Metals are ubiquitous and naturally occurring; however, anthropogenic activities have elevated metal concentrations in river sediments above what would be expected naturally. One of the primary anthropogenic sources of metals in freshwater is contamination by mine tailings, a toxic slurry of waste rock and chemicals left over after ore minerals or coal extraction. Mining waste is most often stored in retention ponds, which sometimes leak or fail. One of the worst mine tailings disasters on record occurred on the Tisza River of Central Europe in the winter of 2000, releasing over 240,000 tons of mine tailings laced with metals into the natural environment. Elevated metals in freshwater can negatively affect fish and other aquatic organisms, raising concerns as to the long-term ecological consequences of the spill. Therefore, the objectives of this dissertation were to describe the ecological implications of metals pollution from mining on freshwater systems through an empirical case study of the Tisza River combined with controlled laboratory experiments of chronic cadmium exposure on a model species, channel catfish (Ictalurus punctatus). Specific research questions discussed in the following chapters include: (1.) What is the current distribution of metals in the lower Tisza River Basin and are metals transported from the main channel to floodplain habitats?; (2.) Is there evidence for bioaccumulation and/or trophic transfer of metals to fish in the Tisza?; (3.) What are angler’s perceptions and fish consumption risks in the lower Tisza river basin?; and (4.) What are the effects of sub-lethal exposure to cadmium on growth, development, metabolism, and stress response in a model fish species? The case study of the Tisza River Basin was conducted during early summer in 2013 and 2014. Water, sediment, and fish were collected from the Tisza River as well oxbow lakes along the lower basin. Samples were screened for metals via GF-AAS following standard methods. Additionally, we conducted interviews at each sampling location to determine fish consumption habits. Controlled laboratory exposures were conducted during summer of 2015. Eggs of channel catfish (Ictalurus punctatus) were fertilized in treatment water and raised to 6 month old fingerlings. Treatments of cadmium included concentrations of 0.5 µg L-1 (control), 2 L-1 (low) and 6 L-1 (high), with endpoints of growth, development, cellular stress, metabolism, and general stress response. Results indicate that lakes with the greatest connectivity trended toward elevated metals; however, chlorophyll a concentrations decreased suggesting dilution of nutrients from surrounding agriculture. River connectivity therefore increases ecosystem health of floodplain lakes by ameliorating eutrophication, but as a trade-off with potential contamination of metals. This may have implications for management strategies in the basin as fish from the oxbow lakes also trended toward higher concentrations of metals compared to the river main stem. Although we did not detect any indication that metals are biomagnifying through the fish community, fish are clearly taking up contaminants from the water and sediment. Additionally, although fish fillets generally fell below human consumption guidelines, surveys collected at field sites indicate disparities between health risks and perceptions of those risks. For example, many people believe that fish from the oxbow lakes present a lower risk for metals than the river main stem, converse to our findings. Exposure of channel catfish to cadmium concentrations similar to those observed in the Tisza Basin indicate that negative physiological effects, such as altered carbohydrate metabolism and subsequently growth, can occur in fish at muscle concentrations below consumption guidelines. Although cadmium in Tisza fish fillets were lower than expected, metals may still be problematic for the basin as they may disproportionately affect early life stages of fish. This could skew life histories of exposed populations compared to unaffected fish, potentially reducing growth rates, size at maturity, reproductive output, and lifespan; indicating the need for more paired field and laboratory assessments of chronic metals exposure.

