Rehabilitation of degraded soils containing aged PAHs based on phytoremediation with alfalfa (Medicago sativa L.)

It has been hypothesized that the sorption of high-molecular-weight PAHs to soil and organic matter matrices reduces PAHs availability to biodegradation and phytoextraction. One of the main objectives of this pot-scale phytoremediation study was to enhance the disappearance of weathered PAHs, mainly recalcitrant benzo[a]pyrene (B[a]P), seeing that a previous 15-month bioremediation strategy simulating landfarming (unvegetated) could not achieve a complete removal of spiked hydrocarbons. In addition, landfarming coupled with sewage sludge compost (SSC) addition led also to salt accumulation and soil pH reduction, which are warning signs for degraded soils. In this subsequent six month phytoremediation study, alfalfa plants (Medicago sativa L.) had a remarkable effect on ameliorating the physicochemical properties of SSC-degraded soils by reestablishing initial pH values (>7.0) and significantly reducing salinity (EC < 500 μS cm−1). These changes were reflected markedly by a steep decrease of soil toxicity toward lettuce root elongation and ostracod survival. Soil rehabilitation coupled with rhizosphere effect also impacted positively on microbial growth and soil enzyme activities as compared to the situation prior to alfalfa planting. The dissipation of residual B[a]P was also enhanced by soil cropping, but only up to 64% for treatments initially spiked at the highest level of 100 mg B[a]P kg−1 soil. Consequently, remaining B[a]P in soil (2.5–4.3 mg kg−1) is not likely to be available for further degradation using non-destructive remediation strategies as applied throughout this long-term study.