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 >150 [micro]m) 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 <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 nonavailable forms are required to minimise micronutrient
Additional keywords: alkalinity, bauxite residue sand, fractionation, micronutrient availability, spiking.
Wastes (Chemical properties)
Bell, Richard W.
|Publication:||Name: Australian Journal of Soil Research Publisher: CSIRO Publishing Audience: Academic Format: Magazine/Journal Subject: Agricultural industry; Earth sciences Copyright: COPYRIGHT 2009 CSIRO Publishing ISSN: 0004-9573|
|Issue:||Date: August, 2009 Source Volume: 47 Source Issue: 5|
|Topic:||Event Code: 320 Manufacturing processes|
|Product:||Product Code: 1051100 Bauxite NAICS Code: 212299 All Other Metal Ore Mining SIC Code: 1099 Metal ores, not elsewhere classified|
|Geographic:||Geographic Scope: Australia|
Globally, >70 Mt of bauxite-processing residue is produced annually, and this production is projected to increase over the coming decade with rising demand for aluminium (Anon. 2007). In Western Australia, Alcoa World Alumina Australia (Alcoa) separates residue into 2 main fractions: sand (> 150 [micro]m) and mud (<150 [micro]m). Residue sand is used for constructing residue disposal areas (RDAs) for long-term storage of residue mud, and the outer sand embankments arc progressively rehabilitated for managing dust emissions and improving aesthetic value of operating and closed RDAs (Li 1998; Courtney and Timpson 2005).
Residue sand represents the primary growth medium for rehabilitating Alcoa's RDAs. However, successful establishment of vegetation in residue sand remains a challenge because of its extreme chemical, physical, and microbiological properties (Meecham and Bell 1977; Gherardi and Rengel 2001, 2003a; Bell et al. 2003; Snars et al. 2003; Courtney and Timpson 2005). Macro- and micronutrient deficiencies have been reported for plants growing in residue sand, even after fertiliser addition (Bell et al. 1997; Jasper et al. 2000; Eastham and Morald 2006). This has been attributed to the very low organic carbon and a dominance of hydrous Fe and/or Al oxides, and the alkalinity of the residue, which strongly influences nutrient reactions with the residue components (Ge et al. 2000; Soumare et al. 2003). Obtaining a better understanding of the mechanisms controlling the distribution of nutrients in response to residue properties will help to develop sustainable fertiliser strategies in achieving successful revegetation.
To date, much of the research on bauxite-processing residues has focused on the role of residue mud in pollution control and micronutrient availability, with little published information on the properties of residue sand. In Western Australia, residue sand represents the primary growth medium for rehabilitating Alcoa's RDAs. Consequently, more information on nutrient availability in this material is required. The objectives of this study were to: (1) quantify micronutrient forms using well-established fractionation schemes, (2) use this information to identify potential micronutrient limitations to plant growth, (3) use plant and soil analysis to verify that micronutrient limitations are manifested as nutrient disorders in the field, and (4) identify the most important residue characteristics controlling micronutrient availability in residue sand.
Materials and methods
Residue sand sources
Residue sand was collected from aluminium refineries in Australia (Alcoa World Alumina Australia's Pinjarra Refinery) and India (MALCO Ltd, Mettur). Three samples were collected from Alcoa: (1) untreated sand direct from the Bayer Process (FRS), (2) sand from a l-year-old rehabilitated site (AGARS1), and (3) sand from a 4-year-old rehabilitated site (AGARS2). Representative portions of the 2 FRS materials were also mixed with gypsum at a rate of 1% (w/w basis) (GARS). Comparing the results for fresh (i.e. no gypsum, FRS) and gypsum-amended (GARS) residue sand should identify the role of cations (Ca v. Na) and alkalinity on nutrient availability.
Due to the very low (<0.5mg/kg) micronutrient concentrations in the residue sands, samples were spiked at rates equivalent to those levels added in the field (per ha mixture of 15kg Zn, 15kg Mn, and 10kg Cu as the sulfate forms of fertilisers and 1.5 kg B as borax incorporated to a depth of 0.25 m). Spiking was undertaken by placing l kg of air-dry (<2mm) residue sand into a polythene bag, applying the micronutrients in solution form, and mixing thoroughly. The bags were moistened to field capacity using triple-deionised water (TDI) and incubated aerobically at room temperature for a week. The moisture content was monitored daily by weighing and TDI was added as required. After 7 days, the samples were air-dried and analysed for micronutrients and basic physical and chemical properties as outlined below.
Particle-size analysis of the residue sand sources was determined using the pipette method (Gee and Bauder 1987). Water-soluble carbonate, bicarbonate, and total alkalinity of residue sand sources were estimated in 1:5 residue sand/deionised water extracts by titrating against standard acid (0.005 M HCl) and alkali (0.50 M NaOH) to a pH equivalence point of 8.30 and 4.50, respectively, using mixed indicators (Rayment and Higginson 1992). All micronutrient-spiked and non-spiked residue sand samples were extracted with 0.005M DTPA-TEA--calcium chloride extractant (pH adjusted to 7.3, Lindsay and Norvell 1978) and analysed for Zn, Fe, Mn, Cu, and Ni (data not shown). Hot 0.01 M Ca[Cl.sub.2] extractable B was determined in 1 : 2 residue to extractant suspension after refluxing for 30min (Bingham 1982). Total micronutrients were extracted by aqua regia digestion (1 : 3 HN[O.sub.3] to HCl) (Melaku et al. 2005). All filtrates were analysed for micronutrient concentrations using ICP-AES.
Sequential fractionation of micronutrients
Sequential chemical fractionation of micronutrients in residue sands was carried out as described in Benitez and Dubois (1999) for micronutrient metals. Residue sand (1 g) was weighed into a 50-mL polypropylene centrifuge tube and each fraction was determined operationally (Table 1). The supenatant from each step was evaporated to near dryness and diluted with 10 mL of 1 M nitric acid. For B fractionation, the sequential extraction procedure outlined by Datta et al. (2002) was used (Table 2).