Mine tailings pose a health risk for populations and ecosystems in the Southwest; this is why effective, and low-cost solutions for the long term are needed. This work is groundbreaking since little information is available with regards to applying greenhouse studies of phytostabilization to the field for mine tailing remediation. Mine tailings from Iron King Mine and Humboldt Smelter Superfund (IKMHSS) site can be considered one of the worst scenarios due to the extreme conditions which prevent the growth of a vegetation cap. The high concentration of metals, such as arsenic and lead, highly acidic, lack of the nutrients carbon and nitrogen in the soil structure, and low microbial communities are factors that negatively affect plant growth. This project provides practical field-scale applications for the use of phytostabilization, which uses plants to create a vegetation cap that stabilizes metals in the root zone while preventing wind and water erosion in mine tailings. The project is divided into three main studies: (1) the assessment of the translation of successful greenhouse results to the field of phytostabilization using compost-assisted direct planting. This includes the use of different rates of compost as an amendment and different desert native plant species in addition to some potential parameters that could be used as indicators of a successful modification of biochemical and physical environment from a disturbed soil towards a more healthy soil when compost assisted direct planting phytostabilization is used; (2) the second study aims to evaluate the effect of the phytostabilization strategy on reducing windborne transport of particle and metal(loids) following the establishment of the vegetation cap. The results indicate that the vegetation resulted from direct planting decreases dust emissions from IKMHSS mine tailings; and (3) the third study focuses on one of the most important requirements for phytostabilization application in the field, the performance of the different plant species selected from the greenhouse studies. This performance was evaluated as the metal accumulation in aerial plant tissue based on metal concentration guidelines from the National Research Council as well as changes in the composition of plant species and canopy cover with time. The results derived from the translation of compost–assisted direct plating based on successful greenhouse results are showing the capacity of this technology on a field scale by maintaining a canopy cover over time that decreases mobilization by not hyper-accumulating metals in the aerial tissue and by preventing windborne particle dispersion with the potential of disrupting contamination pathways.

Particulate and dissolved forms of arsenic and heavy metals are released from legacy mine tailings, particularly in (semi-) arid environments where tailings remain barren of vegetation and therefore highly susceptible to erosion. This leads to contamination of adjacent ecosystems and increased risk to public health. Establishment of a vegetative cap using amendments, such as composted organic matter to enhance plant growth, may be employed to reduce both physical erosion and leaching, but the impacts of such practices on molecular-scale mechanisms controlling metal(loid) speciation and lability remain poorly understood. Here we report on subsurface biogeochemical transformations of metal(loid)s in a phytostabilization field study at a Superfund site in Arizona, USA, where a legacy pyritic tailings (4,000 mg kg^-1 As, 2,438 mg kg^-1 Pb, 6,142 mg kg^-1 Zn, 13.25% Fe, and 11.71% S, averages for the top 0.5 m) has undergone oxidation in the top 1 m. Tailings were amended in the top 20 cm with 10%, 15%, and 20% composted organic matter by mass and seeded with native halotolerant plant species. All field treatments and the uncomposted control received irrigation of 0.36 ± 0.03 mm y^-1 in addition to 0.25 ± 0.16 mm y^-1 of precipitation, resulting in water input of 144% the annual precipitation rate. The field trial incorporated four annual samplings from 2010 – 2013. Sampling consisted of a single core of 90 cm in length and 2.54 cm in diameter collected from each field plot that was subsequently sectioned into 20 cm depth increments for analysis by synchrotron Fe and As X-ray absorption spectroscopy (XAS) coupled with quantitative chemical extraction methods. Subsurface stabilization of arsenic by Prosopis juliflora (mesquite) was investigated by bulk and micro synchrotron XAS and multiple-energy microscale fluorescence mapping combined with chemical digestion of plant samples following 1, 2, and 3 months of growth in greenhouse microcosms as well as 14 and 36 months of growth at the field site. Results indicate persistence of oxidizing conditions following compost amendment in surface tailings despite addition of organic matter, development of heterotrophic microbial communities and irrigation of a poorly draining medium. Compost amendment of 20% corresponded with evidence of higher oxidative pyrite weathering activity at 40-60 cm depth during phytostabilization compared to treatments of 0% or 10% compost for which the highest oxidative pyrite weathering activity was observed closer to the surface at 20-40 cm depth.. Despite observed downward transport of As, Fe, Zn, Mn, Pb, Ni, Cu, Cr, V, and Co during phytostabilization, ≥ 75% of total As was found to be attenuated by ferrihydrite in surface depths. Attenuation of Mn, Co, and Ni was observed below 40 depth by tailings receiving compost amendment relative to the irrigated control. Root associated As(V) was immobilized on the root epidermis bound to ferric sulfate precipitates and within root vacuoles as trivalent As(III)-thiol complexes. Rhizoplane associated ferric sulfate phases were dissimilar from the bulk tailings mineralogy shown by XAS and exhibited a high capacity to scavenge As(V) with As:Fe ratios 2x higher than the compost amended growth medium, indicating a root surface mechanism for their formation or accumulation. Results indicate that arsenate attenuation in semi-arid mine tailings during phytostabilization greatly depends on the presence of high concentrations of Fe(III) (oxyhydr)oxide minerals with a high capacity for arsenic adsorption.

