Bauxite-processing Residue Sand Research Articles

Rhizosphere management by biochar and leaching improved plant performance in fresh bauxite residue sand

High sodicity and Low nutrient retention in bauxite-processing residue sand (BRS) disposal areas restrict sustainable vegetation management in this highly alkaline environment. Although previous investigations have reported the beneficial effect of organic amendments on BRS rehabilitation, little is known about the underlying mechanisms of this complicated process, particularly after supplementary leaching of biochar amended BRS. We have investigated the coupled effect of supplementary leaching process, biochar [aged acidic (AC) vs alkaline pine (PC)] amendment and di-ammonium phosphate (DAP) fertilisation on rhizosphere nutrient dynamic and ryegrass performance in a 116-day glasshouse study. Biochar amendment in DAP + PC and DAP + AC treatments significantly increased and decreased pH (0.3–0.5 units) in all BRS rhizosphere and root-free zones, respectively. The application of alkaline and acidic biochars to BRS have reduced ammonia volatilisation (25–80%) and increased nitrogen retention (9–72%) in comparison with DAP treatment. Supplementary leaching had no significant effect on BRS rhizosphere pH, but reduced the EC values by ca. 62% in biochar-amended treatments. The leachates’ pH remained unchanged (ca. 8) throughout the experiment, while their EC reduced in AC (6 folds) and PC (9 folds) amended treatments, with lower reduction in rhizosphere than root-free zones. The interaction of applied biochars and plant roots generally decreased nutrient leaching from rhizosphere in comparison with root-free zones. Sodium was the dominant cation in the leachate of all treatments and cumulative abundance of exchangeable cations were in the order of Na+ > Ca2+ > K+ > Mg2+ > Al3+. The reduction of BRS salinity has increased plant biomass (ca. 47%) in biochar amended treatments by improving the capacity of fresh BRS rhizosphere for plant establishment.

Aged biochar alters nitrogen pathways in bauxite-processing residue sand: Environmental impact and biogeochemical mechanisms

Low nitrogen (N) content and retention in bauxite-processing residue sand (BRS) disposal areas pose a great challenge to the establishment of sustainable vegetation cover in this highly alkaline environment. The budget and fate of applied N in BRS and its potential environmental impacts are largely unknown. We investigated the effect of combined application of biochars [aged acidic (AC) vs alkaline pine (PC)] and di-ammonium phosphate (DAP) fertiliser on ammonia (NH3) volatilisation, nitrous oxide (N2O) emission and N retention in a 116-day glasshouse study. The application of AC to BRS decreased pH (≈0.5 units) in BRS, while PC biochar increased pH (≈0.3 units). The application of AC reduced NH3 volatilisation by ca. 80%, while PC by ca. 25%. On the other hand, the AC treatment increased N2O emission by 5 folds. However, the N loss via N2O emission in the AC treatment only accounted for ca. 0.4% of applied N. The reduction in BRS pH and increased retention of mineral N due to the presence of oxygen-containing (phenolic and carboxylic) functional groups in AC may be responsible for reduced NH3 volatilisation and increased N2O emission. This study has highlighted the potential of biochar (particularly aged biochar) in improving N retention and minimising environmental impacts in highly alkaline environments.

Biochar amendment and water stress alter rhizosphere carbon and nitrogen budgets in bauxite-processing residue sand under rehabilitation

