The response of partially oxidised acid sulfate soil materials to anoxia.
Acid sulfate soils (ASS) contain iron sulfides (principally pyrite) or the products of sulfide oxidation (White and Melville 1996). Unoxidised sulfidic materials are often referred to as potential ASS. Actual ASS develop when the quantity of sulfuric acid, formed by the oxidation of reduced inorganic sulfur compounds in the ASS material, exceeds the acid-neutralising capacity of that ASS material due to adsorbed bases and easily weatherable minerals (e.g. CaC[O.sub.3]), to the extent that the pH drops below 4.0 (Pons et al. 1982). In broadacre situations the further oxidation and acidification of ASS materials is generally prevented by maintaining an anoxic environment, and this is usually achieved by either flooding or watertable management. Flooding has often been proposed to improve the water quality in ASS landscapes (e.g. Dent 1986), and the prevention of oxidation through watertable management currently appears to be the most cost-effective option in the management of ASS associated with broadacre agriculture in Australia (White et al. 1996). The response of ASS to the exclusion of oxygen as a result of submergence is, however, reported to be highly variable (e.g. Ponnamperuma et al. 1973; Tuong 1993; Konsten et al. 1994).
While the supply of oxygen appears to be the rate-limiting factor in the oxidation of pyrite under acid conditions in the field (Dent 1986), pyrite oxidation may continue in the absence of oxygen as a result of oxidation by ferric iron ([Fe.sup.3+]) formed during the preceding aeration (van Breemen 1993):
(1) Fe[S.sub.2] + 14[Fe.sup.3+] + 8[H.sub.2]0 [left arrow] 15[Fe.sup.2+] + 2S[O.sub.4.sup.2-] + 16H+
Pyrite oxidation will only continue in the absence of oxygen as long as [Fe.sup.3+] is present in the soil solution (White et al. 1997), and although [Fe.sup.3+] is known to be an important pyrite oxidant over a wide pH range (e.g. Moses et al. 1987), it is not until the pH drops below approximately 3.5 that [Fe.sup.3+] becomes sufficiently stable in solution for rapid pyrite oxidation to take place (Evangelou 1995).
In addition to the aim of preventing further pyrite oxidation, the exclusion of oxygen often removes the acidity in partially oxidised sediments as the acidity gets consumed from the reduction of iron(III) oxides, sulfates, and other oxidised species by anaerobic bacteria (Dent 1986). In most moderately acid soils, reduction causes the pH to rise to approximately 7 within a few weeks; however, some ASS may not reach a pH >5 after months of submergence (Ponnamperuma 1972). Factors that have been identified as being responsible for slow reduction, and hence a slow increase in pH, include low content of easily oxidisable organic matter, low content of easily reducible iron, low dissolved sulfate concentration, the adverse effect of low pH on activity of microbes, and poor nutrient status (Ponnamperuma et al. 1973; van Breemen 1976; Berner 1984). While the increase in pH from reduction may improve water quality and reduce Al toxicity, the negative side-effects of reduction may include elevated ferrous iron ([Fe.sup.2+]) and the formation of harmful organic substances, [H.sub.2]S and C[O.sub.2] (Dent 1986; van Breemen 1993).
If reduction regimes become sufficiently intense, sulfate is reduced to sulfide (van Breemen 1976), which then reacts with iron to form iron sulfide minerals. The process of pyrite formation in ASS is poorly understood. Research has shown only that pyrite may be formed via 2 basic mechanisms, which include the reactions of iron monosulfides with both dissolved polysulfides ([S.sub.x.sup.2-]) and [H.sub.2]S (Rickard and Luther 1997). Regardless of the actual mechanism, it is well known that pyrite may form rapidly in sediments (e.g. Howarth 1979; Giblin 1988), and recent research has shown substantial pyrite re-formation at the surface and subsurface in ASS (Bush et al. 2000b; Rosicky et al. 2002).
In this study, 4 ASS materials were allowed to oxidise for various intervals (e.g. 8, 63, and, 126 days) before being subjected to anoxia for up to 120 days to examine their geochemical response. This study aims to examine: (i) the response of a range of ASS materials to anoxic conditions; (ii) whether there were any differences in response due to the degree of prior oxidation and acidification; and (iii) the degree of sulfide re-formation that would occur in these materials under these anoxic conditions over the timeframe of this study.
