Effect of soil amendments on tailings characteristics
Plant growth on mine tailings is facilitated by the addition of soil amendments, which are used to enhance physiochemical conditions for plant growth [4, 20]. Analysis of the HATSF mine tailings revealed high pH, WHC (or poor drainage) and metals, and low OM and nutrients. The combination of these poor physiochemical properties limited the establishment of plants on the unamended tailings.
Assessment of tailings characteristics before and after amendment addition revealed an increase in the OM content of the amended tailings (Table 2), which is consistent with other mine tailings studies [26, 50, 51]. The compost-treated tailings had the highest OM content, and the ash-treated tailings had the lowest. Organic matter is important for soil rehabilitation and reclamation because (1) the organic C provides an energy source for soil microorganisms, which accelerates decomposition and nutrient cycling [4, 7], (2) long-term plant nutrient availability is enhanced because N is in an organic form and is slowly released over time , (3) the OM improves soil physical conditions such as water retention and bulk density [17, 49, 51, 52], and (4) some organic residues (e.g., waste from agri-food industry, seaweed) have high concentrations of base cations, which increase pH and cation exchange capacity . Because of these properties, the longevity of positive restorative effects is often greater when using organic amendments compared to traditional reclamation methods such as inorganic fertilizers [18, 53].
The addition of amendments had a significant effect on tailings pH levels (Table 2). The compost amendment lowered tailings pH, but the effect was suboptimal as the amended tailings remained moderately alkaline (pH > 8). The ash amendment increased the tailings pH level from moderately alkaline to strongly alkaline (pH > 9), which is above the preferred range for most plants (pH 5.5 to 8.5) . In a recent review, Sheoran et al.  reported that mine soil pH range of 6 to 7.5 is adequate for agronomic or horticultural uses of mine sites. Although, in arid environments, it is normal for pH to be slightly to moderately alkaline (pH between 7 and 9) . Abnormally high soil pH can increase mobility of metals such as As, Mo, and Se and limit the availability of P and certain micronutrients (e.g., B, Mn, Fe) [25, 55].
In general, the addition of organic amendments increased the EC of the tailings, with the exception of the blend which had little or no effect (Table 2). In all treatments, EC remained below the critical level of 4 dS m−1 at which plant growth is negatively affected .
The high gravimetric WHC (Table 2) in the tailings indicates water retention was not a limitation; however, substrates with high WHC (> 80%) can have poor drainage leading to anoxic conditions, which can affect root productivity and reduce overall revegetation success . The addition of both amendments reduced the WHC of the tailings, likely because the addition of larger organic particles reduced bulk density and improved drainage of the substrate.
Overall, the compost treatment appeared to provide the most suitable soil conditions for revegetation of the HATSF mine tailings; however, lowered pH from the addition of OM could result in the release of heavy metal cations, such as Ni and Cr, from the substrate, causing potential negative impacts to the surrounding environment and groundwater through leaching and mass flow, especially if the site is irrigated [8, 13]. Under such conditions, environmental and human health risk may be elevated as elements become phytoavailable and enter the food chain through consumption by primary consumers [8, 10].
Changes in soil physiochemical properties following the 90-d growth period were not assessed in this study; however, based on the previous research, we expect plant establishment to aid in further improving the growing conditions of the amended tailings. It is well known that plant establishment improves tailings properties over time by initiating soil development processes such as nutrient cycling, soil aggregation and accumulation of organic C [50, 56, 57]. For instance, Cele and Maboeta  reported significant improvements in soil fertility parameters such as OM, WHC, CEC, and plant nutrients after 12 weeks of Cynodon dactylon (perennial, rhizomatous grass) growth on Fe mine tailings. Furthermore, Antonelli et al.  noted improvements in soil C, N, P content and stable pH levels (around the neutral range) 13 years after revegetating Cu and Mo tailings with agronomic pasture grasses. The latter study noted the role of plants and OM in stabilizing tailings pH levels over time by slowing the rate of carbonate activity and other geochemical weathering processes.
Growth response to soil amendments
The results of the experiment indicate that the addition of amendments, regardless of treatment, improved seedling germination and growth of native bunchgrass species P. spicata and F. campestris on the HATSF mine tailings. Of the treatments investigated, the compost was the most effective at promoting above- and belowground growth of both species. The positive influence of compost on plant productivity was likely a result of improved tailings biological and physiochemical conditions [51, 58]. In two greenhouse studies, Solís-Dominguez et al.  and Gil-Loaiza et al.  reported improvements in tailings pH, EC, organic C, total N and neutrophilic heterotrophic bacteria numbers when compost was applied to acidic mine tailings. The neutrophilic heterotrophic community is thought to play an important role in promoting C and N nutrient cycling by offsetting chemoautotrophic microbial activity (e.g., iron and sulfur oxidation) and is an important indicator of tailings conditions for plant growth [4, 59].
