Field performance of alder-Frankia symbionts for the reclamation of oil sands sites

The Canadian province of Alberta is the world's largest producer of petroleum products from oil sands exploitation. Oil sands process-affected materials (OSPM), such as tailings sand, produced as a result of bitumen extraction, has low fertility, low organic matter content, it is alkaline, compactable, and contains residual hydrocarbons, making it a very inhospitable growth environment. The petroleum industry is currently involved in efforts to revegetate and remediate the tailings sand. One approach used is revegetation of the reclamation sites with Frankia-inoculated alders. Alders are primary succession trees that have the ability to grow in nutrient poor and waterlogged environments, in part because they form a symbiotic relationship with the nitrogen-fixing Actinobacteria, Frankia. In 2005, field trials were established at Syncrude Canada Ltd. The effect of Frankia-inoculated alders on soil quality was evaluated by monitoring the chemical and microbiological characteristics of the soil. The impact on the indigenous microbial community was also studied using hydrocarbon mineralization assays, and molecular approaches, such as denaturing gradient gel electrophoresis (DGGE). Plant parameters (biomass, nitrogen content) were measured to evaluate the impact of Frankia on alder health and growth. After two growth seasons, Frankia-inoculated and non-inoculated alders yielded comparable amounts of plant biomass and there was an increase in hydrocarbon (hexadecane, naphthalene and phenanthrene) mineralization where the reclamation site had been planted with alder-Frankia. The alder rhizosphere samples all had comparable hydrocarbon mineralization rates. DGGE profiles confirmed a change in the microbial communities of the bulk soil between unplanted and alder-Frankia treatments. Soil tests showed that alder-Frankia decreased soil pH (from 7.5 to 6.6, in 2006, and from 8.2 to 7.2 in 2007) and plant-available sodium content (70% reduction), and had a positive impact on soil organic matter content (increase in up to 6 times in alder-Frankia plots). The field results have confirmed that the alder-Frankia combination results in improved remediation capabilities and enhances soil quality. These improvements in soil quality of the reclamation site provide evidence of the potential of alder-Frankia symbionts to be part of a reclamation strategy for the reforestation of the site, and the re-establishment of a balanced ecosystem.

Research highlights▶ Frankia-inoculated alders were successfully used as part of a reclamation strategy. ▶ Greenhouse inoculation improved root nodulation. ▶ Changes in soil quality were observed after only 2 years of plant growth. ▶ Soil bacterial community increased in diversity and hydrocarbon degradation activity.