Materials and methods
Soil sites and sampling
Four ASS materials were collected from 3 coastal floodplain sites in north-eastern New South Wales: McLeods Creek, a tributary of the Tweed River (28[degrees]18'7"S, 153[degrees]30'50"E), Tuckean Swamp (28[degrees]56'00"S, 153[degrees]23'30"E), and Bungawalbin Swamp (29[degrees]06'27"S, 153[degrees]13'30"E) on the Richmond River floodplain. Samples were collected using a Russian D-section corer and the outer layer of each core was removed and discarded to prevent any contamination from the sampling process. The samples were thoroughly mixed and stored in a thick plastic bag to minimise oxidation. Profile sampling depths and some of the characteristics of the ASS materials are shown in Table 1. All samples were in the reduced unoxidised zone, as evidenced by the visual lack of iron and jarosite precipitates, except McLeods Creek A, which had experienced some oxidation and contained iron oxide segregations around vacant root channels.
Laboratory incubation experiments
Incubation experiments are commonly used to identify the presence of potential ASS, in the calculation of lime requirements, and to simulate the natural oxidation behaviour of ASS materials (Dent 1986; Isbell 1996; Soil Survey Staff 1999, 2003; Ward et al. 2002a, 2002b). Incubation was chosen here to simulate: (i) the natural oxidation behaviour of these materials, and (ii) the response of these partially oxidised materials to the exclusion of oxygen that would result from either flooding or watertable management.
Within 24 h of sampling, field-moist duplicate cores were thoroughly mixed and placed in 40-[micro]m-thick plastic bags (35 by 23 cm). Each bag was pressed closely into flat blocks to minimise retained air, then folded and sealed to limit the rate of oxygen diffusion into the samples. All samples had a uniform thickness of 10 mm. The samples were incubated in a dark humid environment at a constant 20[degrees]C. Homogenised subsamples (9 in total) were taken at intervals up to 181 days (e.g. 0, 1, 2, 4, 8, 16, 32, 63, and 181 days). Moisture contents ([[theta].sub.g]) dried at 105[degrees]C were determined on subsamples throughout the experiment. The samples were weighed and remoulded following sampling, and deionised water was added if necessary to maintain field moisture content.
Subsamples were taken from the oxic incubation experiment after 8 and 63 days of oxidation, and placed into a dark, humid, [N.sub.2] environment created by passing a slow stream of >99.9% [N.sub.2] through the incubator at a constant 20[degrees]C. Additional subsamples were also taken from the Bungawalbin and Tuckean Swamp samples after 126 and 135 days of oxic incubation, respectively. As with the oxic incubation experiment, samples were placed in 40-[micro]m-thick plastic bags and had a uniform thickness of 10 mm. Homogenised subsamples (up to 6 in total) were taken at intervals up to 120 days (e.g. 0, 2, 10, 30, 60, and 120 days) after placing the samples into a [N.sub.2] environment. The samples were weighed and remoulded following sampling. A constant slow flow of [N.sub.2] over the samples was maintained throughout the experiment, and the container was purged with [N.sub.2] following subsampling. Moisture contents ([[theta].sub.g]) dried at 105[degrees]C were determined on subsamples at the end of the experiment and showed minimal moisture loss (i.e. [[theta].sub.g]s >85% of the initial moisture content after 120 days of incubation).
All subsamples taken from the incubation experiments were freeze-dried using liquid [N.sub.2] and a Dynavac freeze-drier prior to analysis. These samples were then hand-crushed using a porcelain mortar and pestle under atmospheric conditions. Soil pH and electrical conductivity (EC) were both measured in a 1 : 5 freeze-dried soil/water extract using an Orion 720A glass electrode for pH and a TPS 900C electrical conductivity meter. The reduced inorganic sulfur fraction [which, in most ASS materials from the cast coast of Australia, includes mainly pyrite (Bush and Sullivan 1999; Bush 2000), but also may contain small quantities of marcasite (Sullivan and Bush 1997; Bush 2000) and acid-volatile sulfides (Bush and Sullivan 1997; Bush et al. 2000a)] was measured using the modified chromium-reducible sulfur (CRS) method of Sullivan et al. (2000). The reduced inorganic sulfur fraction ([S.sub.CR]) is a measure of the sulfide fraction in these ASS materials. The rates of sulfide oxidation and re-formation were determined from the rate of [S.sub.CR] decline and increase, respectively, during the experiment. Water-soluble sulfate (S[O.sub.4.sup.2-]) was analysed turbidimetrically using flow injection analysis colorimetry (Lachat QuikChem 8000). Water-soluble iron and aluminium were analysed using atomic absorption spectrometry (AAS) (Varian SpectrAA 220). All water extract samples were centrifuged at 3000 rpm for 15 min and then filtered through 0.45-[micro]m filters prior to analysis; 2 drops of 50% nitric acid (HN[O.sub.3]) were added to the water-soluble iron and aluminium samples for preservation. The reducible iron fraction was extracted using the citrate/dithionite-extrac table iron ([Fe.sub.CD]) procedure of Rayment and Higginson (1992), and was analysed using AAS. It is expected that [Fe.sub.CD] is largely a measure of Fe(III) oxides and hydroxides in these ASS materials, although dissolved iron and jarosite may also be extracted using this technique (Konsten et al. 1994). The total carbon content (%C) of the soil samples (mainly organic carbon in these materials; Ward et al. 2002b) was measured by a LECO 220 Sulfur/Carbon analyser. The standard error was calculated for each of the duplicate samples and standard error bars have been included on all figures; standard error bars are not visible when they lie within the symbol used to depict the data point.
