The effect of hydrocarbon contamination and mycorrhizal inoculation on poplar fine root dynamics

Quantifying the effects of hydrocarbon contamination on hybrid poplar fine root dynamics provides information about how well these trees tolerate the adverse conditions imposed by the presence of petroleum in the soil. Infection by ectomycorrhizal (ECM) fungi may benefit hybrid poplar growing in contaminated soils by providing greater access to water and nutrients and possibly inducing greater contaminant degradation. The overall objectives of this research were to: 1) investigate the relationship between the varying concentrations of total petroleum hydrocarbons (TPH) and nutrients across a hydrocarbon-contaminated site, as well as interactions between these contaminants and physical and chemical soil properties; 2) quantify the effects of these properties on the spatial and temporal patterns of fine root production for Griffin hybrid poplar (P. deltoids x P. petrowskyana c.v. Griffin); and (3) quantify the effect of ectomycorrhizal colonization on hybrid poplar fine root dynamics and N and P uptake when grown in diesel contaminated soil under controlled conditions. A minirhizotron camera provides a nondestructive approach for viewing roots in situ. This camera was used in both the field and growth chamber experiments to provide the data necessary for estimating fine root production. The field study was conducted at Hendon, SK, Canada. Twelve minirhizotron tubes were distributed across the field site and facilitated quantification of fine root production in areas of varying contamination levels. Residual hydrocarbon contamination was positively correlated with soil total C and N, which may suggest that the hydrocarbons remaining in the soil are associated with organic forms of these nutrients or increased microbial biomass. Total fine root production at the site was greater in the 0- to 20-cm depth (1.27 Mg/ha) than the 20- to 40-cm depth (0.51 Mg/ha) in 2004. Fine root production was stimulated by small amounts of hydrocarbon contamination at the field site. Nonlinear regression described fine root production as increasing linearly up to approximately 500 mg/kg TPH, then remaining constant as contamination increased. This trend was most pronounced in the 0- to 20-cm soil layer, with a (r² = 0.915). Stimulation of fine root production in the presence of hydrocarbons has significant implications for phytoremediation. If hybrid poplar can maintain increased root production in hydrocarbon contaminated soils, the rhizosphere effect will be exaggerated and increased degradation of contaminants is likely to occur. Under controlled conditions, colonization of hybrid poplar roots by the ectomycorrhizal fungus Pisolithus tinctorius increased fine root production in a diesel contaminated soil (5000 mg diesel fuel/kg soil) compared to non-colonized trees growing in the same soil. Fine root production was 56.6 g/mˉ² in the colonized treatment and 22.6 g/mˉ² in the non-colonized treatment. In diesel contaminated/ECM colonized treatment, hybrid poplar leaf N and P concentrations after 12 wk were 23.1 and 3.6 g/kg, respectively. In diesel contaminated/non-colonized treatment, N and P concentrations were 15.7 and 2.7 g/kg, respectively. After 12 wk, 5.0% of the initial concentration of diesel fuel remained in the soil of the non-colonized treatment and 6.7% remained in the colonized treatment. Both treatments removed more contaminants from the soil than an unplanted control, which contained 8.9% of the initial diesel fuel concentration after 12 wk. Significantly more hydrocarbons were found sequestered in hybrid poplar roots from the colonized treatment (354.1 mg/kg) than in the non-colonized treatment (102.2 mg/kg). The results of this study indicate that hybrid poplar may be good candidates for use in phytoremediation of petroleum hydrocarbons because of the stimulation of fine root production at low levels of hydrocarbon contamination. However, colonization of hybrid poplar growing in diesel contaminated soil by P. tinctorius inhibited remediation of diesel fuel