Underground Soil Economy: Microbes hidden in the dirt react to UV boost

Where the sun never shines, underground microbial communities still change when there’s extra radiation up above, says an international research team.

In a 5-year test in northern Sweden, aboveground plants didn’t show major responses to experimentally boosted ultraviolet (UV) radiation, says David Johnson of the University of Sheffield in England. Yet the underground microorganisms reacted dramatically. The community doubled its nitrogen storage, Johnson and his colleagues report in the March 7 Nature.

“It’s definitely surprising,” comments Thomas Day of Arizona State University in Tempe, who also studies UV effects.

Microbes that hog nitrogen might eventually alter an ecosystem, explains Davidson. Productivity in plenty of natural landscapes is limited by the nitrogen available in forms that plants can use. Microbes that keep extra nitrogen away from other organisms might make this critical commodity even rarer. The researchers point out that nitrogen could govern how much carbon dioxide far-north ecosystems absorb from the atmosphere and how these lands affect global climate.

In 1992, the researchers set up the experiment–which is still running–on a birch-dotted heath at about 68N in Swedish Lapland.

The team built open frames to hold racks of lights over test plots to supplement UV-B, a major component of sunlight. Fan systems blew extra carbon dioxide into some of the patches.

“This was the first experiment to look at both carbon dioxide and UV-B long-term,” says coauthor Dylan Gwynn-Jones of the University of Wales in Aberystwyth.

The lights and fans mimicked conditions of a 15 percent loss of the atmosphere’s protective ozone and a doubling of atmospheric carbon dioxide, compared with current conditions. After 5 years, the nitrogen content of soil microbes hadn’t responded significantly to the extra carbon dioxide. The microbes in the high-UV-B plots stored nitrogen avidly, and their mass declined to roughly one-third that measured in plots getting normal sunlight.

“We don’t know why yet,” Johnson says. The most likely explanation is that the soil’s microbial species composition shifted, he says. Perhaps the extra UV-B changed the substances that plants exude from their roots, Johnson speculates.

Nigel Paul of the University of Lancaster, England, adds that altered microbial communities on leaf litter might also influence underground activity.