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This research was conducted to find an optimal inoculation way for a pyrene-degrading endophytic Serratia sp. PW7 to colonize wheat for reducing pyrene contamination. Three inoculation ways, which are soaking seeds in inocula (TS), dipping roots of seedlings in inocula (TR), and spraying inocula on leaves of seedlings (TL), were used in this study. Inoculated seedlings and noninoculated seedlings (CK) were, respectively, cultivated in Hoagland solutions supplemented with pyrene in a growth chamber. The results showed that strain PW7 successfully colonized the inoculated seedlings in high numbers, and significantly promoted the growth of seedlings (TS and TR). More importantly, strain PW7 reduced pyrene levels in the seedlings and the Hoagland solutions. Compared to the noninoculated seedlings, the pyrene contents of the inoculated seedlings were decreased by 35.7-86.3% in the shoots and by 26.8-60.1% in the roots after 8-day cultivation. By comparing the efficiencies of decreasing pyrene residues, it can be concluded that TR was an optimal inoculation way for endophytic strains to colonize the inoculated plants and to reduce the pyrene contamination. Our findings provide an optimized inoculation way to reduce organic contamination in crops by inoculating plants with functional endophytic bacteria.

Understanding the release and availability of bound residues of polycyclic aromatic hydrocarbons (PAHs) in the rhizosphere is essential when assessing their fate and risk in the soil environment. However, limited information is available regarding the impact of root-secreted low-molecular-weight organic acids (LMWOAs) on the release of bound PAH residues in soils. The aim of this study was to investigate the LMWOA-influenced release of soil bound PAH residues. An n-butanol extraction procedure was used to evaluate the release of soil bound PAH residues. LMWOAs can dramatically promote the release of bound PAH residues and enhance the extractability and availability of PAHs in soil. The addition of LMWOAs significantly increased the extractable concentrations of ΣPAH and each individual PAH in soils containing bound PAH residues in a LMWOA-concentration dependent manner. Compared with control treatments without addition of LMWOA, the concentrations of butanol-extractable ΣPAH and each PAH in soils following the addition of 10–100 mmol/kg LMWOAs were 54–745% and 58–605% larger after 40 and 80 days, respectively. Citric acid always generated the largest PAH release from soil bound PAH residues, while malic acid generated the lowest release among the three LMWOAs tested. The results of this work provide important information regarding the fate of bound PAH residues in the soil environment, and will be useful for the risk assessment of PAH-contaminated soils and the development of remedial strategies for contaminated sites.