers collect samples that will be analyzed for CO2 plus methane (CH4) and nitrous oxide (N2O), all three of which fluctuate depending on the activity of soil microbes. In addition to the gas emissions, the researchers are tracking moisture levels and temperature, which can affect microbial activity.
“We think that the movement of water into the soil, which generally is good for microbial activity, will enhance microbial activity, and as a result of that you’ll have both more CO2 and more N2O being produced,” Phillips says. He predicts that the holes will lead to lower levels of methane in these areas because aerobic respiration by microbes traps methane.
In the coming weeks, if cicadas aren’t eaten by predators, their bodies will fall to the ground en masse as they die, introducing yet another disturbance to the soil: nutrients. For 17 years, cicada nymphs have been tapped into trees’ roots, sipping on small amounts of water and even smaller amounts of nutrients—including nitrogen—from the xylem. As the cicada carcasses break down, the nitrogen is returned to the soil, becoming available for the tree to reclaim before the next generation of Brood X starts the entire process over again.
“The carcasses are high in nitrogen since insects have lots of chitin in their exoskeletons,” Phillips says. “This chitin is likely an important source of [nitrogen] to the microbes and to the trees. So we anticipate the carcasses will also increase soil [carbon dioxide] and nitrous oxide emissions.”