Nitrogen (N) bioavailability is one of the main limiting factors for microbial activity and vegetation establishment in bauxite-processing residue sand (BRS). Although beneficial effects of biochar on reducing N loss in the early stages of BRS rehabilitation have been observed previously, the underlying mechanisms of this complicated process, particularly the interactions between applied biochar and the plant rhizosphere is largely unknown. This glasshouse study (116 days), investigated the coupled effects of biochar and water stress on N bioavailability in the rhizosphere of ryegrass (Lolium rigidum) grown in BRS amended with di-ammonium phosphate (DAP) fertiliser (at rates of 0 or 2.7 t ha−1) with and without biochar amendment. The applied biochar was characterised as either aged acidic (AC) or alkaline pine (PC) and was mixed with BRS at a rate of 5% v/v under four moisture regimes (50%, 40%, 20% and 7.5% water holding capacity). Amending BRS with AC and PC biochars increased NH4+ retention and decreased cumulative NH3 volatilization within both the rhizosphere and root-free zones compared with fertiliser only treatment. These effects were more pronounced for the AC than PC biochar, suggesting that aged acidic biochar has the great potential for use in rapid establishment of vegetation in BRS disposal areas. The biochar amendment increased cumulative nitrous oxide emissions compared with DAP only treatment, with no significant differences among different moisture regimes. The Control and 20% water holding capacity (WHC) treatment showed the highest dissolved organic carbon (DOC) concentrations compared with other treatments and moisture regimes in the ryegrass rhizosphere, while the highest dissolved organic N concentration were observed in the DAP + AC treatment. Reducing moisture levels below 20% WHC generally decreased microbial biomass carbon (MBC) concentrations and activity in both the rhizosphere and root-free zones of all treatments, while total N generally decreased as moisture levels decreased from 50% to 7.5% WHC. Plant took up more N in the DAP + AC treatment compared with DAP + PC and DAP only treatments, while increasing water stress generally resulted in decreased aboveground biomass.

Forms of Nitrogen Alter Plant Phosphorus Uptake and Pathways in Rehabilitated Highly Alkaline Bauxite Processing Residue Sand

AbstractUnderstanding plant phosphorus (P) uptake and pathways is critical in the rehabilitation of degraded landscapes. This study investigated whether the uptake of P by Lolium rigidum was influenced by forms (inorganic and organic) of nitrogen (N) and whether the forms of N affected P loss by leaching in bauxite‐processing residue sand (BRS). Gypsum amended BRS was grown with L. rigidum and fertilized with P [calcium di‐hydrogen phosphate, Ca (H2PO4)2], plus different forms of N [e.g. control (CK without N addition, glycine (P‐GL), ammonium (P‐AS) and nitrate (P‐KN))]. The experimental treatments were arranged in a complete randomized design. Aboveground biomass P under P‐KN accounted for 47.4% of applied P, but only 26.0% and 25.0% under P‐AS and P‐GL, respectively. However, root P contents showed a different trend where P‐AS (31.9%) was highest followed by P‐GLY (23.0%) and P‐KN (17.1%). Meanwhile, loss of P via leaching was minimal (<5% of applied P) regardless of treatment, which could be because of the high affinity of residue sand for P via specific adsorption sites. This was supported by high percentage recovery of applied P in BRS growth media under P‐GLY (48.1%), P‐AS (37.2%) and P‐KN (32.0%). It is concluded that loss of P via leaching may not be a problem in BRS; however, limited plant P availability may be of more concern. Further, uptake of P by L. rigidum in alkaline BRS was found to be more efficient under nitrate‐N fertilization. Understanding, however, the specific or associated mechanisms involved warrants further investigation. Copyright © 2016 John Wiley & Sons, Ltd.

Shifts in leaf N:P stoichiometry during rehabilitation in highly alkaline bauxite processing residue sand.

Large quantities of sodic and alkaline bauxite residue are produced globally as a by-product from alumina refineries. Ecological stoichiometry of key elements [nitrogen (N) and phosphorus (P)] plays a critical role in establishing vegetation cover in bauxite residue sand (BRS). Here we examined how changes in soil chemical properties over time in rehabilitated sodic and alkaline BRS affected leaf N to P stoichiometry of native species used for rehabilitation. Both Ca and soil pH influenced the shifts in leaf N:P ratios of the study species as supported by consistently significant positive relationships (P < 0.001) between these soil indices and leaf N:P ratios. Shifts from N to P limitation were evident for N-fixing species, while N limitation was consistently experienced by non-N-fixing plant species. In older rehabilitated BRS embankments, soil and plant indices (Ca, Na, pH, EC, ESP and leaf N:P ratios) tended to align with those of the natural ecosystem, suggesting improved rehabilitation performance. These findings highlight that leaf N:P stoichiometry can effectively provide a meaningful assessment on understanding nutrient limitation and productivity of native species used for vegetating highly sodic and alkaline BRS, and is a crucial indicator for assessing ecological rehabilitation performance.

