Spring 1999

[TEXT ARCHIVE WEB-PUBLISHED MARCH 2002.
ORIGINAL PRINT PUBLICATION DATE: SPRING 1999.]

Adventures in the Rhizosphere:
Life Underground

By Alex Blumberg

To the honor roll of world-altering technological innovations, a list that includes the wheel, the cotton gin, and the solid-state transistor, few of us would add the self-scouring steel moldboard plow. We should. John Deere's invention tipped the balance of power once and for all from prairie to farmer. It guaranteed that prairie no longer broke plow, that plow instead broke prairie. And it inscribed on the landscape the first line to the now familiar story of wilderness into fruited plain. But the moldboard plow wrought its greatest transformation on a hidden landscape, the subterranean world of prairie soil.

Michael Miller, an ecologist at Argonne National Laboratory, knows soil. He notices things about soil that most people don't, its composition and chemistry, its smell and weight, cohesion and tilth. He mentions in passing that high metal content can give soil a reddish tinge, the result of iron oxidizing, earth literally rusting. He knows that in every cubic millimeter of soil beneath our feet, nematodes hide and mites hunt; springtails couple and bacteria divide; things grow and die. He knows, in other words, that the soil lives, that it is, in fact, alive by definition. "Without life," he explains, "there is no soil. There's dirt."

Miller knows also that prairie soil, in particular, is a hidden ecosystem rivaling any visible system in complexity. This knowledge causes him to regard the moldboard plow as a tool of environmental calamity on par with the chainsaw and the oil tanker. "The real diversity of the tallgrass prairie is not above ground, but below," he says. "The tallgrass prairie is a rainforest turned upside down."

To find out what inhabits this inverted jungle, it helps to talk to Dr. John Lussenhop, a soil biologist at University of Illinois at Chicago.

He is, among other things, the region's foremost expert on springtails and mites. To envision life underground, Lussenhop recommends shrinking, Alice-like, to about one millionth your original size, roughly one micron (one thousandth of a millimeter) in length, or about the size of the average bacterium. Bacteria coat the surfaces, line the crannies, and plug the interstices of whatever they're eating. They produce CO2 and ooze a slimy film; they clone themselves with abandon, and gorge themselves until food or moisture is gone. When that happens, they either die or go into quiescent mode and lay low until the next food source staggers along and dies. "Bacteria aren't the most mobile of organisms," says Lussenhop. "It's been estimated that the average bacterium lays quietly for six months waiting for breakfast to bump into it."

Occasionally, bacteria find themselves engulfed in the peculiar embrace of the amoeba, chief predator of the unicellular world. An amoeba can wedge its pseudopod into a microscopic crevice 2 microns in diameter and scoop out an individual bacterium, which it then surrounds with its fluid body and consumes for a tasty snack.

Moving up three orders of magnitude, we come to the springtails and mites. The springtails, at a fraction of a millimeter in length, are nonetheless a thousand times larger than the bacteria and amoebae all around them — the same ratio in length as an ant to a person, say, or a person to an aircraft carrier.

"Springtails have white bodies, stubby antennae and two horn-like formations protruding from their last segment," says Lussenhop. "They spend a lot of time walking up and down roots. It's their highway." Along this highway they consume food — fungi, bacteria, and rotting organic matter — and they become food. They share the thoroughfare with diminutive predators, the mesostigmatids, or soil mites.

Mites, Lussenhop explains, "wander along the roots also, snatching unsuspecting springtails up in their cheliscera," the invertebrate equivalent of steely jaws. Lussenhop once blew up a microscopic photograph of a mauled springtail and compared it with the fossilized remains of a Tyrannosaurus kill. "The dentition patterns looked almost the same," he marvels. "These guys are just like T-rex, but on a microscopic scale."

Growing ever bigger, the procession of soil organisms continues, eventually becoming easily visible to the naked eye. Pseudoscorpions, no bigger than a half centimeter, scuttle among the soil particles nabbing mites and springtails in their pincers, piercing them with their deadly fangs. Earthworms glide by, consuming all that they find in front and excreting all they don't like behind. Ants bustle, larvae burrow, and moles tunnel. In one thimbleful of soil, scientists can distinguish roughly 20,000 discrete species of life form. All these organisms don't merely inhabit the soil. Living and dead, they are the soil.

ll soils, of course, contain bacteria and fungi, and most contain mites, nematodes, and springtails. Appreciating what makes prairie soil unique requires a basic understanding of soil structure. Soil is a mixture of sand, silt, clay, and organic matter. Think of the sand, silt, and clay as bricks, and the organic matter as mortar. As living things — roots, leaves, butterflies, woodchucks — die, they are eaten by hordes of microbes living in the soil. These hordes exude a sticky film while they dine. Once the dead root has been nibbled to a small enough piece, says Miller, the sticky film binds the silt and clay around it, and "mummifies it in a matrix of mineral and organic material." The root bit, now sealed in a tomb of clay skins and microbial glues a couple thousandths of a millimeter in diameter, is called a microaggregate.

