Ecology of ecto- and ericoid mycorrhizal systems in petroleum hydrocarbon-contaminated sub-boreal forest soils.

The impacts of petroleum hydrocarbon (PHC) contaminants in northern forest soils are not well understood from either eco-toxicological or microbial ecological perspectives. The purpose of this research was to examine interactions between PHCs and ecto-(ECM) and ericoid (ERM) mycorrhizal communities at the rhizosphere scale, where microbial activities underpin processes such as decomposition, carbon and nutrient cycling, and primary production at landscape scales. Several methodological approaches were simultaneously used to assess changes in physical, chemical and biological properties of plant-soil systems treated with ecologically relevant concentrations (i.e. ~7-22 tonnes ha⁻¹) of PHCs. From microscopy and community fingerprinting (LH-PCR) studies, we found few differences in community structure attributable to PHC contamination. PHC treatment also did not appear to alter broad patterns of C metabolism for either bacterial (CLPP) or ECM fungal (laccase assay) communities. Habitat changes, which generally included increased C:N ratios, slightly more acidic pH, and hydrophobicity or water-logging in organic and mineral layers, respectively, did not appear to inhibit microbial communities. Together, these findings point to resilience within intact mycorrhizal systems, mainly due to sorption of PHCs within organic soil layers and protective properties of the mycorrhizosphere habitat. Soil PHC analysis (GC-FID) showed a general decrease in contaminant levels 16 weeks after PHC treatment, indicating an intrinsic capacity for biodegradation within the soil communities. ECMs appeared to play a vital role in this process through provision of habitat and co-substrates for heterotrophic bacterial communities (i.e. mycorrhizosphere effect) and via secretion of laccase, which opens aromatic ring structures for subsequent bacterial attack. These results emphasize the importance of synergistic functions among microbial guilds with respect to ecological processes. Finally, we found that the spatial patterns of mycorrhizal communities within the rhizosphere depended primarily on properties of the host plant and soil environment. The extent that different properties influences community structure varied between the three groups of microorganisms. This system approach addressed fundamental questions in mycorrhizal ecology by considering PHC pollution as a form of environmental disturbance. Conservation of the integrity of mycorrhizal systems in contaminated forest soils may be key for sustainable management in terms of ecosystem resilience and remediation.