Influence of amendments on acidification and leaching of Na from bauxite processing sand

The effects of addition of acidifying agents (phosphogypsum, iron sulphate, elemental S, ammonium sulphate and biosolids) on pH of bauxite processing residue sand, and on subsequent leaching of Na and other ions was investigated in 6- and 10-week incubation/leaching experiments. Addition of phosphogypsum and iron sulphate caused a reduction in pH and ESP both before and after leaching. Addition of biosolids reduced pH prior to leaching and reduced ESP and increased ECEC before and after leaching. By contrast, addition of elemental S or ammonium sulphate had no effect on pH or ESP either before or after leaching. Addition of soil and/or biosolids to the residue sand did not significantly promote S oxidation or nitrification. In all treatments leaching caused a substantial reduction in pH as well as exchangeable Na, EC and ESP. In control treatments pH was reduced by about two units following leaching. Addition of gypsum and iron sulphate resulted in precipitation of alkalinity and leaching of Na+ with added SO42− as the major balancing anion. By contrast, where there was no initial decrease in pH (e.g. control), Na+ leached with HCO3−/CO32 (i.e. soluble alkalinity) as the main balancing anions and, as a result, residue pH was decreased. This occurred because the mobile Na+ ion was present in residue at concentrations greatly in excess of the CEC of the residue sand. It was concluded that leaving residue sand for a sufficient period of time under leaching conditions is a very important management strategy since it will result in a substantial reduction in pH, EC exchangeable Na and ESP making conditions much more conducive for revegetation.

Shifts in leaf nitrogen to phosphorus ratio of Lolium rigidum grown in highly alkaline bauxite-processing residue sand with differing age of rehabilitation and amendments

Stoichiometric ratios of leaf nitrogen (N) to phosphorus (P) have been used as a tool to characterize ecological problems and ecosystem productivity. This study used leaf N:P stoichiometry to assess nutrient limitation and productivity in alkaline bauxite-processing residue sand (BRS) vegetated with Lolium rigidum. Fresh BRS [amended with either greenwaste compost (BRSgc) or biochar (BRSbc)], and aged BRS [e.g. 5, 7, and 15 years under established rehabilitation (5, 7, and 15YrBRS)] were grown with L. rigidum for 3 months in a controlled environment. Leaf N:P ratios were positively correlated with the pH of growth media [freshly amended BRS (r=0.66, P<0.05) and aged BRS (r=0.89, P<0.001)]. This suggests that increasing pH in BRS limits P utilization by L. rigidum. Leaf biomass decreased as leaf N:P ratio increased (r=−0.71, P<0.001). Leaf N:P ratios (15–17) in BRSgc and 15YrBRS produced the highest leaf biomass. Further, leaf N:P ratios decreased with the age of BRS rehabilitation [e.g. BRS (24.9)>5YrBRS (20.9)>7YrBRS (19.0)>15YrBRS (17.2)]. This suggests that when rehabilitated BRS becomes older, the leaf N:P ratio will shift towards the ratio of the reference sample (i.e. L. rigidum with adequate levels of N and P), demonstrating an improved plant N and P uptake in aged BRS-plant systems. Results highlight that leaf N:P ratios can effectively reflect N and P limitations to productivity in vegetated alkaline BRS, which can be a critical ecological indicator in such environment. These results however, have to be verified with other species (e.g. native shrubs and trees) used for plant establishment of alkaline BRS before using this indicator for assessing ecological rehabilitation performance.