Legacy mine sites are of concern due to their prevalence and associated environmental and human health risks. The United States Bureau of Land Management estimates as many as 500,000 abandoned mine sites in the US (BLM, 2017). Sites requiring costly management and long-term response to the environmental hazardous risks can be designated to a National Priority List (NPL) by the Environmental Protection Agency (EPA) under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) (EPA, 2017). One such site, located in Central Arizona, is the Iron King Mine Humboldt Smelter Superfund Site (IKMHSSS). The site was designated to the NPL in 2008 due to concerns regarding the size of the tailing pile, the proximity of contaminated materials to the town of Dewey-Humboldt and waterways, and the dangerous concentrations of arsenic (As) and lead (Pb) of the tailings (EPA, 2017). Remediation efforts have been ongoing since the designation of the site to the NPL, including sampling, yard soil removal, and distribution of information to the local community regarding risks from the site. The University of Arizona Superfund Research Program (UA SRP) has conducted greenhouse and phytostabilization studies of the site in an attempt to understand the processes and mechanisms employed to stabilize the tailings materials as well as reduce dust emissions from the tailings to the town of Dewey-Humboldt (Gil-Loaiza et al., 2016). This effort has successfully demonstrated a reduction of dust emissions (Sáez, 2016), however chemical changes to the tailings due to phytostabilization are the focus of this research. This work attempts to ascertain whether adverse effects from the method of phytostabilization are observed in the pore waters of the tailing material, in particular the potential for contamination of water sources by mobilized As through chemical or microbiological means. Recent studies have proposed potential mechanisms that can promote mobilization of As by dissolved organic matter (DOM) (Mladenov et al., 2015). Heterotrophic microbial respiration under O2 limited conditions can cause the reduction of Fe3+ to Fe2+, enhancing desorption or dissolution of As from Fe containing minerals (Hasan et al., 2007). Additionally, DOM competes with As for sorption sites at mineral surfaces (Grafe et al., 2002). In this study, batch and column experiments were used to investigate the mechanisms of sequestration and release of As in compost amended mine tailings. Mine tailings were reacted in triplicate in the presence and absence of DOM using plain tailings and radiated tailings for microbiological control and under anoxic and oxic conditions at timescales from ranging from 3 to 900 hours for batch experiments and 1 to 900 pore volumes in column experiments. The highest As release to pore waters was observed under anoxic conditions in the presence of DOM both with microbial activity inhibited and uninhibited through 60Co gamma irradiation after 3 and 910 h of reaction. The release of As from batch experiments was lowest in the control treatment with no DOM added to tailings in both anoxic and oxic treatments after 24 h. Column flow-through experiments were also carried out to better understand the kinetic biogeochemistry of the tailings interacting with DOM. Columns were completed under suboxic conditions to best mimic field scenarios. To test the effect of microbes, control tailing samples were sterilized by 60Co gamma irradiation prior to flowing DOM. Pore volumes (PV) were collected using fractionation equipment from 1 to 900 PVs. The release of As was highest in the presence of DOM after approximately 40 PVs when As release began increasing to its maximum release of 50 μmol l-1. No significant difference between irradiated and non-irradiated tailings was observed in either irradiated or non-irradiated tailings. Lowest release of As to effluent solutions was in the absence of DOM. These results were consistent with the findings from batch experiments. Batch and column experiments show that DOM influences the mobilization of As from mine tailings, and demonstrates the potential risk to proximal ground water resources in the absence of attenuation processes between the oxidized tailings and groundwater.

