Abstract
Phytostabilized highly acidic, pyritic mine tailings are susceptible to re-acidification over time despite initial addition of neutralizing amendments. Studies examining plant-associated microbial dynamics during re-acidification of phytostabilized regions are sparse. To address this, we characterized the rhizosphere and bulk bacterial communities of buffalo grass used in the phytostabilization of metalliferous, pyritic mine tailings undergoing re-acidification at the Iron King Mine and Humboldt Smelter Superfund Site in Dewey-Humboldt, AZ. Plant-associated substrates representing a broad pH range (2.35–7.76) were sampled to (1) compare the microbial diversity and community composition of rhizosphere and bulk compartments across a pH gradient, and (2) characterize how re-acidification affects the abundance and activity of the most abundant plant growth-promoting bacteria (PGPB; including N2-fixing) versus acid-generating bacteria (AGB; including Fe-cycling/S-oxidizing). Results indicated that a shift in microbial diversity and community composition occurred at around pH 4. At higher pH (>4) the species richness and community composition of the rhizosphere and bulk compartments were similar, and PGPB, such as Pseudomonas, Arthrobacter, Devosia, Phyllobacterium, Sinorhizobium, and Hyphomicrobium, were present and active in both compartments with minimal presence of AGB. In comparison, at lower pH (<4) the rhizosphere had a significantly higher number of species than the bulk (p < 0.05) and the compartments had significantly different community composition (unweighted UniFrac; PERMANOVA, p < 0.05). Whereas some PGPB persisted in the rhizosphere at lower pH, including Arthrobacter and Devosia, they were absent from the bulk. Meanwhile, AGB dominated in both compartments; the most abundant were the Fe-oxidizer Leptospirillum and Fe-reducers Acidibacter and Acidiphilium, and the most active was the Fe-reducer Aciditerrimonas. This predominance of AGB at lower pH, and even their minimal presence at higher pH, contributes to acidifying conditions and poses a significant threat to sustainable plant establishment. These findings have implications for phytostabilization field site management and suggest re-application of compost or an alternate buffering material may be required in regions susceptible to re-acidification to maintain a beneficial bacterial community conducive to long-term plant establishment.
Highlights
Phytostabilization is a promising remediation strategy used to mitigate human-induced metal-contamination of the environment from legacy mine wastes through planting directly into the soil to achieve in situ contaminant-containment by reducing wind transport and promoting immobilization of metal(loid)s in the root zone (Gil-Loaiza et al, 2016, 2018)
In a long-term controlled greenhouse mesocosm study using pyritic metalliferous mine tailings from the Iron King Mine and Humboldt Smelter Superfund site (IKMHSS), we showed that pyrite weathering, the bioavailability of Fe and As, and microbial community diversity and phylogenetic composition are closely associated with pH (Valentín-Vargas et al, 2014, 2018)
Phase 3 at time zero and after 1 year of growth had an average pH of 7.4 ± 0.27 and 6.2 ± 1.9 (Table 1), respectively
Summary
Phytostabilization is a promising remediation strategy used to mitigate human-induced metal-contamination of the environment from legacy mine wastes through planting directly into the soil to achieve in situ contaminant-containment by reducing wind transport and promoting immobilization of metal(loid)s in the root zone (Gil-Loaiza et al, 2016, 2018). The harsh conditions frequently associated with these mine tailings in addition to high metal(loid) content include acidic pH, low carbon and nutrient content, poor soil structure, and a lithoautotroph-dominated microbial community (Mendez et al, 2008; Valentín-Vargas et al, 2014), which together severely limit the potential for successful plant establishment for in situ metal(loid) containment. Research has demonstrated successful plant establishment in these materials following amendment with compost that provides a high proton consumption capacity (PCC), nutrients (i.e., carbon, nitrogen, phosphorous), and a heterotrophic microbial inoculum (Solís-Dominguez et al, 2011; Valentín-Vargas et al, 2014, 2018; Gil-Loaiza et al, 2016). A return to acidic conditions, or re-acidification, occurs if the acid-generating potential (AGP) of the substrate exceeds the PCC, posing an obstacle to long term-plant establishment
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