Potential for use of industrial waste materials as filter media for removal of Al, Mo, As, V and Ga from alkaline drainage in constructed wetlands – Adsorption studies

The potential to remove Al, Mo, V, As and Ga from alkaline (pH 8.0–8.6) drainage originating from seawater neutralized bauxite processing residue storage areas using constructed wetland technology was studied in a laboratory study. Bauxite processing residue sand, bauxite, alum water treatment sludge and blast furnace slag were investigated as potential active filter materials. Al was shown to precipitate as Al(OH)3 in the pH range 7.0–8.0 in aqueous solution and 6.0–8.5 in the presence of silica sand particles that provided a surface for nucleation. For V As Mo and Ga, adsorption to the surfaces of the adsorbents decreased greatly at elevated pH values (>pH 6–9). Water treatment sludge and bauxite had a greater ability to adsorb V, As and Mo at high pH (As and V at pH 7–9 and Mo at pH 5–7) than processing sand and slag. Adsorption isotherm data for As and V onto all four adsorbent than processing sand and slag. Adsorption isotherm data for As and V onto all four adsorbent materials fitted equally well to the Langmuir and Freundlich equations but for Ga, and to a lesser extent Mo, the Freundlich equation gave higher R2 values. For all four ions, the maximum adsorption capacity (Langmuir value qmax) was greatest for water treatment sludge. Bauxite adsorbed more Mo, Ga and V than residue sand or slag. The pseudo-second order equation gave a better fit to the experimental kinetic data than the pseudo-first order model suggesting that chemisorption rather than diffusion/exchange was the rate limiting step to adsorption. It was concluded that water treatment sludge and bauxite were the most effective adsorbents and that for effective removal of the target ions the pH of the drainage water needs to be decreased to 6.0–7.0.

Transformation and plant uptake of 15N-labeled fertilizers mediated by ammonia-oxidizing bacteria in alkaline bauxite-processing residue sand amended with greenwaste compost

Ecological restoration of bauxite residue storage areas is a great challenge due to the inherently hostile characteristics (high alkalinity, high salinity and poor nutrient availability). Nitrogen (N) availability and microbial functionality are crucial indicators for successfully restoring ecosystem functions of bauxite residue storage areas. This study examined the pathway and dynamics of 15N-labeled fertilizer [e.g., ammonium sulphate (AS), potassium nitrate (KN), glycine (GL)] and the role of ammonia oxidizing archaea (AOA) and bacteria (AOB) in bauxite residue sand (BRS) amended with greenwaste compost (BRSGC) and without organic amendment (BRSNA) grown with Hardenbergia violacea and Lolium rigidum. 15N losses via volatilization and leaching from applied 15N fertilizer were higher in AS (49–50%) and GL (42–47%) compared to KN (38–44%) treatment under BRSNA. 15N losses via these pathways however, were significantly (P<0.01) reduced under BRSGC. The total residual 15N recovered in BRSGC (34–57%) was significantly higher compared with BRSNA (<2%). Uptake of applied 15N fertilizers by plants was consistently higher in BRSNA for both plants (KN>AS>GL) compared with BRSGC, suggesting an increased sorption or immobilization of applied 15N by greenwaste compost. Significant relationships observed between the AOB abundance under AS and GL treated BRS and 15N uptake by plants and extractable 15NO3–N indicated that AOB plays a significant role in the transformation of 15N and uptake by plants. These results have important implications for developing improved nutrient management strategies and ecological rehabilitation prescriptions in alkaline BRS storage areas.

