Effects of in-situ straw return on PAHs removal and bioavailability depend on their levels in contaminated soil

Straw return, a widespread agronomic practice, substantially affects the fate of polycyclic aromatic hydrocarbons (PAHs) in soils. In-situ straw return may exacerbate soil contamination by enhancing pollutant input and decreasing microbial activity. However, its impact on PAHs contaminated soils with different levels still remains unclear. Here, we investigated the impact of fractions of straw-derived dissolved organic matter (DOM) on PAHs bioavailability in soils, and clarified the effects of in-situ straw return on PAHs removal and bioavailability in PAHs contaminated soils by analyzing FTIR, EEMs, PAHs concentration, PAHs fractions transformation and metagenomics. Our results showed that the DOM fraction with low molecular weight from straw decomposition enhanced HMW PAHs desorption. Only in high contamination soils did in-situ straw return increase PAHs removal rate (4.05 %–6.20 %), bioavailable fractions concentrations (50.22 %–97.62 %) and uptake rate in winter wheat (19.46 %–22.68 %). Metagenomics showed that in-situ straw return promoted PAHs degraders including Nocardioides (153.06 %–316.13 %), Halomonas (91.77 %–262.35 %), and Streptomyces (167.92 %–208.11 %). Our findings indicated that in-situ straw return effectively enhanced PAHs removal in high PAHs contaminated soils by improving DOM concentration and microbial condition. This research provides new insights for bioremediation strategies of high PAHs contaminated soils, but caution is required for low PAHs contaminated soils.