Restoring ecological properties of acidic soils contaminated with elemental sulfur
Graphical abstract
Introduction
Acid deposition and generation occurs worldwide predominately from acid rain (Driscoll et al., 2001) and acid mine drainage (Johnson and Hallberg, 2005). The most extreme cases of acidification often occur where reduced sulfur is oxidized (Johnson and Hallberg, 2005, McTee et al., 2016). Approximately 5–10 Mt of reduced sulfur is stockpiled annually as a byproduct of oil and gas refining, much of which is composed of elemental sulfur (S0) that is stored in blocks (Rappold and Lackner, 2010). Chemolithotrophic and mixotrophic bacteria can oxidize S0 to H2SO4 when H2O and O2 are available (Fliermans and Brock, 1972, Lawrence and Germida, 1988, Suzuki et al., 1999). Consequently, effluent water from blocks of S0 can have pH as low as 0.4, which can contaminate soil and water (Birkham et al., 2010). These blocks likely account for the largest volume of S0 introduced to the environment, but other vectors exist. Skeuse and Spencer (1999) patented a trap and skeet target composed of approximately 53% CaCO3, 6% modifiers, and 41% S0. S0 accumulates where trap and skeet targets fall, which can be at public or private shooting ranges (McTee et al., 2016). Oxidation of S0 in sulfur-based targets resulted in soil pH below 3 in a previous study (McTee et al., 2016). These cases of extreme acidity from the oxidation of S0 represent a recent problem and management strategies need to be developed to restore affected areas.
S0 can be managed by controlling the conditions that allow its oxidation. This would involve manipulating oxygen and water availability, temperature, and the bacterial communities that inhabit particle surfaces (Birkham et al., 2010, Nevell and Wainwright, 1987). The only study to effectively lower oxidation rates used surface additions of NaCl, but NaCl washes away with precipitation (Crescenzi et al., 2006). Management approaches to prevent the conditions that allow S0 to oxidize in situ are not well studied and would be laborious and costly. Rappold and Lackner (2010) even suggested that H2S and S0 could be oxidized to H2SO4, neutralized, and disposed of in the ocean. The shortcomings and challenging logistics of these strategies demonstrate the need for alternatives to restore both soil chemistry and biological communities.
Acidic soils strongly affect both plants and bacterial communities (Fierer and Jackson, 2006, Robson, 1989). For example, acidic soils often have high concentrations of Al3+ and Fe3+ that kill plants (Bowman et al., 2008, De la Fuente et al., 1997), leaving soils bare, and susceptible to erosion (Wong, 2003). The absence of plants and low pH also affect the bacterial communities that are integral in the cycling of nutrients (Lauber et al., 2009, Rousk et al., 2009, Rousk et al., 2010). Bacteria may die or enter dormancy in response to a disturbance (Jones and Lennon, 2010), such as acidification, but their ability to emerge from dormancy in response to restoration of acidic soils needs to be examined.
CaCO3 can rapidly neutralize acidic soils (Robson, 1989, Rappold and Lackner, 2010). But the addition of CaCO3 will not restore nutrients lost from acidification (Bowman et al., 2008, Driscoll et al., 2001, Robson, 1989), so additional soil amendments may be needed. Fertilizer replenishes nutrients and boosts plant yields, which could increase soil organic matter (OM) (Haynes and Naidu, 1998). Organic matter buffers pH in part by increasing concentrations of base cations (Yuan and Xu, 2011), reduces Al3+ toxicity to plants (Haynes and Mokolobate, 2001, Seco et al., 2014), and can increase the functional diversity of microbes (Bending et al., 2002). Effective types of OM for restoration include compost, biochar, and chitin. Compost increases plant growth at shooting ranges, partly by reducing the bioavailability of Pb2+ and Zn2+ (Siebielec and Chaney, 2012). Biochar helps plants establish in contaminated soils because it absorbs and retains toxic substances (Beesley et al., 2011). Chitin, which comprises the exoskeleton of arthropods, has been used to immobilize contaminants and increase pH in acid mine water (Daubert and Brennan, 2007, Robinson-Lora and Brennan, 2010).
It is unknown how to manage acidic soils that receive continual inputs of acid from the oxidation of S0. It is also unknown how these management strategies might influence soil quality and the plant and bacterial communities that help maintain a healthy ecosystem. Our objective was to determine to what degree various soil amendments could restore soil chemistry, plant growth, and bacterial communities. Soils were amended with CaCO3 by itself, and in combination with fertilizer, compost, biochar, and chitin, which were then incubated in a greenhouse. After one week, we analyzed soil chemistry and planted Poa nevadensis in half of the containers. After ten weeks, we analyzed soil chemistry again, measured plant biomass, and characterized the bacterial communities. Results were compared to a contamination gradient in the field that had been limed with CaMg(CO3)2 two years prior. Three questions were addressed: 1) how do soil amendments change soil chemistry over time, 2) which soil amendments facilitate the greatest plant growth, and 3) do bacterial communities recover when acidic soils are amended? We hypothesized that CaCO3 alone would restore soil chemistry, plant growth, and bacterial diversity because pH is often the most important driver of these properties. We also hypothesized that organic amendments would enhance plant growth by decreasing concentrations of Al3+ and stabilizing soil pH over time.
Section snippets
Soil collection
We collected soil at a former sporting clay range (Bitterroot Sporting Clays) in the Bitterroot Valley, Montana (46° 41′N, 114° 02′W; elevation 970 m). From 1999 to 2006, the range used trap and skeet targets that contained S0 which caused soil pH to fall below 3 in places (see McTee et al., 2016 for a description of the site). We collected soil (0–15 cm) with a trowel in several areas within five contaminated (termed Contaminated) and five uncontaminated sites (termed Reference hereafter).
Greenhouse: effect of amendments on soil chemistry
Amended treatments clustered separately from Unamended and Reference treatments at one week (Fig. 1a), whereas all amended treatments except CaCO3 clustered with the Unamended treatment at ten weeks (Fig. 1b). Importantly, no treatments clustered near the Reference soil despite a recovery in pH.
At one week, the pH of all soils that received a CaCO3 amendment increased to values equal to the pH in the Reference treatment, but by ten weeks, the pH decreased in all amended soils (Fig. 2).
Multiple applications of CaCO3 may be necessary to remediate S0 contamination
Amending contaminated soils with CaCO3 alone and in combination with fertilizer, biochar, and compost did not generate soil chemical profiles resembling uncontaminated (Reference) soils (Fig. 1). This may be explained, at least partly, by much greater inherent concentrations of Ca2+ and SO42– in contaminated soils compared to uncontaminated soils, and CaCO3 additions increased Ca2+ concentrations further. It is uncertain if elevated concentrations of these ions would actually impair restoration
Conclusions
Using greenhouse experiments and a field survey, we showed that CaCO3 can neutralize acidic soils contaminated with S0, reduce concentrations of Al3+, Mn2+, and Fe, enable plant growth, and increase bacterial richness and diversity to near that of uncontaminated soil. This occurred even though soil amendments did not restore the chemical profile of uncontaminated soils (Fig. 1). Organic and fertilizer amendments increased soil fertility but may provide too little benefit at too high a cost to
Acknowledgements
Sequencing was performed by the IBEST Genomics Resources Core at the University of Idaho and was partly supported by NIH COBRE grant P30GM103324. All other research efforts were funded by MPG Ranch. We thank Chris Harris for assisting with the setup of the experiment, Dave Patterson for lending statistical advice, and Sean Gibbons for discussing techniques to analyze bacterial communities. We are grateful to Ward Laboratories for being helpful with our questions regarding analyses and three
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