Storage of mine tailings and waste concrete imposes economical and environmental impacts. Researchers have attempted to reuse wastes as construction material by utilizing ordinary Portland cement (OPC) to stabilize them. This method, however, has a number of limitations related to OPC. In this research, a recent technology called geopolymerization is used to stabilize mine tailings and concrete waste so that they can be completely recycled and reused. The research includes three main parts. The first part studies the effect of different factors on the mechanical properties, micro/nano structure, and elemental and phase composition of mine tailings-based geopolymer binder. The second part investigates the feasibility of producing geopolymer bricks using mine tailings. The physical and mechanical properties, micro/nano structure, durability, and environmental performance of the produced bricks are studied in a systematic way. Moreover, the enhancement of the mine tailings-based geopolymer bricks by adding cement kiln dust (CKD) is studied. The third part of the research investigates the recycling of the fines fraction of crushed waste concrete to produce binder through geopolymerization in order to completely recycle concrete waste. The results indicate the viability of geopolymerization of mine tailings by optimizing the synthesis conditions. By properly selecting these factors, mine tailings-based geopolymer bricks can be produced to meet the ASTM standard requirements and to be environmentally safe by effectively immobilizing the heavy metals in the mine tailings. The physical and mechanical properties and durability of the mine tailings-based geopolymer bricks can be further enhanced by adding a small amount of CKD. The results also show that the fines fraction of crushed waste concrete can be used together with fly ash to produce high performance geopolymer binder. Incorporation of calcium in the geopolymer structure and coexistence of the calcium products such as CSH gel and the geopolymer gel explains the enhancement of the mine tailings-based geopolymer bricks with CKD and the high performance of geopolymer binder from the waste concrete fines and fly ash. The research contributes to sustainable development by promoting complete recycling and utilization of mine tailings and concrete waste as construction material.

<p>Tailings are a fine-grained waste product produced during the metal recovery process. Tailings consist mostly of different silicates but also sulfides (e.g. pyrite), since 100 % metal recovery is not possible. Freshly processed tailings are deposited in large impoundments. If the mine tailings in the impoundments are exposed to water and oxygen, the sulfides will oxidize and release acidity and metals such as Fe, Cu, Zn, and Pb. The sulfide mineral oxidation reactions are catalyzed by sulfur and iron oxidizing bacteria (principally <i>Acidithiobacillus ferrooxidans</i>) that oxidize ferrous iron to ferric iron, which then oxidizes pyrite. When the leachate produced by this process discharges from the impoundment, it is called acid mine drainage, which may lead to the pollution of adjacent streams and lakes.</p><p>The intention with this thesis is to investigate and quantify mineral weathering processes and element release rates occurring in water-saturated and soil-covered sulfidic mine tailings. The study was performed in different batch and column experiments in room temperature and in the laboratory. The batch experiments were conducted for ca. three months and investigated: a) microbial and abiotic sulfide oxidation in freshly processed tailings under oxic conditions at pH 2-3 and pH 8, b) microbial oxidation of pure pyrite grains at pH 2-3 under different oxygen concentrations ranging from anoxic to oxic conditions. The column experiments, consisting of unoxidized tailings in water-saturated columns, were conducted for up to three years. In these experiments, an oxygen-saturated solution was continually pumped into the column inlet, and investigated: a) differences in oxidation rates between tailings of two different grain sizes, b) factors affecting element discharge rates, acid neutralization, and sulfide oxidation, c) the effect of ions released in a soil cover on release rates in the tailings.</p><p>Sulfide oxidation processes within the batch experiments were limited by surface kinetics. The microbial oxidation of pure pyrite at atmospheric conditions produced the most rapid rate, while the microbial oxidation of pure pyrite at anoxic conditions was slower by 1.8 orders of magnitude. Microbial and abiotic oxidation of pyrite in freshly-processed tailings resulted in pyrite oxidation rates that were intermediate between these two extremes. The results from the microbial experiments with pure pyrite indicated a positive correlation between the concentration of dissolved oxygen, ferric iron and bacterial cells (at a total cell concentration > 10⁶ cells/mL and a dissolved oxygen concentration ≥ 13.2 µM), which implies an interdependence of these factors. The results from these batch experiments support the indirect mechanism for microbial oxidation by the ferric oxidation pathway. Pyrite oxidation rates estimated from the batch experiments may be comparable with oxidation rates in the unsaturated zone and at the groundwater table in a tailings impoundment.</p><p>Acid neutralization reactions in the column experiments resulted in the release of base cations to the column leachate. Calcite was the most important neutralizing mineral despite that it was only present in minor amounts in the tailings. It was confirmed that acidity forced the calcite dissolution. Element release rates in the column experiments were controlled by the availability of dissolved oxygen, which was a function of the water flow rate into the column. These column experiments also showed that the results are comparable with results from field studies, justifying the use of column experiments to study processes within tailings impoundments.</p>