All the samples were extracted in triplicate and analysed for their micronutrient concentrations using ICP-AES. Simple correlation analysis was performed to examine the relationship between micronutrient fractions, their availability, and residue properties.
Plant and soil sampling from revegetated areas
The relationship between micronutrient availability in residue sand and plant nutrient status was investigated by sampling leaves and sand from the 2003 rehabilitated area at Alcoa's Pinjarra refinery. Rehabilitation established in 2003 had received 225 t gypsum/ha incorporated to a depth of ~1.5 m and inorganic fertiliser at 2.57 t/ha incorporated to a depth of ~0.25 m. The inorganic fertiliser was principally di-ammonium phosphate (DAP) at 1500kg/ha (supplying 300kg P and 265kg N/ha) plus granulated [K.sub.2]S[O.sub.4] at 300 kg K/ha; CuS[O.sub.4] ( 10 kg Cu/ha); granulated ZnS[O.sub.4] at 16 kg Zn/ha; MgS[O.sub.4] at 30 kg Mg/ha; granulated MnS[O.sub.4] at 15kgMn/ha; NaMo[O.sub.4] at 0.25kgMo/ ha; and granulated borax at 1.5 kg B/ha.
Residue rehabilitation used many of the native species found in coastal south-west Western Australia. Of the >60 species present, 4 were chosen to reflect differences in growth form and physiology, and a range of leaf symptoms indicative of nutrient deficiencies. The plants selected were Hardenbergia comptoniana (Hc), a vigorous creeping ground cover legume; Acacia cyclops (Ac), a legume shrub; Grevillea crithmifolia (Gc), a proteaceous shrub; and Eucalyptus gomphocephala (Eg), a tree.
Plants were sampled from both the upper and lower half of the embankment. Recently matured leaf blades from 15-25 plants were sampled for each species (Reuter and Robinson 1997). If a species displayed potential nutrient-deficient symptoms, separate leaf samples were taken from plants with and without symptoms for comparison and to provide a guide to critical nutrient levels because deficiency criteria for coastal species of Western Australia are generally not available. Plant samples were initially washed then oven-dried at 70[degrees]C and ground before N, P, K, S, Na, Ca, Mg, Cl, Cu, Zn, Mn, Fe, and B analysis. Residue sand samples were taken from 0-.010 m depth from under plant canopies and in gaps between canopies. Soil samples from each sampling location (n = 10) were bulked and air-dried before analysis. Soil extractions were undertaken using standard methods for: total N (Bremner and Mulveny 1982), N[O.sub.3]-N, N[H.sub.4]-N (Rayment and Higginson 1992); Colwell extractable P (Rayment and Higginson 1992) and K; KCl-extractable S (Rayment and Higginson 1992); organic carbon (Rayment and Higginson 1992); electrical conductivity (EC, 1 : 5); pH in Ca[Cl.sub.2] and water (1 : 5); DTPA-extractable Cu, Zn, Fe, and Mn (Lindsay and Norvell 1978); phosphorus retention index (PRI, Rayment and Higginson 1992); exchangeable Al, Ca, Mg, Na, and K (Tucker 1985); hot-Ca[Cl.sub.2] extractable B (Bingham 1982). Simple correlation and regression studies were performed to relate the influence of residue properties on micronutrient transformation and availability.
Results and discussion
Characterisation of residue sand sources
Samples from both Australia and India were predominantly sand (>950 g/kg), although the Indian residue sand had higher silt and clay content (>20g/kg) than the Australian source (>8.0g/kg). The water content at field capacity varied between 110 and 161 g/kg. The Indian sample retained more water than the Australian samples due to its higher proportion of fine material. The fresh residue sand samples from both Australia and India were extremely alkaline, saline, and sodic (Table 3).
Adding gypsum lowered the initial pH and EC irrespective of the residue source. In the Indian sample, gypsum addition reduced pH from 10.3 to 9.79 and in the Australian sample from 10.3 to 9.80. This reduction may be attributed to precipitation of hydroxides and carbonates by [Ca.sup.2+] ions (Barrow 1982; Bell et al. 1997; Courtney and Timpson 2005). Samples from 1- and 4-year-old rehabilitation exhibited a lower pH (7.98 and 8.18), which reflects the effects of past gypsum incorporation and rainfall leaching on removal of water-soluble alkalinity. Gypsum addition produced a 4-fold reduction in EC relative to fresh samples, presumably via the formation of insoluble calcium carbonates and hydroxides. The [EC.sub.1 : 5] varied between 0.10 and 1.70 dS/m and ageing reduced the salt status to favourable levels for plant growth. Values of [EC.sub.1 : 5] were consistent with those reported in other residue characterisation studies (Meecham and Bell 1977; Bell et al. 1997; Courtney and Timpson 2004, 2005).
Organic carbon content was very low in both the Australian and Indian samples (<4.0 g/kg), but the AGARS1 samples had higher values than fresh sand. This may be attributed to additions of poultry manure (from past management programs), wood mulch used for dust suppression, and recycled plant litter.
Samples of fresh residue sand from India and Australia exhibited very high total water-soluble alkalinity (61.8 and 56.6gCaC[O.sub.3]/kg). These concentrations are 6-fold higher than the optimal range reported for plant growth (<10 g CaC[O.sub.3]/kg, Bell 1981). Carbonate represented >85% of total water-soluble alkalinity of the FRS, while bicarbonate was the dominant source of alkalinity in GRS. Gypsum dissolution and subsequent precipitation of C[O.sub.3] as CaC[O.sub.3] is considered the most likely cause for the decline in water-soluble carbonate (Barrow 1982; Courtney et al. 2003). Aged residue sand exhibited water-soluble alkalinity levels 10-15 times lower than fresh samples, with bicarbonate being the dominant inorganic carbon ion. Water-soluble carbonate alkalinity was nil in aged residue sand sources, which indicated the presence of sufficient Ca ions supplied by gypsum to precipitate C[O.sub.3], in addition to removal by drainage water (Courtney and Timpson 2004; Xenidis and Harokopou 2005).