Results and discussion
Characteristics of the ASS materials
The ASS materials had a light clay texture, with the exception of the Bungawalbin ASS, which was a peat. The pH of the ASS materials at the start of the incubation experiment ranged between 3.2 and 8.2 (Table 1), with the Bungawalbin and Tuckean Swamp samples being very acidic (pH <4.0) despite a lack of evidence of previous oxidation (i.e. iron segregations). This acidic/unoxidised phenomenon is commonly observed in ASS backswamp environments, and has probably resulted from downward acid diffusion from the overlying oxidised actual ASS layers (Rosicky et al. 2000). However, it is possible that some sulfide oxidation may have occurred in these materials under acidic conditions without the formation of iron and/or jarosite precipitates (Fanning et al. 2002). The reduced inorganic sulfur (i.e. the [S.sub.CR] fraction) is mostly pyrite for these ASS materials, with the exception of the Bungawalbin ASS material which contains abundant marcasite as well as pyrite (Sullivan and Bush 1997; Bush 2000), and initially ranged between 1.96% (McLeods Creek A) and 5.44% (Tuckean Swamp) (Table 1). The total carbon content of the materials ranged between 1.7% (McLeods Creek A and B) and 11.8% (Bungawalbin) (Table 1). All samples showed an increase of [less than or equal to]0.1% soluble Ca upon peroxide oxidation, indicating essentially an absence of effective CaC[O.sub.3] in these ASS materials (Ward et al. 2002b). The water-soluble sulfate (S[O.sub.4.sup.2-]) and reducible iron ([Fe.sub.CD]) concentrations at the start of the incubation experiment are also shown in Table 1.
Soil acidification during incubation experiments
All 4 ASS materials showed acidification in the oxic incubation experiments (Fig. 1). The rate of acidification generally decreased markedly when the ASS materials were placed in the anoxic environment (Fig. 1). The time taken for the rate of acidification to decrease compared with that of the oxic samples varied depending on the ASS material and the degree of oxidation the sample had undergone prior to being placed into the anoxic environment. When the non-peat ASS samples were placed into the anoxic environment after 8 days of oxidation, they acidified at the same rate as the oxic samples for up to approximately 10 days. However, when these samples were placed into the anoxic environment after 63 days of oxidation, they acidified at the same rate as the oxic samples for a considerably longer period before the rate of pH decline in the anoxic ASS materials decreased. For example, the McLeods Creek A 63-day oxidised samples when subject to anoxic conditions acidified at the same rate as the oxic samples for 30 days before the rate of acidification in these anoxic samples decreased (Fig. 1a).
[FIGURE 1 OMITTED]
The response of the Bungawalbin peat material to the anoxic environment also varied depending on the degree of oxidation (Fig. 1c). When the peat was placed into the anoxic environment after 8 days of oxidation, an initial increase in pH was observed, followed by slow acidification for 60 days. However, when the peat ASS was placed into the anoxic environment after both 63 and 126 days of oxidation, further acidification did not occur, and a slight pH increase was observed. The Tuckean ASS materials also showed a slight increase in pH when placed into the anoxic environment after 135 days of oxidation (Fig. 1d).
The slight increase in pH that was observed when some of these ASS materials were placed into the anoxic environment indicates that some acid-consuming processes occurred in this environment. However, acidification still continued in the majority of the ASS samples after being placed in the anoxic environment, indicating that for most of the samples, the acidifying processes continued (albeit at much slower rates than with the oxic samples) despite the absence of oxygen.
