Growth tolerance and remediation potential of six plants in oil-polluted soil

The aim of this study was to screen plant species for remediation of oil-polluted soil in a semi-arid loess area. Pot experiments were conducted to test six indigenous plant species: ryegrass (Lolium perenne), tall fescue (Festuca ovina), wheatgrass (Agropyron cristatum), alfalfa (Medicago sativa), erect milkvetch (Astragalus adsurgens), and caragana (Caragana korshinskii). Loessial soil was spiked with crude oil at five pollution levels (0, 0.5, 1, 2, and 4%, w/w). We analyzed plant growth parameters at different stages, soil oxidoreductase activities, and oil degradation rate under different treatments. Soil pollution by oil negatively affected the germination rate, plant height, and biomass of all six species while inhibiting soil dehydrogenase and catalase activities. These inhibitory effects increased with increasing oil pollution level, with 2% and 1% being the critical levels at which plant growth and soil oxidoreductase activities were significantly inhibited, respectively. Both oxidoreductase activities in the rhizosphere soil were significantly higher than those in the bulk soil, which led to a considerable increase in the degradation rate of total petroleum hydrocarbons (TPHs) in the rhizosphere. The plants showed varying remediation effects in the oil-polluted soil. Erect milkvetch and caragana showed potential for remediation of soil below the 0.5% pollution level, alfalfa for soil below the 1% pollution level, and ryegrass, tall fescue, and wheatgrass for soil below the 4% pollution level. The oil pollution level was negatively correlated with plant height, biomass, oxidoreductase activities, and TPH degradation rate for the six plants (p