The use of plants, including trees, to remediate oil-contaminated soils : a review and empirical study

Soil contamination can result in soil degradation, bring great loss to agricultural production and pose threat to human health. Many of the soil contaminants are petroleum hydrocarbons (PHCs) derived from crude oil or refined petroleum products. Phytoremediation which relies on plants and their associated microorganisms to remove contaminants is cost-effective and applicable to treat a wide variety of soil contaminants. Besides trees, herbaceous plants are widely and effectively used in the remediation of PHC contaminated soils. Greenhouse studies have found that Galega orientalis co-inoculated with Rhizobium galegae and plant growth promoting bacteria (PGPB) benefiting soil with nitrogen fixation is able to remediate PHC contaminated soils. The FP7 ‘‘Legume-Futures’’ remediation field experiment was established at Viikki experimental farm, University of Helsinki in 2009 in order to test the practical applicability of the greenhouse results in a field scale. In a split-plot design, crop (Galega orientalis, Bromus inermis, Galega orientalis + Bromus inermis, bare soil control) treatments were designated the main factor, oil (±) and PGPB (±) the sub-factors in factorial combination with four replicates. Soil samples were taken at four time points from July 2009 to May 2011. Soil total solvent extractable material (TSEM) was extracted and measured by the gravimetrical method as a direct indicator of oil content. Physiochemical properties (pH, EC, total C and N and C/N ratio) of soil samples (taken in July 2009 and Nov. 2010) were determined. The losses of total C and TSEM between July 2009 and Nov. 2010 were calculated to estimate the differences crops and PGPB brought in oil treated plots. Crop dry matter yields were determined. The changes of soil microbial population, bacterial diversity and community structures were studied by the 16S rRNA gene based community fingerprinting method LH-PCR. Bioremediation and physical removal were the main processes of oil removal in our experiment. Climate factors (e.g. temperature and precipitation) had an overriding influence on the removal of oil in our study. Soil condition with a neutral pH and C/N ratio in our field was optimal for biodegradation of hydrocarbons. The changes in soil microbial total DNA, diversity and community structure were sensitive indicators of soil contamination and recovery. Crop (Galega orientalis and Bromus inermis) and PGPB treatment had no significant effect on soil physiochemical and microbiological properties nor on the removal of oil in our experiment, which largely differed from our hypothesis. Resource competition between crops and microorganisms might have resulted in the better oil remediation in bare soils than in vegetated soils. Nevertheless, crops were found to have a high tolerance to oil contamination and surprisingly, the oil contamination seemed to increase the growth of both crop species. Bromus in mixture plots (without commercial nitrogen fertilization) had better yield than in pure plots (with commercial nitrogen fertilization) as a result of biological nitrogen fixation of Galega orientalis and Rhizobium galegae. Therefore the mixture of galega and bromus can be suggested to be applied in future phytoremediation projects.