Microaggregates can clump together like soap bubbles or add layers like a pearl or both. Doing any of these things lets them drop the micro and go by the less fussy, more universal term, aggregate. Aggregates can be thought of as the essential building blocks of the soil, since a clump of soil dropped in water will dissolve only to the aggregate level. In the space between aggregates, the tunnels and pores that filigree the ground below, the microscopic life of the soil goes about its business. The nuances of aggregate size, stability and composition are known collectively as a soil's "crumb structure." To fully grasp this concept's importance, it's instructive to compare the soil in a cornfield to the soil in a virgin prairie.

The roots of many prairie plants plunge 20 feet or more into the ground. On the roots live Michael Miller's specialty, mycorrhizal fungi. These fungi send out tendrils called hyphae that have a binding effect on the soil. Together with the slimy residue of bacteria and a slurry of water and silt, they form gigantic tangles called macroaggregates. Gigantic, of course, is a relative term. To earn macroaggregate status, tangles must measure at least 250 microns across. That's .25 millimeters, about the size of a head of a small pin. Miller and co-workers have photographed macroaggregates and x-rayed them. Magnified, they resemble a bit of gum that's been rolled in a dirty rain gutter. In the prairie, macroaggregates make up 95 percent of the soil. In a cornfield the percentage drops to 35. The ramifications of this 60-point spread are huge.

First, large aggregates add tilth to the soil, or to use Michael Miller's personal jargon, "they make it fluffy." Prairie soil, having a crumb structure of bigger constituent parts, is "fluffier" than agricultural soil, which is another way of saying it's less dense. This may seem an academic distinction, but Vermont Prairie, a one-acre remnant in Will County, highlights its real-world ramifications. Plotted as a cemetery nearly 150 years ago, Vermont Prairie escaped the plow. It persists as a tiny island of 10,000 year-old ecosystem amid a sea of 20th-century agriculture. During the past 100 years, the soybean and cornfields around the prairie have been slowly collapsing, losing their fluff like a down pillow under a sprinkler. The prairie, by contrast, has maintained the tilth of the soil below it. The result is a nearly two-foot differential between the level of the Vermont Prairie and the surrounding cropland. It juts like a squat, rectangular butte from the flat and sagging plain. "A lot of people think the level subsided due to erosion," says Miller, "and that's part of it, but mainly, it's just the soil in the fields growing more dense."

"Here's a second difference between the prairie soil and cultivated soil," says Miller. "There's roughly twice the biomass below prairie as below corn." Again the reason has to do with crumb structure. Imagine a jar of steel ball bearings. In between the bearings, there are spaces, say, where tiny ants could roam. Now imagine a jar of iron filings. There's no room for ants in between those particles. The same is true with soil. The bigger the aggregates, the more room for life to teem.

The added living space leads to what Miller sees as another key distinction. Prairie soil hosts far greater biodiversity. He has empirical evidence to support this claim — he'll find 10 to 12 species of mychorrhizal fungi in a sample of prairie soil, for example, but no more than five or six in a cornfield. But mostly he interprets signs. Miller, after all, is an ecologist: he specializes in viewing systems as a whole. Certain systems tend to support highly diversified flora and fauna, others less so. Diverse systems vary, but from coral reef to rainforest, they all possess one common element — a large number of niches.

"In a cornfield," Miller explains, "all the aggregates are small, leading to uniform pore spaces in the soil." There are far fewer roots constantly burrowing and dying, fewer insects tunneling and worms digging. In other words, he says, "you've only got a few niches."

Under a prairie, by contrast, "you've got an animal that died here, a ground squirrel that dug a hole over there, a bluestem that rooted here, all this life doing all these different things," gesticulates Miller. "This leads to a high degree of diversity below ground." Add to this a crumb structure of large and craggy macroaggregates forming countless pores and openings throughout the soil, and you've got a lot of niches. You've got so many niches, in fact, that Miller thinks the prairie soil might be one of the greatest repositories of biodiversity in the world.

