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Army of Microscopic Life Probed for Health of Planet's Soil

Bijal P. Trivedi
National Geographic Today
March 4, 2002
 
Diana Wall stands in her garden surrounded by bizarre and savage animals
that are eating, chewing, tearing, and grinding the dead plants around
her. Some have six legs; others, armor-like shells, bristly bodies, or
large, multifaceted ruby eyes.

Wall can't see any of these
creatures because they are microscopic.



A soil ecologist at Colorado State University in Fort Collins, Wall is fascinated with the creatures that facilitate rotting by breaking down plant litter, enabling the nutrients to be recycled for use by other organisms. Now she is part of an international effort to identify and name all the critters like these in 20 countries.

The project, called the Global Litter Invertebrate Decomposition Experiment (GLIDE), is part of the International Biodiversity Observation Year, now underway. The aim of GLIDE is to improve understanding of how the various species in different types of soil—mites, nematodes, millipedes, and others—contribute to rates of decomposition.

Such knowledge is important because the armies of microscopic creatures that live in the soil help it maintain its fertility, its porous structure—which is essential for filtering water—and its role in Earth's cycling of carbon and nitrogen.

"For so long, anything underground was this big black box and, with the exception of farmers, very few people were interested in the organisms in the soil," said Wall. Now, because human activities are changing the natural systems of the planet dramatically, it's crucial to learn whether essential forms of biodiversity are being lost, she explained.

Picturing Decomposition

Wall and an international team of scientists are conducting experiments to determine what organisms participate in the decomposition process under various conditions and at different sites.

Wall's team at CSU put identical piles of alfalfa grass into more than 2,000 mesh bags, then mailed the bags to ecologists in 20 countries. The bags were placed in a total of 30 different ecosystems; for example, in the hot Namibian desert, humid oceanic broadleaf forests of Tasmania, temperate fields of Poland, and semiarid scrub plains of the United States.

Soil consists in large part of decomposed surface litter, said Wall. By placing these mesh bags filled with litter in different environments we can get a good sampling of the diverse fauna that crawls from the soil to break it down.

"We want to know whether the same organisms are chowing down on [the alfalfa]," said Mark Dangerfield, director of the Key Center for Biodiversity and Bioresources at Macquarie University in Sydney, Australia.

The scientists collected the bags several months later and measured the amount of rotting that had occurred. The tiny organisms were then extracted from the partially decomposed litter and collected in bar-coded vials and mailed to a laboratory in Australia.

The lab, operated by a company called BioTrack, is a spin-off venture of Macquarie University that monitors biodiversity for government agencies, private companies, and other clients. BioTrack uses microscopes and advanced digital cameras to detect and record the microscopic animals in each sample.

"Soil is vital to ecosystem," said Dangerfield. "The question is, how much biodiversity is needed in the soil to have it work—as a filter for toxins, as a nutrient recycler, and as a water filter?"

Getting a "Signature"

The scientists are not interested primarily in classifying different mites in a sample, for example, as a taxonomist might do. Instead, they want to find out how many different species are represented in soil from a particular ecosystem and what those species are—essentially, the "biodiversity signature" of a given environment, said Dangerfield.

The signatures should help scientists identify exactly what organisms are associated with fertile soil and which are missing from less arable soils.

So far, BioTrack has received vials from the United States, United Kingdom, Canada, Australia, Russia, Taiwan, Brazil, Namibia, Germany, and Burkina Faso. Samples from five of these countries have been analyzed and contain about 17,000 organisms, possibly representing thousands of species, said Dangerfield.

Taxonomy is a long and laborious task, requiring painstaking work to classify plant and animals. BioTrack's identification process, however, is relatively rapid.

The organisms that Dangerfield's lab is working to pinpoint in the contents of a vial are typically only about a half to one millimeter long. Once detected with a microscope, they are photographed to produce high-resolution 3-D digital images.

The magnification is so high that it's possible to see every hair on one of these microscopic creatures, which is important because an extra bristle here or there could indicate a different species.

Then comes the sorting process, for which the digital images are crucial. As new samples arrive, the contents of the vials are compared with those of other samples already recorded with pictures in the database. Dangerfield hopes to put this data on the Web so anyone around the world could access the images for comparison with the organisms in local soils.

Toward Baseline Biodiversity

"GLIDE will be particularly useful for introducing a baseline biodiversity for many regions," said Brian Boag, a soil ecologist at the Scottish Crop Research Institute in Dundee.

Knowing the baseline biodiversity of a region will make it easier to measure the impacts of invasive species or human activities, said Boag.

His research has included studies of how the invasive New Zealand flatworm has affected the ecology of soil in Scotland and Northern Ireland, where the worm has displaced native earthworm populations and had a dramatic impact on farmland.

The New Zealand flatworm has infested 70 percent of Northern Ireland in just over a decade. Without the native earthworms, which are important in churning up soil and aiding the cycling of nutrients, many areas of land have become waterlogged and filled with plants that thrive in wet conditions.

Biological components of soil also play a major role in some soil problems, like salinity, that have been viewed as largely the result of chemical and physical changes.

In many regions of Australia, for example, salt buildup from irrigation is a widespread problem for farmers because most plants will not grow in highly saline soils. Heavy irrigation also drowns many common soil organisms. The loss of these creatures amplifies the effects of the salt, which remains concentrated in the top layer of the soil, said Dangerfield.

Similarly, heavy application of inorganic fertilizer kills a large proportion of soil biodiversity. As the number of soil organisms decreases, so does the rate of decomposition; this, in turn, reduces the amount of nutrients, making the soil less fertile, said Dangerfield. Farmers then become even more dependent on chemical fertilizers.

"We need to know how to restore soil," said Wall, "but without knowing what creatures are present, it is hard to say how to improve soil biology."

Dangerfield believes that BioTrack's primary customers will eventually be farmers and forestry officials, who critically need to understand the nature of soil in certain ecosystems. "We hope to match the biodiversity on different farms with different soil management practices, then see which 'signature' is most beneficial," said Dangerfield.

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