Effects of amendments and fertilization on plant growth, nitrogen and phosphorus availability in rehabilitated highly alkaline bauxite‐processing residue sand

AbstractThe effects of organic–inorganic amendments and nitrogen‐phosphorus (NP) fertilization (NH4NO3 plus Ca (H2PO4)2) on ryegrass (Lolium rigidum) growth, and nitrogen (N) and phosphorus (P) availability in highly alkaline bauxite‐processing residue sand (BRS), were examined in a pot experiment. The BRS used was either unamended (control) or amended with organic (e.g. greenwaste compost and biochar) or inorganic (e.g. zeolite) materials at a rate of 10% v/v. BRS from 15 years of rehabilitation (15YRRH) was also used as the second control. NP fertilizer was applied at different rates. The experimental set up was arranged in a two factorial complete randomized design. BRS with zeolite and 15YRRH at NP fertilizer rates of 2.0 and 2.5 t/ha produced the highest dry matter, leaf N concentration and N uptake by ryegrass, which were significantly higher (P &lt; 0.05) than the other treatments, suggesting the potential of zeolite in providing stability of applied N fertilizer in BRS. Further, BRS with biochar at NP rates 2.0 and 2.5 t/ha can also be suitable amendments as they enhance growth and also improved the N and P supplying capacity of BRS. Ryegrass leaf P concentration and P uptake were above the critical P values in the 15YRRH compared with organic–inorganic amended BRS, suggesting that time is important for better P uptake from the residue. It is concluded that zeolite and biochar combined with appropriate NP fertilizer rates can improve plant growth and provide a source of nutrients for ryegrass establishment in bauxite residue storage areas. The results need to be tested in field conditions before being advised in farming practice.

Plant phosphorus availability index in rehabilitated bauxite-processing residue sand

Background and aims Soil phosphorus (P) indices that have been originally developed and applied to agricultural soils for predicting P uptake by plants were examined in a pot experiment to determine the most suitable index for P availability in bauxite-processing residue sand (BRS).

Selecting a nitrogen availability index for understanding plant nutrient dynamics in rehabilitated bauxite-processing residue sand

Understanding nutrient dynamics in non-typical soil materials such as bauxite-processing residue sand (pH>10; EC>30dSm−1) is critical for developing fertilizer strategies and evaluating ecological restoration performance. Indices relating nitrogen (N) concentration in soil to plant N uptake are well-established for natural soils but their application to non-typical soils has received little attention. This study investigated a range of soil-based methods [i.e. 2M KCl extractable inorganic N (NH4+, NO3−), potentially mineralizable nitrogen (PMN), and 0.01M CaCl2 extractable N] to identify their suitability for describing soil–plant N relations in highly alkaline bauxite-processing residue sand. Nitrogen availability indices were measured under laboratory (pot trial) and field conditions. Pot trial was established using residue sand that had been amended (10%, v/v, basis) with various organic (greenwaste compost, biochar and biosolids) and inorganic (zeolite) materials. Both the field study and pot trial showed that 2M KCl extractable NO3−-N was most highly correlated with plant biomass N compared with the other N availability indices. Findings from this study suggest that 2M KCl extractable NO3−-N can be used as a soil quality indicator in developing fertilizer management strategies and assessing ecological status of the residue storage areas.

Impacts of greenwaste biochar on ammonia volatilisation from bauxite processing residue sand

The objective of this study was to test the suitability of greenwaste biochar to aid nitrogen (N) retention in rehabilitated bauxite-processing residue sand (BRS). Bauxite residue sand was collected from the Alcoa of Australia Pinjarra refinery. The pH of BRS was adjusted to values of 5, 7, 8 and 9 and subsequently amended with different rates (1, 5, 10 and 20 %, w/w) of greenwaste biochar. The loss of N via NH3 volatilization following addition of di-ammonium phosphate (DAP) was determined using an acid trapping method. At low pH (5), increasing pH rather than adsorption capacity, resulting from biochar addition, caused greater losses of N through volatilization from BRS. In BRS with medium pH (7, 8), increasing adsorption capacity, induced by biochar addition, played the more dominant role in enhancing adsorption of NH 4 + -N /NH3-N and lowering NH3 volatilization. In the BRS with high pH (9), the majority of NH 4 + -N /NH3-N pools was lost via NH3 volatilization due to the strong acid-base reaction at this pH. It is concluded that the interaction of changes in pH and adsorption capacity induced by greenwaste biochar addition affects the availability and dynamics of NH 4 + -N/ NH3-N in BRS amended with DAP.

