Spatial heterogeneity and oil pollution structured the soil microbial community in salt marshes in Barataria Bay, Louisiana, USA, eight years after the Deepwater Horizon oil spill

•Oiled marshes contained distinct soil microbial communities eight years later.•Spatial location and petroleum residue governed the structure of soil microbial communities.•The most abundant bacterial indicator for heavily oiled marshes was genus KCM-B-112.•Myxobacteria were indicators of reference sites. The Deepwater Horizon (DWH) oil spill significantly impacted salt marsh ecosystems, with numerous repercussions observed in the subsequent years. However, the long-term effects of chronic oil exposure on soil microbial communities remain unexplored. This study, conducted in 2018, aimed to identify how the legacy of heavy oiling from the DWH spill in 2010 influenced the soil microbial community. We surveyed vegetation and soil variables while simultaneously employing high-throughput 16s rRNA gene sequencing to examine microbial communities in soils across 21 marsh sites that exhibited varying degrees of initial oil contamination following the DWH incident. The effects of these variables, including geographic location, on taxonomic and phylogenetic metrics of community composition were disentangled using variance partitioning analysis and permutational ANOVA. Results showed a strong and significant longitudinal trend in microbial community composition that was partially confounded with degree of initial oil exposure and contemporary vegetation biomass. However, significant indicator species for heavily oiled sites included oil-degrading bacteria, and there were significant differences in microbial community composition among marshes that received variable levels of initial oiling after controlling for this spatial effect. The level of soil petroleum residues that remained in 2018 had significant explanatory power for community composition after controlling for location, whereas the effects of vegetation biomass were largely confounded with longitude. Collectively, our results indicate that persistent oil pollution as well as unidentified spatial processes, possibly associated with spill-induced alterations in erosion or wave dynamics, played a role in structuring soil microbial communities within this ecosystem.