Tailings are a fine-grained waste product produced during the metal recovery process. Tailings consist mostly of different silicates but also sulfides (e.g. pyrite), since 100 % metal recovery is not possible. Freshly processed tailings are deposited in large impoundments. If the mine tailings in the impoundments are exposed to water and oxygen, the sulfides will oxidize and release acidity and metals such as Fe, Cu, Zn, and Pb. The sulfide mineral oxidation reactions are catalyzed by sulfur and iron oxidizing bacteria (principally Acidithiobacillus ferrooxidans) that oxidize ferrous iron to ferric iron, which then oxidizes pyrite. When the leachate produced by this process discharges from the impoundment, it is called acid mine drainage, which may lead to the pollution of adjacent streams and lakes. The intention with this thesis is to investigate and quantify mineral weathering processes and element release rates occurring in water-saturated and soil-covered sulfidic mine tailings. The study was performed in different batch and column experiments in room temperature and in the laboratory. The batch experiments were conducted for ca. three months and investigated: a) microbial and abiotic sulfide oxidation in freshly processed tailings under oxic conditions at pH 2-3 and pH 8, b) microbial oxidation of pure pyrite grains at pH 2-3 under different oxygen concentrations ranging from anoxic to oxic conditions. The column experiments, consisting of unoxidized tailings in water-saturated columns, were conducted for up to three years. In these experiments, an oxygen-saturated solution was continually pumped into the column inlet, and investigated: a) differences in oxidation rates between tailings of two different grain sizes, b) factors affecting element discharge rates, acid neutralization, and sulfide oxidation, c) the effect of ions released in a soil cover on release rates in the tailings. Sulfide oxidation processes within the batch experiments were limited by surface kinetics. The microbial oxidation of pure pyrite at atmospheric conditions produced the most rapid rate, while the microbial oxidation of pure pyrite at anoxic conditions was slower by 1.8 orders of magnitude. Microbial and abiotic oxidation of pyrite in freshly-processed tailings resulted in pyrite oxidation rates that were intermediate between these two extremes. The results from the microbial experiments with pure pyrite indicated a positive correlation between the concentration of dissolved oxygen, ferric iron and bacterial cells (at a total cell concentration &gt; 106 cells/mL and a dissolved oxygen concentration ≥ 13.2 µM), which implies an interdependence of these factors. The results from these batch experiments support the indirect mechanism for microbial oxidation by the ferric oxidation pathway. Pyrite oxidation rates estimated from the batch experiments may be comparable with oxidation rates in the unsaturated zone and at the groundwater table in a tailings impoundment. Acid neutralization reactions in the column experiments resulted in the release of base cations to the column leachate. Calcite was the most important neutralizing mineral despite that it was only present in minor amounts in the tailings. It was confirmed that acidity forced the calcite dissolution. Element release rates in the column experiments were controlled by the availability of dissolved oxygen, which was a function of the water flow rate into the column. These column experiments also showed that the results are comparable with results from field studies, justifying the use of column experiments to study processes within tailings impoundments.

Extensive research conducted over the last decade has demonstrated the great potential of phytostabilization for the reclamation of abandoned mine tailing piles. The right combination of plant species and soil amendments can facilitate the growth of a permanent vegetative cover on the tailings that will help minimize the mobilization of metal-bearing particles by means of wind dispersion and water erosion. Despite previous research efforts, the diversity and potential role of microbial populations inhabiting the root zone of the plants on the stabilization of the metal(loid) contaminants remains mostly unresolved. The study presented in this dissertation represents one of the first comprehensive efforts aimed to understand the ecology and dynamics of microbial communities colonizing both bulk and rhizosphere tailings during phytostabilization as an initial step towards elucidating the role of microbes in the stabilization of metal(loid) contaminants during the remediation treatment. This study was divided into two main projects: (1) the first aimed to monitor the temporal variations in functional and taxonomic diversity of prokaryotic populations in acid-generating metalliferous mine tailings during phytostabilization to determine how they respond to and/or influence changes in environmental parameters and to identify key patterns in their composition that may serve as bioindicators of soil health and the success of the remediation treatment; and (2) the second aimed to expand our understanding of the dynamics of root-associated bacterial, fungal and archaeal communities during mine tailing phytostabilization and how the dynamic behavior of the communities correspond to the growth of plants, the addition of soil amendments, and fluctuations in environmental conditions. The results presented here demonstrate that different microbial groups respond differently to changes in environmental conditions during phytostabilization, suggesting that by monitoring the behavior of specific microbial groups in the systems (as bioindicators) we may be able to assess the effectiveness of the remediation treatment. Furthermore, the results from the taxonomic and functional analysis of the microbial communities served as the basis for the development of a model that explains the ecology and distribution of dominant microbial groups in the tailings that may significantly contribute to the oxidation of iron-sulfides, the production of acid mine drainage, and to facilitate plant establishment and survival during phytostabilization.

