Evaluation of Agriculture-Based Phytoremediation in Pacific Island Ecosystems Using Trisector Planters

ABSTRACT

It is difficult to directly evaluate the efficacy of phytoremediation of petroleum hydrocarbon contaminants embedded in deep soil layers, especially if the contaminants are of relatively low concentration and are unevenly distributed. This report describes the greenhouse and laboratory experiments carried out to evaluate a field demonstration project. A trisector planter was designed to simulate field conditions, including soil profiles and field management of the trees selected. The third or bottom section of the planter was spiked with known quantities of 6 diesel-fuel components and the reduction of their concentrations was monitored after 200 days under the influence of the plant root systems. Results are statistically compared; among the three tree species used, milo (Thespesia populnea) and kou (Cordia subcordata) are more effective than false sandalwood (Myoporum sandwicense) in reducing the concentration of the spiked contaminant. Enumerations of populations of hydrocarbon-degrading microoorganisms in the bottom section suggest that biodegradation may be affected by the response of microorganisms to both the “close rhizosphere” (soil within 1 mm of the root) and the “expanded rhizosphere” (soil in the bottom section after root removal). Root exudates leached from the upper sections could be responsible for the expanded rhizosphere effect in the bottom section.

Keywords:

• bioremediation
• deep soil contamination
• PHC
• PAH
• rhizosphere
• soil microorganisms

ACKNOWLEDGMENTS

Funding for this project was provided by the U.S. Army Environmental Quality Technology Program, Agricultural Bioremediation Project, administered by the U.S. Army Engineer Research and Development Center, Environmental Laboratory, and the U.S. Army Engineer District, Honolulu, HI. The authors wish to acknowledge the support of CH2M HILL, Inc., and the generous assistance of Chittaranjan Ray, Jim Jordahl, and Mark Madison in various phases of the project.

Notes

^a This soil was used in the middle section of the trisector planters.

^b The sandy loam below the silt layer was wet at the time of collection and showed signs of anaerobiosis (black spots). This soil was used in the bottom section of the trisector planters and was spiked with six selected hydrocarbons.

*For definitions of Rc and Re, see text.

^a Only means of counts in the rhizosphere of the same plant in the same column should be compared. Means followed by different letters in the same column are significantly different (P < 0.01) according to the Wilcoxon rank sum test.

^b Rc/S or Re/S = close- or expanded-rhizosphere effect = number of microorganisms in the respective rhizosphere/number of microorganisms in unplanted soil. The rhizosphere effects were calculated using the antilog of the values in the table.