With the exception of Fe, micronutrient concentrations were well below levels regarded as inducing plant deficiencies (Munshower 1994; Gherardi and Rengel 2001). This finding supports the practice of micronutrient addition for rehabilitation of bauxite-processing residue areas. However, no significant increase in concentration of extractable micronutrients was observed, with additions equivalent to current field application rates of B, Cu, Mn, and Zn (Table 4). Gypsum amendment reduced B and Cu extractability by more than 40 and 14%, respectively, while it increased the extractability of Mn, Fe, and Zn in the Australian residue sand sample. The increased extractability of Fe, Mn, and Zn following gypsum addition may be associated with the removal of these cations from weakly held forms associated with the carbonate-bound fraction.
Negligible organic matter, coupled with abundant carbonate and hydrous Fe and Al oxides and hydroxides, suggests that inorganic reactions may be largely responsible for poor availability of micronutrients (Fuller and Richardson 1986; Gherardi and Rengel 2003c). Extractable levels of all micronutrients were lower in 4- than 1-year-old sand and the decrease in extractability was 20% for Fe, 50% for Cu, 92% for Zn, 89% for Mn, and 58% for B. This suggests that declining nutritional status over time may be an issue with respect to optimal plant growth.
Manganese deficiency has been cited as an important limiting factor for vegetative plant growth on residue sand by many workers (Gherardi and Rengel 2001, 2003a, 2003b; Courtney and Timpson 2004); however, the current study indicates that in addition to Mn, deficiencies of Zn and B may also need to be addressed in the rehabilitation of RDAs. The Indian sample had higher extractable Mn, Cu, and B, while the Australian sample was richer in Zn and Fe, suggesting that bauxite ore mineralogy has a significant bearing on the levels of micronutrients in residues. Correlation analysis of extractable micronutrients with residue properties showed that with the exception of Fe, all micronutrients were negatively correlated with increasing pH. Poor correlation was observed between total Mn and Fe and DTPA-extractable forms of these elements. Despite this, total concentration was found to be a reliable indicator for Zn, Cu and B extractability. Alkalinity of the residue sand was negatively correlated with Zn and Cu but was not related to levels of other micronutrients. More than 45% of variation in Zn and Cu extractability was explained by total alkalinity and exchangeable Na. Despite the low levels of organic matter in the residue, there was a positive and significant correlation of organic carbon with Zn, Mn, and B availability.
Iron was the most abundant micronutrient element followed by Mn > Cu > Zn > B. With the exception of B, the Indian samples contained much higher total micronutrient concentrations (Fe, Mn, Zn, Cu) than the Australian samples. Gypsum addition increased total Zn while it reduced Cu content. The increase in Zn may be directly related to gypsum addition since the gypsum used in this study contained Zn (see Table 3). Except for B, all micronutrients were higher in 1- than 4-year-old residue sand, which may be due to the recent fertiliser input and less-complete plant uptake.
Sequential fractions of micronutrients
Zinc fractionation followed the order: residual>Fe and Mn oxide bound>organic matter bound=carbonate bound> exchangeable fraction. The contribution of the residual fraction to total Zn was >75% in the Indian source sand and <40% in Australian source (Table 5).
The predominance of Zn in the residual fraction has been reported by other workers for soils (Yasrebi et al. 1994; Ma and Uren 1995). Ma and Uren (1995) found that although Zn was primarily concentrated in the residual fraction, it was also present at significant concentrations in the Fe-Mn oxide bound fractions. Higher concentrations of Zn in Fe and Mn oxide fraction were associated with aged residue sand followed by fresh and gypsum-amended Indian samples. Fe and Mn oxide bound Zn accounts for >60 and 20% of total Zn in fresh and gypsum-amended Australian residue sand, respectively.
Gypsum addition produced an inconsistent change of Zn levels in the Fe oxide fraction, but did increase Zn in the Mn oxide bound fraction. Preferential sorption of Zn by Fe and Mn oxides under alkaline pH has been reported by others for soils (Shuman 1985; Luo and Christie 1998; Silveira et al. 2006). A positive and significant correlation was found between Fe and Mn oxide bound Zn with pH (r = 0.793**, 0.816**), organic carbon (r = 0.696**, 0.655**), and total alkalinity (r = 0.471** 0.509**). Levels of Zn bound to organic matter ranged from 1.67 to 6.67 mg/kg. These values are regarded as being very low, and contribute to <1% of the total Zn in the Indian sample and 5-6% in the Australian sample. However, the extractability of organically bound Zn declined between 1- and 4-year-old sand sources, possibly through re-distribution into less-available fractions. Zn bound to carbonates ranged from 2.30 to 34.3mg/kg, with highest concentrations in the 1-year-old residue sand. These results indicate that Zn has a moderate affinity for carbonate surfaces, particularly CaC[O.sub.3] and/or it has precipitated as Zn carbonate (Prasad et al. 2006).
A strong positive correlation was found between carbonate bound and Fe-Mn oxide bound fractions of Zn (r = 0.914** and 0.943**, Table 6). For non-spiked samples, concentrations of Zn in the exchangeable fraction were negligible (<0.02 mg/kg). These findings are consistent with those reported by Luo and Christie (1998). Low extractability of Zn from carbonate and exchangeable fractions might be due to higher Fe and Mn oxide bound Zn, which acts as major active sites for Zn sorption under alkaline pH (Hseu 2005; Silveira et al. 2006). A positive and significant correlation was observed with organic carbon (r = 0.620**), DTPA Zn (r = 0.857**), carbonate (r = 0.862**), and Fe-Mn oxide bound Zn fractions (r = 0.778** and 0.811**). The pH and total alkalinity of residue sources was correlated with all Zn fractions except the residual fraction. Total Zn and organic carbon content of residue sand significantly and positively correlated with Zn fractions.
