THERE'S an old stump near our home that we often stop to admire. It symbolizes a major challenge to environmental science.Nature's recycling team has been working for many years to transform its substance into new soil. This is a critical process that keeps Earth's living systems going. Environmental scientists would like to enlist the recycling microbes to do the same job with chemical spills and other organic pollution. But here they face what ecologist Eugene L. Madsen of Cornell University considers one of the most intractable and important problems in his field - showing that nature's recycling agents actually do break down the pollutants. News reports about using bacteria to clean up nasty spills seldom mention this uncertainty. Yet, writing in the journal Environmental Science and Technology, Dr. Madsen explains that reliable on-site assessment is the key to using natural processes to clean up humanity's messes. Until scientists know what goes on at a contaminated site - as opposed to laboratory studies - they won't know how to develop biodegradation processes into effective pollution-fighting technology. It's one thing to spray oil-eating bacteria on a contaminated site and quite something else to show that this actually does some good. Madsen illustrates the point by comparing the natural and the artificial situations. He notes that, when an old log decays, there is a well-documented degradation process. The biomass of the wood disappears to be replaced, in part, by a growing biomass of microbes. Eventually, higher organisms - such as the mosses and bugs on my old stump - move in. This all takes place in a well-defined volume. It is easy to account for what happens to the material involved. When the mass of the wood diminishes and the mass of microbes increases, this shows that biodegradation has occurred. It's a different story when you spill a chemical - say gasoline - on the ground. The material evaporates, disperses into soil, gets in water, and otherwise eludes a tidy accounting by scientists. It's easy to upset the delicate and intricate balance of natural decay processes when examining them. "The problems of extrapolating from laboratory data to the field have never been solved," Madsen says. It will take concerted effort to devise reliable ways to tell how effective biodegradation is at a spill site. Pooling knowledge from careful study of many such sites will help. It also will take teamwork by a variety of specialists - including those who can unravel the genetics of the organisms involved - to give a clear picture of how well the natural recycling processes deal with pollution. Meanwhile, when you walk by an old stump, give it a quiet salute. What's happening there has planet-wide implications.