Plants growing in the ash treatment did not appear healthy; this can be attributed to the lack of N coupled with increased pH levels, which created less favorable conditions for plant growth. In high pH environments, bacterial and fungal activity is restricted, which has a negative effect on nutrient cycling [55, 60, 61]. Because of these properties, it is possible that the nutrients (namely P and K) contained in the ash were not available for plant uptake. The data suggest that incorporating the very strongly alkaline ash material into the alkaline tailings was not an effective method for optimizing plant growth. In a recent review assessing remediation technologies on high pH bauxite residue, Santini et al.  suggested inoculating the substrate with alkaline-resistant microorganisms (alkaliphiles) to enhance nutrient cycling. A combination of the ash amendment with an inoculation treatment should be investigated as an option for enhancing the tailings conditions. Assessment of post-treatment nutrient concentrations and neutrophilic heterotrophic bacteria numbers would have revealed more insight as to the limitations of the ash material as a soil amendment.
Root-to-shoot ratios were around 1 for the compost treatment, which indicates balanced biomass allocation and adequate nutrient availability in the amended substrate . Generally, when nutrients are limiting, plants will allocate more resources to their roots, which increases the root-to-shoot ratio . The high root-to-shoot ratios of plants growing in the ash-amended tailings can be explained by the lack of N in the growing medium, which may have forced plants to allocate more effort into root production at the cost of shoot production. The positive response to compost addition (Fig. 2) suggests that N was a limiting factor for plant growth on these tailings. Several studies have underscored the importance of soil N in mine reclamation because it is an essential plant nutrient, yet it is often limiting in mine soil ecosystems [51, 56, 57].
With respect to plant growth and overall productivity, P. spicata outperformed F. campestris on all treatments including the control (Figs. 1a, 2). This can be partially attributed to the ability of P. spicata to germinate under a wider range of conditions compared to other grassland species . The results were consistent with a recent field study where Carlyle  reported higher relative growth rates and shoot and root biomass for P. spicata compared to F. campestris.
Effect of soil amendments on plant metals uptake
Shoot and root concentrations of select metals were assessed for both species after 90 d of growing in the amended tailings. The results indicated high shoot concentrations of Mo, which exceeded maximum tolerable level for cattle  in all treatments, most notably when the ash amendment was used (Fig. 3e). The uptake in Mo on the amended tailings is attributed to the alkaline pH (8.3 to 9.3) of the mixtures, which led to increased mobility of Mo in the soil. According to Goldberg et al. , Mo availability increases with increasing pH due to weaker adsorption of Mo to clay minerals under alkaline conditions. The enhanced Mo uptake in plants grown on the ash treatment can be attributed to a combination of the high pH and P content relative to the other treatments. Neunhäuserer et al.  mention that P additions to soil can reduce the sorption of Mo to clay minerals, thereby increasing the metal’s availability to plants. In the same study, sewage sludge applications of pH 12.1 increased extractable Mo on contaminated pasturelands but decreased plant uptake; it was suspected that the effect was due to other cations such as Ca2+ interacting with Mo to compete for plant absorption. Doran and Martens  found similar effects of soil pH on Mo uptake when growing M. sativa in a fly-ash amendment. In the Neunhäuserer et al.  study, Mo uptake increased in the field due to increased moisture availability. In the current study, Mo uptake may decrease in the field due to lack of moisture, high soil and air temperatures, and other abiotic constraints expected in a semiarid environment .
Although Mo is an essential trace element for plants and animals, elevated levels in forage can lead to molybdenosis (induced Cu deficiency) when ingested by cattle or other ruminants [17, 66]. The condition is influenced by relative concentrations of Cu, Mo and sulfur  and can be detrimental to ruminant health. In general, the risk of molybdenosis increases when the Cu: Mo ratio is < 2:1 . In the current study, Cu: Mo ratios ranged from 0.1 to 0.7 (calculated from Fig. 3b, e), with the lowest value being for F. campestris growing on the ash-amended tailings. As the ratios were all well below the tolerance level, the grasses would not be ideal for ruminant forage under the conditions studied.