The extractability of Mn followed the order: residual fraction > Fe oxide bound > Mn oxide bound > carbonate bound > organic matter bound > exchangeable (Table 7). On average, 40-66% of total Mn was associated with the residual fraction and 12-45% in Fe-Mn oxide bound fractions. Strong retention of Mn by the oxide fraction has previously been found for both soil and bauxite residue sand, and has also been related to the low affinity of Mn for organic compounds (Goldberg and Smith 1984; Tong et al. 1995; Gherardi and Rengel 2001). Samples from India exhibited higher Mn concentrations in the residual and Fe Mn oxide bound fractions than the Australian sample. Oxide-bound fractions showed significant positive correlations among themselves (r [greater than or equal to] 93%) and also were highly correlated with exchangeable Na (r = 0.873**, 0.896**) and total Mn status (r = 0.962**, 0.975**).
Exchangeable and organically bound Mn contributed <0.5% of the total Mn, while carbonate-bound fraction accounted for 2-7% of total Mn. A relatively high proportion of Mn was associated with carbonate-bound forms, which might be due to higher CaC[O.sub.3] content of residue sand sources, or through precipitation as MnC[O.sub.3]. Strong association of Mn with the carbonate fraction was reported for soils by Tong et al. (1995) and for bauxite residue by Gherardi and Rengel (2001). There was a strong relationship between carbonate bound Mn and DTPA Mn (r=0.883**, Table 6) and exchangeable Mn (r=0.895**), suggesting that carbonate bound Mn might be an important source of plant-available Mn.
Adding gypsum significantly increased the concentration of exchangeable and residual Mn, while it markedly lowered organic matter and oxide-bound fractions. Displacement of Mn by Ca from exchange sites and further precipitation as MnC[O.sub.3] under alkaline pH could be the possible reasons for enhanced extraction of the carbonate-bound fraction and not the other 2 plant-available fractions (McBride 1994; Gherardi and Rengel 2001, 2003a; Zinati et al. 2001). One-year-old residue sand samples had higher extractability of all fractions except the Mn oxide bound fraction, which may be due to enhanced rhizosphere activities and Mn reduction in the presence of root exudates (Uren 1981; Gherardi and Rengel 2001). Further ageing reduced extractability of all Mn fractions but increased residual Mn, which indicated the possible conversion of all plant-available fractions into non-available forms with time. Except for carbonate-bound Mn, distribution into various fractions was not significantly influenced by residue properties such as pH, total alkalinity, and exchangeable Ca. However, exchangeable Na exhibited a positive relationship with all non-available pools of Mn (r=0.85**).
More than 40% of total B occurred in the residual pool, followed by readily soluble (16-20%)>organically bound (13-17%)>oxide bound (6-15%)>specifically adsorbed (3.6-6.5%) and hot water soluble fractions (2-8%, Table 8). Relative to the micronutrient metals, readily soluble and organic matter bound fractions comprised a high proportion of total B, while residual B was relatively low. All residue sand sources had levels of hot water soluble boron (<0.50mg/kg) too low for normal growth of most plants (Bell 1999) and values ranged from <0.04 to 0.21 mg/kg.
The high proportions of readily soluble B observed with all residue sources could be due to weak sorption on negatively charged surfaces of Fe and A1 oxides under alkaline pH (Goldberg 1997; Datta et al. 2002), as extractable B was positively related to pH (r = 0.921**). Gypsum addition favoured extraction of the Ca[Cl.sub.2]-B fraction and aging reduced B extractability. Total alkalinity and exchangeable Na were significantly correlated with readily soluble B (r=0.745**, 0.910**). Low levels of specifically adsorbed B may be due to very low organic carbon and clay content of residue sand sources. Aged samples had higher values of specifically adsorbed B than fresh residue, but 2003 samples had reduced extractability of this fraction compared to 2006 samples (data not shown). No significant correlations were observed between the specifically adsorbed B fraction and residue properties but the specifically adsorbed B fraction was positively correlated with all other B fractions. This indicates that the fraction might have originated from weakly bonded sites of both organic and inorganic constituents of residue sand sources. Similar relationship between soil properties and specifically adsorbed B was reported by Xu et al. (2001) for a range of Chinese soils.
Oxide-bound fractions of B were higher in fresh residue sand than gypsum-amended and aged samples, which can be explained with the positive effect of Fe2[O.sub.3] in adsorbing B as B(O[H.sub.3]) and B(O[H.sub.4]) (Su and Suarez 1995; Xu et al. 2001; Datta et al. 2002). Positive correlations were found between oxide-bound and all other B fractions. None of the residue sand properties was significantly correlated with the oxide-bound B fraction. Considerable proportions of organically bound B were extracted from all residue sand sources despite the low soil organic carbon status. The highest levels of organically bound B were associated with 1-year-old residue sand, which may be due to poor decomposition and mineralisation of the added organic amendments (Hou et al. 1994). Ageing reduced extractability of the organically bound B fraction by 50% and was positively and significantly correlated with organic carbon (r=0.550**), exchangeable Na (r=0.521**), and Ca[Cl.sub.2] B (r=0.701**). A strong positive correlation existed between residual fraction and total B (r=0.935**) but surprisingly there were no significant correlations between residual B and properties of residue sand sources. Gypsum addition slightly increased the readily soluble fraction but decreased other fractions such as oxide-bound, organically bound, residual, and specifically adsorbed B. Indian residue sand had lower extractability of all B fractions than Australian residue sand. A positive response to B addition was observed for specifically adsorbed, residual, and total B content of residue sand sources but not on other fractions.
Plant nutrient status
In Acacia cyclops, reduced leaf size, curling, crinkling, and malformation of leaves resembling B deficiency were observed with concomitant low leaf B values. Other Acacia cyclops plants showed symptoms resembling Mn deficiency and contained low leaf Mn values. By contrast, Hardenbergia comptoniana and Eucalyptus gomphocephala exhibited vigorous growth and dark green leaves with no specific symptoms resembling micronutrient deficiencies.
