When Joyce Kilmer penned his paean to the tree, he didn’t know the half of it.
Oh, any tree can be lovely to the eye and sport nests of robins (or, in Hawai`i, mynahs or manikins) in its hair. What Kilmer didn’t celebrate are trees’ amazing ability to scrub the soil where they live, even when the “earth’s sweet flowing breast” may be contaminated by spills of oil and gas.
That ability, which so far seems to have escaped the notice of poets, has been the subject of much recent work by University of Hawai`i scientists, enlisted by the Army in an effort to determine what trees or other plants might be the best candidates to clean up petroleum-contaminated soil near the coast at Hickam Air Force Base on O`ahu.
Two of the scientists – Wen Hao Sun, a post-doctoral researcher in the university’s Department of Molecular Bioscience and Bioengineering, and Francoise Robert, associate professor in the Department of Microbiology – presented their findings at a January conference sponsored by the university’s Water Resources Research Center.
The result? If a tree’s beauty is measured by its soil-cleansing properties, then the handsome kou (Cordia subcordata) and sturdy milo (Thespesia populnea), both generally thought to be Polynesian introductions to the islands, are fairer than kiawe (Prosopis pallida), ironwood (Casuarina equisetifolia), and coral trees (Erythrina variegata). One grass (buffelgrass, Cenchrus ciliaris) and three shrubs (false sandalwood, or Myoporum sandwicense, beach naupaka, or Scaevola sericea, and oleander, or Nerium oleander) were also tested by Sun and Robert, but they, too, were overshadowed by kou and milo.
Sun’s work had two key phases. First, he determined which plants were most tolerant of diesel at two concentrations (5,000 and 10,000 parts per million) and salinity (with salt concentrations in the soil of 1 percent and 2 percent). Milo, kou, ironwood, oleander, naupaka, and false sandalwood all were able to tolerate the high concentrations of salt and diesel.
Sun then placed cuttings in specially designed growing tubes called trisector planters — narrow, deep containers with strata of soil similar to those found at the contaminated site at Hickam. The bottom layer of sandy loam was spiked with six representative components of diesel (out of more than 200 components). After 200 days, the soil was tested and the six selected components were measured in the planted soil as well as in soil with no plants to test the effectiveness of the various species in hastening decay of the hydrocarbons.
The winners of this beauty pageant were milo and kou, which substantially accelerated the breakdown of some diesel components, compared to the rate of their breakdown in the unplanted soil.
Robert’s work looked at the actual process of decomposition, or breakdown, of diesel products. This is accomplished through the action of microorganisms among tree roots.
For her experiments, Robert censused populations of bacteria in the rhizosphere, or root zone, of six plants (kou, milo, kiawe, oleander, beach naupaka, and false sandalwood) and in unplanted soil from Sun’s experiments. The bacteria included three types known to degrade diesel or individual diesel constituents. She used soil treated with 1 percent salt and contaminated with diesel at a concentration of 10,000 parts per million (1 percent) and after 98 days, tested the soil again for the presence of bacteria and contaminants. “We wanted to operate at the level that was near the limit of plant tolerance,” she says.
In a summary of her work, Robert suggests that the plants most efficient at removing diesel get this quality from the particular type of substances in their root exudates. The exudates “diffuse out of the plant roots,” she explains. Exudates “may be all kinds of things, but usually they include some sugars, amino acids, proteins, enzymes, and hormones. They may also include phenolic compounds, which turn on the mechanism of the bacteria for the degradation of petroleum products.”
When the bacteria-laced roots of these plants reach contaminated soil, the subset of bacteria that break down diesel start feeding on the diesel rather than their normal diet of “root exudates,” Robert speculates, “because the root exudates are less attractive than the contaminant.”
At the same time the lab work was being done, an Army contractor, CH2M Hill, planted trees at Hickam to see how they performed in the field. But by no means is the lab work done, according to Robert.
“We still have further lab work to do to see what happens when the microbes are faced solely with root exudates as food, and then with both root exudates and diesel. We need to ascertain they switch from the root exudates to diesel,” she says. “We also need to determine the differences between the root exudates of the efficient and inefficient plants in the process of degrading diesel.”
— Patricia Tummons
Volume 13, Number 9 March 2003
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