Effect of amendment of bauxite processing sand with organic materials on its chemical, physical and microbial properties

The effects of addition of a range of organic amendments (biosolids, spent mushroom compost, green waste compost and green waste-derived biochar), at two rates, on some key chemical, physical and microbial properties of bauxite-processing residue sand were studied in a laboratory incubation study. Levels of exchangeable cations were not greatly affected by additions of amendments but extractable P was increased significantly by mushroom and green waste composts and massively (i.e. from 11.8 to 966 mg P kg −1) by biosolids applications. Levels of extractable NO 3 −–N were also greatly elevated by biosolids additions and there was a concomitant decrease in pH. Addition of all amendments decreased bulk density and increased mesoporosity, available water holding capacity and water retention at field capacity (−10 kPa), with the higher rate having a greater effect. Addition of biosolids, mushroom compost and green waste compost all increased soluble organic C, microbial biomass C, basal respiration and the activities of β-glucosidase, L-asparaginase and alkali phosphatase enzymes. The germination index of watercress grown in the materials was greatly reduced by biosolids application and this was attributed to the combined effects of a high EC and high concentrations of extractable P and NO 3 −. It was concluded that the increases in water storage and retention and microbial activity induced by additions of the composts is likely to improve the properties of bauxite-processing residue sand as a growth medium but that allowing time for soluble salts, originating from the organic amendments, to leach out may be an important consideration before sowing seeds.

Surface charge characteristics and sorption properties of bauxite-processing residue sand

Bauxite-processing residue sand (BRS) is the primary growth medium used to rehabilitate Alcoa’s residue storage areas (RSAs) in south-west Western Australia. This material is typically coarse-textured, highly saline, highly alkaline, extremely sodic, and deficient in plant nutrients. To develop appropriate fertiliser strategies for optimising rehabilitation performance, a fundamental understanding of the surface charge and nutrient retention properties of BRS is essential. The contribution of permanent (σp) and variable (σv) charge to the overall magnitude and sign of the surface charge, and ammonium (NH4) and phosphorus (P) sorption, as a function of pH were studied. Samples of BRS were obtained from Alcoa’s Kwinana (KW), Pinjarra (PJ), and Wagerup (WG) Refineries. Each sample exhibited predominantly variable charge (σv ≈ 8–12 cmol/kg at pH 12), and negligible permanent negative charge (σp ≈ 0.2 cmol/kg). The point of zero net charge (PZNC) was observed at pH 6.96, 6.89, and 5.98 for the KW, PJ, and WG samples, respectively. These values are consistent with those reported for soils dominated by Fe and Al oxides and hydroxides but containing negligible organic matter. Solution and adsorbed NH4 decreased with increasing pH (pH 7–11) for BRS. It was suggested that ammonia volatilisation was a major loss pathway for NH4 applied to BRS. Phosphorus sorption decreased with increasing pH for each BRS. It was suggested that the presence of competing anions (i.e. carbonate) and increasing negative surface charge density were the major causes for this behaviour. The results from this study have major implications for the selection of suitable types of fertilisers (particularly nitrogen) for rehabilitating alkaline BRS.

