Phytoremediation of hydrocarbon-contaminated soil using plants adapted to the western Canadian climate
Robson, Diana Bizecki
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As hydrocarbon contaminated sites occur throughout Canada, and are threats to ecosystem and human health, techniques to remediate them are needed. Phytoremediation is the use of plants and their associated microorganisms to degrade, sequester or contain contaminants in soil. This technique is gaining in popularity due to low cost and minimal soil disturbance. However, there are several barriers to implementation of phytoremediation biotechnology in Canada. Only a few species of plants that are native or non-native in Canada have been tested for hydrocarbon tolerance and/or degradation ability. Furthermore, the reason why some plants are more tolerant of hydrocarbons than others, and whether tolerant species also increase hydrocarbon degradation is still unknown. A series of experiments were executed to examine hydrocarbon tolerance in plants including botanical field surveys and growth chamber experiments. Contaminated field plots had significantly higher soil pH, carbon to nitrogen ratio and bare ground, and lower total nitrogen, available phosphorus and litter cover. Mean diversity was 0.52 at the uncontaminated and 0.45 at the contaminated plots even though species richness was similar. Mean species similarity between contaminated and uncontaminated plots was only 31.1 % and cover similarity 22.2%. The most common species observed on contaminated field soil were kochia (Kochia scoparia (L.) Schrad.), wild barley (Hordeumjubatum L.), salt grass (Distichlis stricta (Torr.) Rydb.), Canbyi bluegrass (Poa canbyi (Scribn.) Piper), western wheatgrass (Agropyron smithii Rydb.) and slender wheatgrass (A. trachycaulum var. trachycaulum (Link) Malte). Species that were non-native, non-mycorrhizal, annual, biennial, large seeded and that reproduced by seed only were significantly more common on contaminated plots. Species that were non-mycorrhizal, self-pollinated, large seeded and that reproduced by seed only formed significantly more plant cover on contaminated plots, while woody species and those with unassisted or bird-dispersed seeds formed less. The species with the highest survival after five weeks in a variety of crude oil contaminated soils included one native and four non-native grasses, two native and three non-native legumes and two native forbs. All plants grown in potting soil contaminated with 5,000, 10,000 and 50,000 ppm crude oil had significantly lower total biomass and relative growth rates (RGR) compared to the control except Indian breadroot (Psoralea esculenta Pursh). As the crude oil concentration increased from 5,000 to 50,000 ppm total biomass became more strongly negatively correlated (from r=-0.674 to r=-0.939) with RGR. In hydrocarbon-contaminated field soil, total biomass and RGR of eight species with seed masses covering four orders of magnitude were significantly lower than in uncontaminated soil. Both seed size (r=0.976) and RGR (r=-0.916) were strongly correlated with performance in hydrocarbon-contaminated field soil. Those species with large seeds and slow RGR were more tolerant than species with small seeds and high RGR. The two most tolerant species were Indian breadroot and crested wheatgrass (A. pectiniforme R. & S.). After 16 weeks of growth crested wheatgrass was a better hydrocarbon degrader than Indian breadroot as the quantity of five unidentified hydrocarbons was significantly lower in the rhizosphere soil of the former species. When these two species were grown together in the same pot, individual crested wheatgrass plants produced 16 mg more biomass on average than when grown in single species pots, suggesting that interplanting a legume and a grass benefits growth. Hydrocarbon tolerance in plants was related to resource use. Plants possessing stress-tolerant traits performed better on hydrocarbon-contaminated soil than those species with competitive or ruderal traits. This is because the soil of contaminated sites had low fertility and/or adverse soil chemistry. The most tolerant species were not necessarily good at hydrocarbon degradation.