Based on previous studies (Bell et al. 1997; Gherardi and Rengel 2003a, 2003b; Eastham et al. 2006), Mn deficiencies were expected in plants grown on bauxite residue sand. However, results from this study revealed little evidence of Mn deficiency, apart from Acacia cyclops. Both cereal rye and triticale, which showed Mn deficiency in earlier studies (Bell et al. 1997; Eastham et al. 2006), are expected to be Mn-efficient, relative to most cereal crops. Hence, the lack of Mn deficiency in Hardenbergia comptoniana and Grevillea crithmifolia (data not shown) suggests a high level of Mn efficiency exists in the native species. Grevillea crithmifolia (data not shown) in particular contained very high leaf Mn (and Fe) concentrations. Many proteaceous species produce proteoid roots that mobilise nutrients such as Mn and P in the root-zone by organic acid secretion (Shane and Lambers 2005). Grevillea crithmifolia appears to be able to acquire Mn and Fe efficiently, suggesting the production of proteoid roots when grown in residue sand. Acacia cyclops appeared most susceptible to low Mn, with possible Mn deficiency in plants showing symptoms in phyllodes. Many eucalyptus species are sensitive to Mn deficiency on alkaline soils, and leaf concentrations <15-20 mg/kg dry weight are considered to be marginal for growth (Dell et al. 2001). Based on this critical range, Eucalyptus gomphocephala throughout residue rehabilitation areas may be expected to exhibit deficient levels of Mn (data not presented). Earlier studies by Bell et al. (1997) did not determine B levels in the test species, triticale, or in soils. Eastham et al. (2006) reported that B concentrations in cereal rye were adequate for growth in the first season after fertiliser application. By contrast, Eastham and Morald (2006) found B levels in cereal rye below critical levels. Acacia in particular had low leaf B, and leaves with symptoms in 2003 rehabilitation commonly contained very low leaf B. In 2006 and 2005 rehabilitation, low leaf B in leaves of Grevillea crithmifolia with symptoms were indicative of B deficiency (data not shown), but in 2003 rehabilitation areas, this species appeared to be free of B deficiency. One case of low B (along with low Zn) in Eucalyptus gomphocephala leaves was found in 2005 rehabilitation (data not shown). The hot Ca[Cl.sub.2] extractable B was below detection limits in residue in 2003 rehabilitation areas.
Eastham et al. (2006) found that cereal rye contained deficient levels of Zn in leaves despite the addition of 15 kg ZnS[O.sub.4]/ha at planting. Bell et al. (1997) found no indication of Zn deficiency in triticale when grown on residue sand with inorganic nitrogen-phosphorus based fertiliser added, although DTPA-extractable Zn levels were noted to be low. Soil extractable Zn levels were extremely variable, with several very high values from 2005 and 2004 areas (data not shown) and a very low value in 2003 areas. Hence, further research is needed on the risks and prevalence of Zn deficiency in residue sand. However, in 4-year-old residue rehabilitation sites, Zn levels had dropped relative to those in 1-year-old residue and may indicate a future risk of its deficiency. Like Mn deficiency, Zn deficiency is likely to be expressed in some species more than others due to the differential Zn acquisition characteristics of plants from residue sand.
Despite the alkaline pH and low levels of DTPA-extractable Fe, there were no confirmed cases that suggested Fe deficiency in the revegetated plants. Hence, the forms of Fe in residue sand apparently allow root acquisition of adequate Fe even under alkaline pH. The extractable Cu levels were low in most residue samples. However, leaf Cu status was not associated with deficiency symptoms except in Grevillea crithmifolia at 2006 and 2005 rehabilitation areas, which had low leafCu associated with symptoms (data not shown). However, the symptoms in these plants could not be attributed to low Cu alone.
Next to Mn, B and Zn limitations were the major micronutrients affecting plant growth in the revegetation sites. Furthermore, the mechanisms responsible for maintaining adequate available Fe and Cu for plants under alkaline pH conditions warrant further investigation. Use of geochemical speciation models such as PHREEQ may provide valuable insight to key mechanisms affecting plant available forms of micronutrients.
Samples of fresh residue sand from both Australia and India exhibited characteristics (highly saline, highly alkaline, and sodic) that may be expected to cause severe limitations to the availability of plant micronutrients. Gypsum addition markedly improved residue sand characteristics by reducing alkalinity (hence pH), and replacing Na with Ca as the dominant solution and exchangeable cation. Fresh residue sand had higher total alkalinity (>85-90%), which was primarily due to the dominance of water-soluble carbonate. In contrast, ageing reduced the pH along with a shift from carbonate to bicarbonate alkalinity. Ageing reduced extractability of all micronutrients, especially of Zn and Mn followed by B. Plant-available micronutrients were very low in all residue sand samples despite high total concentrations, which was also confirmed through the low leaf Zn, Mn, Cu, and B concentrations of plant species grown in the revegetated sites especially with A. cyclops.
More than 80-90% of Mn, Zn, and Cu existed in residual and Fe and Mn oxide bound pools. Ageing reduced all Zn and Cu fractions while it increased non-available Fe, Mn, and B forms. Residue properties such as pH, organic carbon, exchangeable Na, and alkalinity were correlated with Zn availability but only few significant correlations were observed with Fe, Mn, and B. Although Fe and Mn oxide bound fractions are not expected to be immediately available to plants, altering the residue properties like pH, alkalinity, and exchangeable Na and redox conditions can modify these fractions resulting in enhanced micronutrient availability and their redistribution. Management strategies based on chemical reactions observed in this study that control plant nutrient availability should be considered central to developing sustainable rehabilitation programs on residue sand.
The authors wish to acknowledge the support provided for this study by the Australian Government through the Australian Leadership Award Fellowship (ALA) and for the support provided by Alcoa World Alumina Australia.
Manuscript received 12 September 2008, accepted 27 April 2009
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Chitdeshwari Thiyagarajan (A,B), I. R. Phillips (C), B. Dell (D), and Richard W. Bell (B,E)
(A) Department of Soil Science and Agricultural Chemistry, Tamil Nadu Agricultural University, Coimbatore 641 003, Tamil Nadu, India.
(B) School of Environmental Science, Murdoch University, Murdoch, WA 6150, Australia.
