A ground-breaking lab

A DIRTY JOB: Alexander Friend observing activity in a window at the rhizotron.

Alexander Friend walks up to a stainless steel door, twists some latches holding it into the wall, and lifts the 7-kg rectangle out of its hole and onto the floor, revealing a sideways 152 x 101 cm window into the earth. This is window 17 of the 24 in the US Department of Agriculture Forest Service's Houghton, Michigan rhizotron - an underground lab named for the Greek word for root. Like the others, window 17 is divided into 15 panels that can be removed easily to take samples, inject dye, or whatever else a scientist might think of. Today, there's a centipede making its way by one of the panels, as well as scattered roots, probably from a nearby sugar maple.

Northern Michigan isn't known for its temperate winters, but the soil in the Houghton region doesn't freeze because of the constant snowpack. So on a December day, there's plenty of activity in the window. "I think it's really cool to be in here with snowpack outside and have all this life going on," says Friend, a USDA Forest Service research ecologist, as he watches a pill bug - otherwise known as a wood louse - go by. Most of the panels are still just dirt. "Wait two or three years," says Kurt Pregitzer, a professor at Michigan Tech, down the street, and director of the Ecosystem Science Center. "You'll be amazed at what you see. There's a whole food web down here. A lot of the soil animals, when you actually see them, look like Spielberg, like Hollywood hired a soil scientist. Most people ignore it."

Rhizotrons have been used for many years to study agricultural growth, but not usually forest growth, says Friend. The Houghton rhizotron follows the natural topography, sloping upward from the entrance. The indoor space is set at 40°F (4.4°C) to match the ambient temperature of the soil as closely as possible. The team planted white pine, trembling aspen, and sugar maple on the right side of the rhizotron, and left the left side's trees alone. Inside, infra-red lights can be switched on instead of visible lights, which tend to scare the bugs away from the glass.

The rhizotron, which cost $500,000 to build, offers a number of advantages over other methods of studying soil, says Friend. "The ability to study soil in a spatially explicit manner is very powerful," he says. "When you take soil cores, you average out effects, and you may miss a lot of signals." There are some limitations; for example despite the fact that the glass facing the soil is coated with an inert substance, its presence probably affects root growth in some way, so it may not be useful to study roots right on the glass.

The group is still doing mostly inventory, not experimentation, while the soil near the rhizotron returns to its natural state. (To see what happened when they buried a vole they found nearby, see the slideshow.) The first things the researchers saw were worm burrows. "That means spaces for bugs to grow, and for water to flow," says Friend.

The worms of northern Michigan aren't native. The native species were pushed south by glacial movements and replaced by European invasive species including Lumbricus terrestris - a.k.a. the night crawler - which can grow larger than 20 cm. As they burrow vertically into the earth, they slough off cells and exude waste, changing the soil chemistry. They also pull freshly dropped leaves right into their burrows, says research ecologist Erik Lilleskov.

One of the motivations for building the facility was global climate change, he says. Forests are considered good carbon sinks, and two-thirds of the carbon in forest ecosystems is in the soil. Earthworms shift carbon from the dead leaf layer to the soil, but no one knows how, says Friend. "Human civilization really depends on ecological processes in soil that sustain the terrestrial services we depend on," Pregitzer says. One of the fundamental problems, he says, is understanding the mechanisms that control the recycling of organic matter. "Historically, we haven't been able to observe that," he says.

Pregitzer likens the rhizotron to a sea-going oceanographic vessel. The facility will be joined by a nearby mesocosm, a system of pots embedded in the ground whose conditions can be manipulated at will. Researchers could attach pressure transducers to roots in the rhizotron, to determine how they would behave in urban settings, for example. Lilleskov is building a time-lapse photography system that will allow him to take high-resolution images every 60-90 minutes all the way down to the level of the hyphae of the fungi growing on roots. "We can make the soil come alive," he says. "This is educationally as well as scientifically valuable."

Friend encourages others to contact him about conducting projects at the facility. Grants from agencies from the US Department of Energy to the USDA to NASA could be a part of the research, he notes: "One of the things we fantasize about is being able to sense from space what's going on in the soil," he says. "The sky's the limit," he says of the highly earthbound lab.