Behaviour and dynamics of di-ammonium phosphate in bauxite processing residue sand in Western Australia—II. Phosphorus fractions and availability

The production of alumina involves its extraction from bauxite ore using sodium hydroxide under high temperature and pressure. This process yields a large amount of residue wastes, which are difficult to revegetate due to their inherent hostile properties--high alkalinity and sodicity, poor water retention and low nutrient availability. Although phosphorus (P) is a key element limiting successful ecosystem restoration, little information is available on the availability and dynamics of P in rehabilitated bauxite-processing residue sand (BRS). The major aim of this experiment was to quantify P availability and behaviour as affected by pH, source of BRS and di-ammonium phosphate (DAP) application rate. This incubation experiment was undertaken using three sources of BRS, three DAP application rates (low, without addition of DAP; medium, 15.07 mg P and 13.63 mg N of DAP per jar, 100 g BRS; and high, 30.15 mg P and 27.26 mg N per jar, 100 g BRS), and four BRS pH treatments (4, 7, 9 and 11 (original)). The moisture content was adjusted to 55% water holding capacity and each BRS sample was incubated at 25 degrees C for a period of 119 days. After this period, Colwell P and 0.1 M H(2)SO(4) extractable P in BRS were determined. In addition, P sequential fractionation was carried out and the concentration of P in each pool was measured. A significant proportion (37% recovered in Colwell P and 48% in 0.1 M H(2)SO(4) extraction) of P added as DAP in BRS are available for plant use. The pH did not significantly affect 0.1 M H(2)SO(4) extractable P, while concentrations of Colwell P in the higher initial pH treatments (pH 7, 9 and 11) were greater than in the pH 4 treatments. The labile fractions (sum of NH(4)Cl (AP), bicarbonate and first sodium hydroxide extractable P (N(I)P)) consisted of 58-64% and 70-72% of total P in the medium and high DAP rate treatments, respectively. This indicates that most P added as DAP remained labile or moderately labile in BRS, either in solution, or in adsorbed forms on the surface of more crystalline P compounds, sesquioxides and carbonate, or associated with amorphous and some crystalline Al and Fe hydrous oxides. In addition, differences in the hydrochloric acid extractable P and the residual-P fractions among the treatments with and without DAP addition were relative small comparing with other P pools (e.g., NaOH extractable P pools), further indicating the limited capacity of BRS for fixing P added in Ca-P and other most recalcitrant forms. P availability in the original BRS without addition of DAP was very low, mostly in recalcitrant form. It has been clearly demonstrated that significant proportions of P added as DAP could remain labile or moderately labile for plant use during the rehabilitation of bauxite-processing residue disposal areas. There was limited capacity of BRS for fixing P in more recalcitrant forms (e.g., Ca-P and residual-P). Concentrations of most P pools in BRS increased with the DAP application rate. The impact of the pH treatment on P availability varied with the type of P pools and the DAP rate. It is recommended that the development of appropriate techniques for more accurate estimation of P availability in BRS and the quantification of the potential leaching loss of P in BRS are needed for the accurate understanding of P availability and dynamics in BRS. In addition, application of organic matters (e.g., biosolids and biochar, etc.) to BRS may be considered for improving P availability and buffering capacity.

Behaviour and dynamics of di-ammonium phosphate in bauxite processing residue sand in Western Australia—I. NH3 volatilisation and residual nitrogen availability