The first part of the thesis investigates the enhancement of fly ash-based geopolymer with alkali pretreated sweet sorghum fiber. The unconfined compression, splitting tensile and flexural tests were conducted to investigate the mechanical properties of geopolymer composite. The results indicate that the inclusion of sweet sorghum fiber slightly decreases the unconfined compressive strength (UCS), however, the splitting tensile and flexural strengths as well as the post-peak toughness increase with the fiber content up to 2% and then decrease thereafter. A durability test program containing 10 wet/dry cycles was performed to evaluate the long-term performance of the geopolymer composite related to wet/dry cycling. The results indicate that both the UCS and the splitting tensile strength of the geopolymer composite progressively decrease with the number of wet/dry cycles. The second part of the thesis investigates the utilization of biopolymers to stabilize MT for dust control. First, a fall cone method was adopted to evaluate the Atterberg limits and undrained shear strength of MT stabilized with biopolymers. The results indicate that the inclusion of biopolymers increases both the liquid limit and the undriained shear strength of MT. Two new equations are proposed for predicting the undrained shear strength of MT based on liquid limit and water content, and liquidity index. Second, an experimental program including moisture retention, wind tunnel and surface strength tests was performed to evaluate the effectiveness of biopolymer stabilization for dust control. The results indicate that biopolymers are effective in enhancing the moisture retention capacity, improving the dust resistance, and increasing the surface strength of MT. Third, a durability test program containing 10 wet/dry cycles was applied to MT samples treated with biopolymer solutions of different concentrations. The results show that the dust resistance of MT samples progressively decreases with the number of wet/dry cycles. Finally, experimental and numerical studies on the unconfined compressive strength (UCS) of MT stabilized with biopolymer were carried out. It is found that inclusion of biopolymer into MT favors the increase of adhesion between MT particles and thus the increase of the UCS of MT.

Three experiments were conducted to examine restoration of disturbed land in Arizona. The first experiment attempted to revegetate abandoned farmland by direct seeding native seeds and using various soil preparation techniques, amendments, and weeding of Salsola iberica. Only irrigation and weeding had a significant effect on seed germination and canopy cover. Irrigation increased plant cover on plots, but weeds dominated the cover. A seedbank study conducted near the end of the second growing season found the soil was dominated by weeds and contained few viable native seeds. The results illustrate the difficulty of establishing native plants on abandoned desert farmland due to the dominance of weedy species, the presence of salts in the soil, and the lack of adequate soil moisture.The second experiment, a lysimeter study, tested the efficacy of different evapotranspiration (ET) soil cover designs for stabilization of acidic copper mine tailing piles. The study evaluated the effectiveness of capillary barriers (CB) to contain the waste found in tailings and different plants to revegetate the piles. The ET covers reduced infiltration of water into tailings. Copper concentrations increased significantly in plant tissue grown on the ET covers compared to plants grown in the greenhouse. Plants did not exhibit signs of phytotoxicity and concentrations were below levels toxic to all domestic animals except sheep. The CB did not reduce water infiltration into the tailings or upward migration of copper into the soil cover. Vegetation is vital to an effective ET cover. A mix of transplanted shrubs and seeded grasses and forbs establish long-term, sustainable vegetation.The third experiment examined the influence of biosolids on the bacterial communities within mine tailings by bacterial counts and bacterial diversity. The diversity of neutral copper mine tailings two weeks after biosolid application was compared with that of desert soil via cloning and sequencing of PCR amplified community 16S rRNA. Culturable heterotrophic plate counts (HPC) increased following biosolid addition. Total direct counts exceeded HPC by approximately two orders of magnitude. Overall, biosolid-amended tailings contained large numbers of bacteria diverse in nature and with many of the traits of normal desert soil bacterial communities.