At the moment, we can't know for sure. The fact is, scientists have yet to name the vast majority of below-ground life. Ed Zaborski, a soil invertebrate ecologist with the Illinois Natural History Survey, conducted a study of mites in corn and soybean fields in Ohio. "You'd think this system would be well-known, but it turned out that 20 percent of the mites I collected were new," he says. "We think that in North America as a whole, only 15 percent of the total number of mites have been described." And it's not just mites. "Take nematodes, a group that functionally, at the ecosystem level, is much more important than soil mites," Zaborski continues. "We've described thousands of species, but people say there may be millions."

Prairie soils also differ from agricultural soils in their nutrient loads. A soil gets its nutrients from the break-down of organic matter by bacteria and fungi. Michael Miller and other soil scientists measure a soil's nutrients by extracting them into water or salt solution, boiling the solution down, and weighing what's left.

For example, take nitrogen. "If you do a water extraction, you'll find, at least initially, that cornfield soils contain much higher nitrogen levels," Miller says. "But if you crush both soils with a rolling pin, incubate them for several weeks, and repeat the extraction, similar amounts of nitrogen emerge from the prairie soil the second time." The cornfield soil, by contrast, has almost nothing left.

The answer to this seeming mystery lies in the microaggregates. Remember the entombed piece of root at the microaggregate's center? The root bit is, in a sense, reserve fuel for the soil. Its nutrients are released only when its microaggregate is broken open and microbes digest it into fertilizer. By crushing the soil with a rolling pin, Miller unpacked all the preserved bits of organic matter and allowed the microbes to make them into plant food.

Scientists refer to organic matter in the soil as organic carbon. And the process whereby microscopic blobs of carbon — root, leaf, or woodchuck — get preserved inside pacs of silt, slime, and mychorrhizal hyphae, is called carbon sequestration. Prairie soils, because they're so good at forming aggregates, capture a portion of all the carbon that passes through them. Most of every ground squirrel or butterfly or compass plant that dies on the prairie goes into the air as CO2 or into the cell walls of bacteria. But some of it gets stuck inside a microaggregate before the bacteria or fungi can finish it off.

The effect of carbon sequestration is that when the soil is undisturbed, available nutrient levels are relatively low. Of course, there's always something digging, burrowing, rooting, or shifting in the prairie. The activity constantly breaks down the microaggregates and releases their nutrients, but at a slightly slower rate than new microaggregates are formed. For 10,000 years, the Midwestern prairie soil, like a thrifty pensioner, set aside more carbon than it spent. Then along came the steel moldboard plow and blew the savings virtually overnight.

The productivity of the land, the vast, flat, treeless expanse of it, the size of the corn it shot up year after year, must have fired the settler imagination. It's easy to sympathize with 19th-century man, tempted as he must have been to see this magically fertile earth as proof that he was fulfilling God's plan. Of course, we now know that he was really just raiding God's storage bins. A century and a half later, things are a bit different, agriculturally speaking. The binge of the mid-19th century is over. The soil is still deep and dark, but, as already noted, certainly denser, less teeming, poorer in organic material. "We've gone from soil that was 10 percent carbon to soil that is less than three percent," says Miller. "We didn't farm the soil, we mined it."

The most profound consequence of John Deere's plow has come to light only recently, in the era of global warming. The effort to determine the origins of all the excess CO2 in the atmosphere is called mass balancing. On one side of the equation, scientists place all new carbon in the atmosphere. On the other side they place carbon in all the fossil fuels that have been burned since the industrial revolution. For many years the equations didn't balance; more new carbon hung in the atmosphere than could possibly have come out the ends of tail pipes. "Much of the unknown was explained," claims Miller, "by the plowing of prairie soils. A good portion of the elevated CO2 levels in the world comes from the breakdown of the North American prairies."

It's awesome to consider the intricacy of this web — a pluck here, near the blind and groping bacteria on a bluestem root, sends tremors strong enough to melt the polar ice caps. It's scary how much the well-being of our planet depends on a world beneath our feet that most of us aren't aware of. Who knows how many species we'll eventually discover at work beneath our remnant prairies, or if we'll ever be able to count all the inhabitants of this inverted rainforest? Who knows how irreversibly John Deere's plow has altered things, how much has been lost, how much remains? But one thing is for certain, if any of the millions of organisms inhabiting the soil had hands, the fate of the world would be in them.