Post-reclamation microbial diversity and functions in hexachlorocyclohexane (HCH) contaminated soil in relation to spontaneous HCH tolerant vegetation

The toxicity, volatility and persistence of the obsolete organochlorine pesticide hexachlorocyclohexane (HCH), makes reclamation of contaminated areas a priority for the health and welfare of neighboring human communities. Microbial diversity and functions and their relation to spontaneous vegetation in post-excavation situations, are essential indicators to consider in bioaugmentation or microbe-assisted phytoremediation strategies at field scale. Our study aimed to evaluate the effects of long-term HCH contamination on soil and plant-associated microbial communities, and whether contaminated soil has the potential to act as a bacterial inoculum in post-excavation bioremediation strategies. To scrutinize the role of vegetation, the potential nitrogen fixation of free-living and symbiotic diazotrophs of the legume Louis tenuis was assessed as a measure of nutrient cycling functions in soil under HCH contamination. Potential nitrogen fixation was generally not affected by HCH, with the exception of lower nifil gene counts in excavated contaminated rhizospheres, most probably a short-term HCH effect on early bacterial succession in this compartment. HCH shaped microbial communities in long-term contaminated bulk soil, where we identified possible HCH tolerants such as Sphingomonas and Altererythrobacter. In L. tenuis rhizosphere, microbial community composition was additionally influenced by plant growth stage. Sphingobium and Massilia were the bacterial genera characteristic for HCH contaminated rhi-zospheres. Long-term HCH contamination negatively affected L. tenuis growth and development. However, root-associated bacterial community composition was driven solely by plant age, with negligible HCH effect. Results showed that L. tenuis acquired possible HCH tolerant bacteria such as the Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium dade, Sphingomonas, Massilia or Puntoect which could simultaneously offer plant growth promoting (PGP) benefits for the host. Finally, we identified an inoculum with possibly HCH tolerant, PGP bacteria transferred from the contaminated bulk soil to L. tenuis roots through the rhizosphere compartment, consisting of Mesorhizobium foci, Neorhizobium galegae, Novosphingobium lindaniclasticum, Puntoeu agglomerans and Lysobucter bugurensis. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study evaluates the effects of long-term HCH contamination on soil and plant-associated microbial communities, as well as the potential of contaminated soil to act as a bacterial inoculum in post-excavation bioremediation strategies. Results show that microbial communities in long-term contaminated bulk soil were shaped by HCH, while the composition of root-associated bacterial communities in L. tenuis was mainly influenced by plant growth stage.