(C) Alcoa World Alumina Australia, PO Box 172, Pinjarra, WA 6208, Australia.
(D) School of Biological Science and Biotechnology, Murdoch University, Murdoch, WA 6150, Australia.
(E) Corresponding author. Email: email@example.com
Table 1. Summary of extraction procedures used for micronutrient (Zn, Fe, Mn, and Cu) fractionation after Benitez and Dubois (1999) Micronutrient fractions Procedure Exchangeable 1 g residue sand, 10 mL 0.50M Mg[Cl.sub.2] (pH 7.0) shaken for 90min, centrifuged at 1019G for 15 min Carbonate bound Residue, 10 mL of 1.0M ammonium acetate (pH 5.0), shaken for 90 min, centrifuged at 1019G for 15 min Fe oxide bound Residue, 20 mL of 1.0 M HON[H.sub.2] x HCl (pH 3.0) prepared in 25% (v/v) C[H.sub.3]COOH, shaken for 90 min, centrifuged at 1019G for 15 min Mn oxide bound Residue, 20mL of 1.0M HON[H.sub.2] x HCl (pH 3.0) prepared in 0.50 M HCl, shaken for 90 min, centrifuged at 1019G for 15 min Organically bound Residue, 10 mL of 0.10M sodium pyrophosphate, shaken for 90min, centrifuged at 1019G for 15 min Table 2. Summary of extraction procedures used for B fractionation after Datta et al. (2002) B fractions Procedure Readily soluble B 5 g residue sand, 10mL 0.01 M Ca[Cl.sub.2], shaken for 16h at 25[degrees]C, centrifuged at 6708G for 30 min Specifically adsorbed B Residue from the above step, 10mL 0.05 M K[H.sub.2]P[O.sub.4], shaken for 1 h at 25[degrees]C, centrifuged at 6708G for 30 min Oxide bound B Residue, 20mL 0.175 M ammonium oxalate, shaken for 4 h, centrifuged at 6708G for 15 min Organically bound B Residue, 20 mL 0.5 M NaOH, shaken for 24 h, centrifuged at 6708G for 15 min Residual Concentration of mineral-bound B was estimated after subtracting the sum of all fractions from the total Total Residue, 3 mL aqua regia, wet digestion overnight, and later digested at 130[degrees]C for 2 h, filtered, diluted with 0.50 M nitric acid Table 3. Selected chemical properties of fresh and aged residue sand sources FRS, Fresh residue sand; CJFRS, gypsum/amended residue sand; AGARS 1, aged gypsum/amended residue sand, 2006 (1 year old); AGARS2, aged gypsum/amended residue sand, 2003 (4 years old). Values in parentheses are standard errors of the mean of 3 replications. The composition of gypsum applied (by aqua regia) was (mg/kg): 640 P, 1530K, 10 4000 Ca, 26 700 Na, 10.6 Zn, 317 Fe, 0.14 Mn, 1.38 Cu, 33.1 B Properties India Austrial FRS GFRS FRS pH TDI water, 1:5 10.2 (0.029) 9.79 (0.078) 10.3 (0.017) Sat. paste extract -- -- 10.3 (0.026) EC (dS/m) TDI water, 1:5 1.58 (0.032) 1.19 (0.058) 1.70 (0.043) Sat. paste extract -- -- 19.0 (0.848) Organic C (g/kg) 2.7 (0.45) 2.9 (0.31) 1.7 (0.17) Exch. Na (cmo[/kg) 13.6 (0.29) 11.8 (0.29) 7.30 (0.58) Soluble Na (mg/L) 13062 (1562) Water soluble alkalinity (g CaC[O.sub.3]/kg) Total 61.8 (1.33) 20.9 (2.09) 56.6 (0.48) Bicarbonate 5.32 (1.18) 13.1 (1.61) 2.40 (0.45) Carbonate 56.5 (2.05) 7.72 (0.750) 54.2 (0.208) Properties Austrial GFRS AGARS1 AGARS2 pH TDI water, 1:5 9.80 (0.032) 7.98 (0.061) 8.18 (0.050) Sat. paste extract 9.83 (0.040) 8.12 (0.012) 8.61 (0.455) EC (dS/m) TDI water, 1:5 0.40 (0.009) 0.35 (0.012) 0.10 (0.003) Sat. paste extract 4.21 (0.202) 2.15 (0.049) 1.19 (0.133) Organic C (g/kg) 2.4 (0.24) 3.6 (0.24) 2.4 (0.36) Exch. Na (cmo[/kg) 3.15 (0.52) 0.90 (0.52) 0.12 (0.02) Soluble Na (mg/L) 3121 (156) 391 (14.0) 194 (25.0) Water soluble alkalinity (g CaC[O.sub.3]/kg) Total 13.8 (0.96) 3.23 (0.209) 3.75 (0.313) Bicarbonate 8.55 (0.418) 3.23 (0.209) 3.75 (0.313) Carbonate 5.21 (1.16) Nil Nil Table 4. DTPA/extractable copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) and Ca[Cl.sub.2]/extractable boron (B) status (mg/kg) in residue sand sources with and without addition of respective micronutrients FRS, Fresh residue sand; GFRS, gypsum-amended residue sand; AGARS 1, aged gypsum-amended residue sand, 2006 (l year