The metals Cr and Ni exceeded the soil quality guidelines in tailings but were not analyzed for in plant tissues. Available metal concentrations (Table 3) and pH of the substrates analyzed indicate these metals were not readily available for plant uptake, as noted in other studies with similar pH values [13, 25]. Rodríguez-Vila et al. , the addition of compost (pH ~ 9) and biochar (pH ~ 10) to copper mine tailings, reduced the ‘phytoavailable’ amounts of Ni in the soil as determined by CaCl2 extraction techniques. They determined that a mixture of 19% compost and 1% biochar was most effective at reducing the Ni concentrations in foliage of B. juncea. At the same site, Forján et al.  reported reduced Ni uptake when biochar was used as an amendment. Aluminum concentration of the tailings was notably high (Table 2), and when treated with ash the material had ideal conditions for the formation of soluble Al in the form of aluminate (Al(OH)4–1), which can cause soil toxicity . This may be a reason for the stunted growth and poor plant health observed on the ash treatment. According to Hodson , some plants are able to tolerate excessive levels of Al and other metals by avoiding shoot uptake and concentrating them in their roots. Both plant species used in this study accumulated substantially more Al in their roots (up to seven times) compared to their shoots (TF values 0.07–0.3), which provides some indication of their tolerance to Al (Table 4). Our results suggest that these species may be useful for remediating tailings and other mine wastes that are high in Al.
While changes in available metals or chemical speciation in the tailings following amendment addition and plant establishment were not assessed in the current study, we would expect the compost treatment to have had a positive effect on metals immobilization due to the increase in root biomass and OM present in the rhizosphere. (The roots provide a surface for metal sorption and organic C offers binding sites for complexation with metals.) For example, the key metals in this study, Cu and Mo, have a strong tendency to sorb to clay minerals and bind to organic products on amended soils [29, 69]. Bolan et al.  observed an increase in Cu adsorption and formation of Cu-organic complexes on sandy mineral soils amended with organic manure. In Neunhäuserer et al. , Mo availability decreased with OM, clay minerals, and Fe-oxide content, and increased with addition of P-rich fertilizer amendments.
Evaluation of bunchgrasses for phytostabilization
Vegetation plays a key role in the physical stabilization of barren tailings [4, 7]. Aboveground, the plant shoots, stems and leaves provide cover which protects against wind and water erosion [8, 10]. Belowground, the plant roots act to amalgamate the loose material and reduce the vertical transport of soil particles and contaminants carried by water through the soil profile . We suspect that the observed improvement in seedling emergence and biomass productivity (Figs. 1, 2) with the addition of soil amendments would translate to enhanced physical stabilization in the field. In theory, the plants with higher shoot and root biomass would be more capable of protecting the ground surface from wind erosion and stabilizing the surficial soils .
TF and BCF indices are used for measuring metal accumulation in plant tissues and characterizing plant species strategies for growing in metal-contaminated substrate [7, 13, 33]. Generally, plants with TF values < 1 and root BCF values > 1 are considered excluder plants, whereas those with TF and BCF values > 1 are accumulator or hyperaccumulator plants, depending on the total concentration of the metal sequestered in shoots (generally above or below 1000 mg/kg) [8, 33]. Suitable candidate plant species for phytostabilization have high biomass and are excluders: those which minimize shoot accumulation without limiting root uptake [7, 10, 35], allowing for metals to be stabilized in the rhizosphere and preventing potential translocation to shoots or leaching to groundwater . In the current study, TF values generally remained below 1 for most of the metals investigated, except for Mn, Mo and Zn which exceeded (or were close to) the threshold for all treatments (Table 4). Root BCF values also remained below 1 except for Mo and Zn. These results suggest that both species are accumulators of Mo and Zn; however, with no significant difference in shoot Zn concentration or exceedance of maximum tolerable level for cattle, the effect of amendments on Zn is negligible. Similar TF values were obtained by Forján et al.  for Zn uptake on copper mine tailings amended with a mixture of compost and biochar.
In addition to promoting physical stabilization and translocation of metals from the soil into root and shoot tissues, plants encourage phytostabilization by immobilizing metals in the rhizosphere through a variety of biogeochemical processes . On metal-contaminated soils, OM and plant roots act to reduce available metals via precipitation, root sorption, or formation of metal–organic complexes . These products are typically less soluble compared to their previous form, allowing the metals to be stabilized in place . Root exudes also play an important role in modifying metal availability in the rhizosphere by influencing redox conditions and facilitating microbial transformation of cations .
Despite adequate biomass production, the high shoot concentration of Mo coupled with the TF and root BCF values indicates that both P. spicata and F. campestris are accumulators of Mo and therefore are not suitable for phytostabilization of the HATSF tailings under these conditions. A better use of the grasses would be for phytoextraction where shoot uptake is encouraged (i.e., high TF values) and vegetation is subsequently harvested [36, 70].