Australia is the largest producer of bauxite in the world, with an annual output of approximately 62 million metric dry tons in 2007. For every tonne of alumina, about 2 tonnes of highly alkaline and highly saline bauxite-processing residue are produced. In Western Australia, Alcoa World Alumina, Australia (Alcoa) produces approximately 15 MT of residue annually from its refineries (Kwinana, Pinjarra and Wagerup). The bauxite-processing residue sand (BRS) fraction represents the primary material for rehabilitating Alcoa's residue disposal areas (RDAs). However, the inherently hostile characteristics (high alkalinity, high salinity and poor nutrient availability) of BRS pose severe limitations for establishing sustainable plant cover systems. Alcoa currently applies 2.7 t ha(-1) of di-ammonium phosphate ((NH(4))(2)HPO(4); DAP)-based fertiliser as a part of rehabilitation of the outer residue sand embankments of its RDAs. Limited information on the behaviour of the dominant components of this inorganic fertiliser in highly alkaline BRS is currently available, despite the known effects of pH on ammonium (NH(4)) and phosphorus (P) behaviour. The aim of this study was to quantify the effects of pH on NH(3) volatilisation and residual nitrogen (N) in BRS following DAP applications. The sponge-trapping and KCl-extraction method was used for determining NH(3) volatilisation from surface-applied DAP in samples of BRS collected from each of Alcoa's three Western Australia Refineries (Kwinana, Pinjarra, Wagerup) under various pH conditions (pH 4, 7, 9 and 11). Following cessation of volatilisation, the residual N was extracted from BRS using 2 M KCl and concentrations of NH (4) (+) -N and NO (3) (-) -N were determined by flow injection analysis. The quantities of NH(3) volatilised increased dramatically as the pH increased from 4 to 11. Much of the N lost as NH(3) (up to 95.2%) occurred within a short period (24 h to 7 days), particularly for the pH 9 and 11 treatments. Concentrations of residual NH (4) (+) -N recovered in DAP-treated BRS at the end of the experiment decreased with increasing pH. This finding was consistent with increasing loss of N via volatilisation as pH increased. The concentration of NO (3) (-) -N was very low due to no nitrification in BRS. The pH was a key driver for NH(3) volatilisation from DAP-treated BRS and primarily controlled N dynamics in BRS. Results indicate that NH(4) not adsorbed by BRS was highly susceptible to volatilisation. The likely lack of nitrifying bacteria did not allow conversion of ammonium to nitrate, thereby further exacerbating the potential for loss via volatilisation It was demonstrated that the pH is the key factor controlling the loss of inorganic N from BRS. Although volatilisation was considerably lower at pH 4, achieving this pH reduction in the field is not possible at present. Findings from this study highlight the need to better understand which forms of N fertiliser are most suitable for use in highly alkaline BRS. Although pH reduction is the most likely means of stopping NH(3) volatilisation in BRS, it is economically and operationally unfeasible to add sufficient acidity for adequately lowering pH in the BRS for revegetation. More attention on forms of fertilisers more suitable to highly alkaline, microbially inert soil conditions appears to be warranted.

Micronutrient fractionation and plant availability in bauxite-processing residue sand

Bauxite-processing residue must be disposed of in specifically designed facilities for long-term management. Consideration of alkalinity, salinity, sodium content, and poor nutritional status is essential for successful rehabilitation of residue disposal areas (RDA). The aim of this study was to examine the availability and distribution of the micronutrients, B, Cu, Fe, Mn, and Zn, in (i) fresh bauxite-processing residue sand (particle size &gt;150 μm) with and without gypsum amendment, and (ii) aged residue sand from a 4-year-old rehabilitated RDA that had received past gypsum and fertiliser addition. Samples of fresh residue sand from India and Australia exhibited high alkalinity, high salinity, and sodicity. Gypsum addition significantly lowered pH, soluble Na, and alkalinity. Aged residue sand had low levels of all micronutrients, with low extractability for Zn and Mn followed by B, Cu, and Fe. Fractionation showed that 30–78% of Zn and Mn and 40–60% of B existed in non-available (residual) forms. The next most dominant fractions were the Fe and Mn oxide-bound and carbonate-bound fractions. Plant-available fractions (i.e. exchangeable and organically bound) contributed &lt;1% of the total concentration. Total concentration was found to be a reliable indicator for Zn, Cu, and B extractability but not for DTPA-extractable forms of Fe and Mn. Leaf analysis of vegetation grown on aged residue sand indicated deficiencies of Mn and B. Results demonstrated that bauxite-processing residue sand contained very low levels of B, Mn, and Zn and these concentrations may be limiting to plant growth. Distribution of micronutrients among chemical pools was significantly influenced by pH, organic carbon, exchangeable Na, and alkalinity of residue. Nutrient management strategies that account for the characteristics of residue sand need to be developed for residue rehabilitation. Importantly, strategies to limit the conversion of nutrients to non-available forms are required to minimise micronutrient disorders.