old); AGARS2, aged gypsum-amended residue sand, 2003 (4 years old). Values in parentheses are standard errors of the mean of 3 replications India Australia FRS GFRS FRS Without micronutrient addition Zn 0.070 (0.01) 0.070 (0.01) 0.127 (0.02) Mn 0.420 (0.02) 0.590 (0.02) 0.020 (0.00) Fe 2.16 (0.42) 1.29 (0.07) 5.21 (0.07) Cu 0.153 (0.02) 0.133 (0.007) 0.175 (0.014) B 0.237 (0.023) 0.202 (0.02) 0.220 (0.023) With micronutrient addition Zn 0.080 (0.01) 0.070 (0.01) 0.153 (0.018) Mn 0.460 (0.04) 0.660 (0.004) 0.060 (0.01) Fe 2.69 (0.07) 1.35 (0.04) 6.15 (0.37) Cu 0.160 (0.00) 0.140 (0.00) 0.244 (0.012) B 0.253 (0.029) 0.235 (0.03) 0.340 (0.04) Australia GFRS AGARS1 AGARS2 Without micronutrient addition Zn 0.216 (0.02) 9.01 (0.87) 0.867 (0.07) Mn 0.030 (0.00) 1.70 (0.13) 0.250 (0.03) Fe 7.75 (0.35) 3.21 (0.37) 1.83 (0.04) Cu 0.142 (0.008) 0.284 (0.061) 0.271 (0.013) B 0.127 (0.018) 0.380 (0.053) 0.153 (0.037) With micronutrient addition Zn 0.30 (0.03) 12.2 (1.412) 1.08 (0.121) Mn 0.081 (0.02) 1.81 (0.19) 0.260 (0.01) Fe 8.35 (0.29) 3.68 (0.12) 2.41 (0.53) Cu 0.187 (0.03) 0.444 (0.05) 0.417 (0.04) B 0.160 (0.03) 0.473 (0.04) 0.207 (0.05) Table 5. Distribution of Zn into various fractions (mg-kg) in residue sand sources with (+Zn) and without (-Zn) addition of Zn FRS, Fresh residue sand; GFRS, gypsum-amended residue sand; AGARS l, aged gypsum-amended residue sand, 2006 (l year old); AGARS2, aged gypsum-amended residue sand, 2003 (4 years old). Values in parentheses are standard errors of the mean of 3 replications India Australia FRS GFRS FRS Exchangeable -Zn <0.02 <0.02 <0.02 +Zn <0.02 2.30 (0.30) 6.00 (1.50) Carbonate-bound -Zn 2.30 (0.33) 2.30 (0.33) 3.00 (0.58) +Zn 3.70 (0.88) 3.30 (0.67) 3.33 (0.88) Fe-oxide bound -Zn 54.0 (1.15) 48.7 (0.30) 17.3 (0.67) +Zn 58.0 (2.31) 61.3 (5.21) 28.0 (5.03) Mn-oxide bound -Zn 11.3 (1.33) 9.30 (0.67) 7.33 (0.67) +Zn 12.7 (1.76) 12.0 (1.15) 8.67 (0.67) Organic matter bound -Zn 2.00 (0.58) 2.0 (0.0) 1.67 (0.33) +Zn 2.30 (0.33) 2.70 (0.33) 3.67 (0.88) Residual -Zn 219 (3.5) 231 (0.6) 11.7 (2.40) +Zn 230 (11.0) 247 (16.2) 15.0 (1.15) Total -Zn 288 (6.01) 293 (4.41) 41.0 (2.08) +Zn 308 (6.01) 328 (10.1) 65 (6.77) Australia GFRS AGARS1 AGARS2 Exchangeable -Zn <0.02 <0.02 <0.02 +Zn 6.00 (1.20) 3.00 (0.60) 5.70 (0.90) Carbonate-bound -Zn 2.33 (0.33) 29.3 (2.73) 10.3 (0.33) +Zn 2.67 (0.33) 34.3 (3.48) 10.7 (0.67) Fe-oxide bound -Zn 17.3 (1.76) 116 (3.30) 64.0 (2.30) +Zn 22.0 (1.15) 166 (6.70) 72.3 (4.33) Mn-oxide bound -Zn 6.67 (0.67) 32.7 (2.67) 16.7 (1.76) +Zn 8.00 (2.00) 43.3 (6.36) 19.3 (2.91) Organic matter bound -Zn 3.00 (0.00) 6.00 (0.00) 2.67 (0.33) +Zn 3.00 (0.58) 6.67 (0.33) 3.67 (0.67) Residual -Zn 19.0 (1.15) 125 (8.2) 36.3 (2.85) +Zn 23.0 (0.58) 161 (19.1) 39.0 (2.89) Total -Zn 47.0 (1.67) 310 (10.4) 125 (5.78) +Zn 65 (0.67) 415 (11.6) 144 (9.45) Table 6. Relationship between residue properties and Zn and Mn fractions * P < 0.05; ** P < 0.01; n = 36 (6 residue sand sources, 2 rates of spiking, 3 replications) Residue Zn properties Exch. Carb. Fe oxide Mn oxide bound bound bound pH -- 0.824 ** 0.793 ** 0.816 ** Organic carbon -- 0.563 * 0.696 ** 0.655 ** Total alkalinity -- 0.558 * 0.471 * 0.509 * Exch. Na -- 0.613 ** 0.335 0.509 * Exch. Ca -- 0.057 0.228 0.082 DTPA Zn -0.330 0.960 ** 0.888 ** 0.876 ** Total Zn 0.699 ** 0.520 ** 0.759 ** 0.597 ** Residue properties Org. Residual Total bound pH 0.759 ** 0.157 -0.243 Organic carbon 0.620 ** 0.470 * 0.681 ** Total alkalinity 0.529 ** 0.206 -0.064 Exch. Na 0.513 * 0.700 ** 0.360 Exch. Ca 0.150 0.467 0.446 DTPA Zn 0.857 ** 0.121 0.552 ** Total Zn 0.423 * 0.875 ** 1.00 Residue Mn properties Exch. Carb. Fe oxide Mn Oxide bound bound bound pH -0.755 -0.693 ** 0.463 0.503 * Organic carbon 0.689 0.677 ** 0.168 0.176 Total alkalinity -0.574 -0.334 0.481 * 0.493 * Exch. Na -0.444 -0.257 0.873 ** 0.896 ** Exch. Ca 0.313 