What is mulch leaving behind?

Study finds that decaying leaves contain chemicals in same class as DDT and PCBs

Steven Schultz

Princeton NJ -- It has always seemed so natural: Autumn leaves turn beautiful colors, fall to the ground, then decay into the rich mulch that gardeners prize as the ultimate nourishment for a new season of plant life.

Princeton geochemist Satish Myneni has shown that ordinary leaf mulch contains chlorinated compounds that were previously thought to come only from pesticides and other man-made pollutants. Future studies will reveal whether these natural compounds are toxic, as are many of the man-made forms.


A study by Princeton geochemist Satish Myneni, however, has revealed a new side to this ageless cycle. As leaves and other plant materials die and decay, they naturally develop increasing concentrations of chemicals that, while possibly harmless, belong to the same class that includes the toxic pollutants DDT and PCBs.

In an article published in the Feb. 8 edition of Science, Myneni reported that common leaf mulch from around the world contains high concentrations of chemicals called organochlorines, which were previously believed to come only from pesticides and other man-made products. Earlier studies by other researchers had found some natural organochlorines, but they were not thought to be a ubiquitous part of the ecosystem.

In an accompanying perspective article, William Casey of the University of California-Davis called Myneni's work a "stunning result" with "important social and scientific implications."

The study did not identify the specific kinds of organochlorines present in leaf mulch or assess their toxicity. Nonetheless, the results may have implications for evaluations of pollution, raising the possibility that at least some portion of organochlorine pollutants are actually natural products.

The finding also may require scientists to rethink other aspects of the ecosystem. Organochlorines tend to be stable, long-lived compounds, which is one reason they are such persistent pollutants. Myneni's discovery that they are everywhere suggests that chlorination of organic molecules may help soils store certain forms of carbon and other key elements longer than previously thought. An accurate understanding of storage lifetimes is critical for predicting the consequences of pollution and climate change.

The research does not mean that common dirt is suddenly dangerous, said Myneni. "We don't know the toxicity of these natural compounds," he said. He noted that some organochlorines are relatively benign, and even some toxic ones could be bound up with other natural leaf compounds that render them harmless to people.

Nonetheless, his finding already has Myneni thinking about the inevitable time when his son, now one and a half, stuffs his first handful of fresh dirt into his mouth. "Now I definitely will not allow my son to go and eat dirt -- at least until we know what the constituents are," he said.

At the same time, Myneni said his finding does not mean that man-made organochlorines are any less toxic than has been shown by decades of study. Since an early edition of his paper appeared online last month, Myneni has received e-mails from people in industry asking if some pollutants might now be considered more natural and safe than before. Myneni emphasized that his research in no way contradicts extensive data about the toxicity of organochlorine pollutants, nor the very reliable tests used to find them in soils.

"Those compounds are toxic. Period. Our research is not going to change that," he said.

The key to Myneni's discovery was his use of a seemingly unlikely tool. He analyzed samples of leaves, mulch and soil using X-rays generated by a powerful particle accelerator, the Stanford Synchrotron Radiation Laboratory in Stanford, Calif. When exposed to sufficiently powerful X-rays, different chemical compounds produce unique "signatures" in the way they alter the beams. When Myneni tested his samples, they produced a signature unique to organochlorines.

Myneni started with samples from California's Redwood Forest, then collected from New Jersey's Pine Barrens and, to avoid human contamination, from Antarctica. He even included samples from his backyard and the Princeton campus. "We also just walked out of the synchrotron and collected a few samples," he said. "Everything showed this signal."

Myneni then tested materials over the course of a year as they changed from fresh leaves to increasingly decayed organic matter. When fresh, the material contained chlorine mainly in the form of inorganic minerals and ions such as those in salt. As the material decayed, the inorganic chlorine formed chemical bonds with carbon in complex organic molecules.

At the most advanced stage of this process, organic soils contained concentrations of organochlorines in the range of 17 to 70 parts per million. "Nearly all the chlorine present in the plant is converted to organochlorines," he said. "These concentrations are extremely high."

It is not clear why these compounds have not shown up in routine tests for chlorine-based pollutants. One possible explanation, said Myneni, is that the natural organochlorines are bound to very large and complex organic molecules, such as tannins and lignins, that essentially hide the organochlorines. Such coupling of molecules may also render natural organochlorines harmless to humans, even though they might be very toxic on their own.

Method a breakthrough

Myneni's use of the particle accelerator has key advantages over conventional tests. The device allowed him to study the samples without any more preparation than crumbling them up and putting them in the path of the radiation beam. Standard methods of looking for organochlorines involve extensive chemical manipulation to extract the compounds, followed by exposure to visible light or infrared radiation. That process could either destroy the natural compounds or fail to extract them.

"Satish's real breakthrough was the establishment and application of this method," said Casey. "We now have a new method that can show the formation of organochlorines in their natural environment."

Myneni was not looking for organochlorines when he began his study four years ago as a postdoctoral researcher at the Lawrence Berkeley National Laboratory. He was looking for sulfur and phosphorus and kept experiencing interference in his signal from chlorine, which is next to sulfur on the periodic table of the elements. He spent a year trying to find something wrong with his experiment.

Myneni, who came to Princeton as an assistant professor of geosciences in 1999, is now starting on studies to identify the particular organochlorines that are present in mulch and to assess their toxicity. With the help of graduate student Rachel Reina, he also has begun tests in which he is applying natural enzymes to leaves to recreate the chlorine transformation in the lab.

That work will involve collaborations with faculty from geosciences, chemistry, biology and environmental engineering. "The great thing about Princeton is we have so many people with complementary expertise," he said.


February 18, 2002
Vol. 91, No. 16
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