0.126 0.260 0.277 DTPA Zn 0.951 ** 0.883 ** 0.049 0.031 Total Zn -0.155 -0.073 0.962 ** 0.975 ** Residue properties Org. Residual Total bound pH 0.465 0.466 0.467 Organic carbon 0.196 0.143 0.159 Total alkalinity 0.437 0.351 0.422 Exch. Na 0.877 ** 0.857 ** 0.883 ** Exch. Ca 0.359 0.467 0.376 DTPA Zn 0.051 0.052 0.054 Total Zn 0.966 ** 0.960 ** 1.00 Table 7. Distribution of Mn into various fractions (mg-kg) in residue sand sources with (+Mn) and without (-Mn) addition of Mn FRS, Fresh residue sand; GFRS, gypsum-amended residue sand; AGARS 1, aged gypsum-amended residue sand, 2006 (1 year old); AGARS2, aged gypsum-amended residue sand, 2003 (4 years old). Values in parentheses are standard errors of the mean of 3 replications India Australia FRS GFRS FRS Exchangeable -Mn 0.73 (0.13) 2.10 (0.12) 0.30 (0.06) +Mn 1.77 (0.52) 2.90 (0.44) 0.63 (0.12) Carbonate bound -Mn 12.7 (0.47) 10.8 (0.58) 7.33 (0.37) +Mn 15.6 (0.42) 11.5 (0.33) 9.47 (1.67) Fe oxide bound -Mn 4990 (577) 4010 (509) 136 (8.11) +Mn 5560 (335) 4670 (410) 174 (19.7) Mn oxide bound -Mn 1889 (60.9) 1613 (96.2) 53.0 (8.66) +Mn 2052 (81.0) 1999 (58.0) 76.0 (4.80) Organic matter bound -Mn 83.5 (8.41) 81.8 (2.08) 1.67 (0.42) +Mn 120 (10.0) 86.8 (1.15) 2.27 (0.45) Residual -Mn 6021 (50.0) 7443 (953) 124 (26.2) +Mn 7078 (987) 8463 (848) 148 (10.2) Total -Mn 12 994 (577) 13 161 (1093) 301 (28.0) +Mn 17 161 (1878) 19 161 (1590) 411 (22.0) Australia GFRS AGARS1 AGARS2 Exchangeable -Mn 0.73 (0.12) 7.37 (0.28) 1.77 (0.03) +Mn 1.03 (0.18) 8.53 (0.50) 2.30 (0.12) Carbonate bound -Mn 4.00 (0.98) 25.4 (0.87) 12.1 (0.67) +Mn 5.03 (0.59) 32.1 (3.18) 16.5 (0.33) Fe oxide bound -Mn 32.0 (8.66) 187 (12.02) 72.0 (4.37) +Mn 47.0 (5.70) 220 (17.3) 83.0 (4.60) Mn oxide bound -Mn 81.0 (8.00) 35.0 (2.31) 9.47 (0.703) +Mn 101 (1.50) 39.0 (2.40) 13.0 (1.70) Organic matter bound -Mn 1.13 (0.23) 3.10 (0.00) 0.77 (0.12) +Mn 1.83 (0.28) 3.50 (0.21) 0.83 (0.09) Residual -Mn 184 (24.6) 104 (15.0) 202 (26.2) +Mn 220 (15.5) 121 (22.5) 235 (19.4) Total -Mn 302 (17.0) 327 (30.0) 297 (30.0) +Mn 376 (22.0) 424 (26.0) 351 (22.0) Table 8. Distribution of B into various fractions (mg-kg) in residue sand sources with (+B) and without added B (-B) FRS, Fresh residue sand; GFRS, gypsum-amended residue sand; AGARSI, aged gypsum-amended residue sand, 2006 (1 year old); AGARS2, aged gypsum-amended residue sand, 2003 (4 years old). Values in parentheses are standard error of the mean of 3 replications, n.d., Not detected India Australia FRS GFRS FRS Hot water soluble -B n.d. n.d. 0.127 (0.007) +B n.d. n.d. 0.150 (0.02) Readily soluble -B 0.30 (0.0) 0.31 (0.01) 0.34 (0.01) +B 0.33 (0.03) 0.34 (0.02) 0.37 (0.01) Specifically adsorbed -B 0.09 (0.01) 0.09 (0.02) 0.10 (0.01) +B 0.10 (0.01) 0.13 (0.03) 0.15 (0.03) Oxide bound -B 0.19 (0.01) 0.17 (0.01) 0.31 (0.01) +B 0.22 (0.01) 0.21 (0.02) 0.35 (0.03) Organically bound -B 0.28 (0.04) 0.23 (0.04) 0.35 (0.02) +B 0.31 (0.03) 0.28 (0.03) 0.37 (0.03) Residual -B 0.76 (0.12) 0.58 (0.15) 1.05 (0.23) +B 0.86 (0.11) 0.99 (0.06) 1.29 (0.21) Total -B 1.63 (0.10) 1.52 (0.16) 2.0 (0.25) +B 1.82 (0.08) 2.06 (0.04) 2.27 (0.20) Australia GFRS AGARS1 AGARS2 Hot water soluble -B 0.067 (0.007) 0.160 (0.01) 0.060 (0.0) +B 0.080 (0.01) 0.210 (0.02) 0.070 (0.0) Readily soluble -B 0.36 (0.0) 0.45 (0.01) 0.42 (0.0) +B 0.39 (0.01) 0.51 (0.04) 0.45 (0.02) Specifically adsorbed -B 0.09 (0.01) 0.14 (0.01) 0.09 (0.02) +B 0.13 (0.02) 0.16 (0.01) 0.12 (0.02) Oxide bound -B 0.27 (0.05) 0.32 (0.02) 0.16 (0.02) +B 0.34 (0.06) 0.36 (0.02) 0.19 (0.04) Organically bound -B 0.31 (0.04) 0.87 (0.07) 0.33 (0.04) +B 0.34 (0.04) 0.95 (0.10) 0.41 (0.01) Residual -B 0.94 (0.16) 1.13 (0.18) 1.49 (0.35) +B 0.82 (0.32) 1.30 (0.01) 1.45 (0.18) Total -B 1.96 (0.12) 2.25 (0.13) 2.50 (0.23) +B 2.01 (0.33) 2.44 (0.03) 2.62 (0.13)
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