Sulfide oxidation and re-formation during incubation experiments
All 4 ASS materials showed considerable sulfide oxidation during oxic incubations (Fig. 2). The long-term rate of sulfide oxidation and acidification both showed a similar trend during the oxic incubation experiment, indicating that acidification in these materials is largely controlled by the rate of sulfide oxidation. The rate of sulfide oxidation decreased considerably when all the partially oxidised ASS materials were placed into an anoxic environment (Fig. 2). This response was expected, as the rate of pyrite oxidation (the predominant sulfide mineral in these materials) is usually largely controlled by the oxygen availability (Dent 1986; Ward et al. 2004a). However, as also indicated by both the acidification of these materials (Fig. 1) and the decrease in sulfide concentration (Fig. 2), sulfide oxidation generally continued, although at restricted rates, when these materials were placed into anoxic environments.
[FIGURE 2 OMITTED]
There was minimal further sulfide oxidation with the McLeods Creek B sample after placing this ASS material into an anoxic environment after 8 days of oxidation (Fig. 2b). Sulfide oxidation continued with the McLeods Creek A, Bungawalbin, and Tuckean Swamp samples under these conditions (Fig. 2a, c, d); sulfide oxidation was still observed 120 days after being placed into an anoxic environment with the Tuckean Swamp sample (Fig. 2d). The response of the ASS materials to the anoxic environment after 63 days of oxidation also varied between the materials. The Bungawalbin material showed sulfide oxidation for the initial 10 days after being placed into the anoxic environment, followed by sulfide re-formation over the subsequent 50 days (ScR increased from 2.14 to 2.61%). McLeods Creek A showed a similar trend, with sulfide oxidation during the initial 30 days after being placed into the anoxic environment, followed by probable slight sulfide re-formation over the next 30 days ([S.sub.CR] increased from 1.51 to 1.57%). Although some sulfide re-formation was initially observed with the Tuckean Swamp ASS after being placed into an anoxic environment following 63 days of oxidation, no significant change in sulfide concentration was observed after 60 days of incubation. McLeods Creek B also showed minimal change in sulfide concentration when placed into the anoxic environment after 63 days of oxidation. When the ASS from Bungawalbin and Tuckean Swamp were placed into the anoxic environment after 126 and 135 days of oxidation, respectively, both materials showed an initial increase in the sulfide concentration; however, the sulfide formed during the initial 2 days was subsequently rapidly oxidised.
The temporal variation in the citrate/dithionite-extractable iron fraction ([Fe.sub.CD]) (Fig. 3) was negatively correlated with the change in sulfide concentration. When substantial sulfide re-formation occurred in the Bungawalbin ASS (i.e. when placed into the anoxic environment after 63 days of oxidation), the [Fe.sub.CD] content decreased (Fig. 3c). The water-soluble sulfate concentration also decreased as sulfide concentration increased, but it was generally a poor indicator of sulfide re-formation (Fig. 4). It is most likely that sulfate reduction was masked by other processes controlling the water-soluble sulfate concentration. For example, previous research has shown an increase in sulfate on submergence as a result of the reduction of basic ferric sulfate and displacement of adsorbed sulfate (Nhung and Ponnamperuma 1966). In addition, insoluble basic ferric sulfates may be reduced back to pyrite (Ivarson et al. 1982), and therefore a reduction in the water-soluble sulfate concentration is not necessarily observed in association with sulfide re-formation.
[FIGURE 3-4 OMITTED]
The water-soluble iron and aluminium concentrations (Figs 5 and 6) were negatively correlated with pH, being detected only at pH values less than ~4.0, where they became increasingly soluble. When the ASS materials were placed into an anoxic environment the rate of production of these water-soluble oxidation products, including sulfate, usually decreased considerably (Figs 4, 5, and 6). However, there was usually a minimal increase in pH and elevated concentrations of these soluble oxidation products were maintained in the highly acidic ASS materials. Inundation should therefore be more successful on neutral or alkaline ASS materials where the amount of soluble oxidation products such as sulfate, iron, and aluminium available for export will be less than for ASS materials that are already highly acidic from oxidation.
[FIGURE 5-6 OMITTED]
Factors controlling sulfide oxidation
The rate of sulfide oxidation in an ASS is largely dependent on the oxygen concentration (Dent 1986), which in turn depends on both the rate of oxygen diffusion and oxygen consumption (Bronswijk et al. 1993). The oxygen concentration is particularly important in controlling the rate of bacterially catalysed sulfide oxidation in highly acidic ASS (i.e. pH <4.0) (Ward et al. 2004a). While placing these ASS materials into an anoxic environment prevented further oxygen diffusion, any oxygen present in the materials prior to being placed into the [N.sub.2] environment would still be available for sulfide oxidation. This may explain why sulfide oxidation continued in some cases despite the prevention of further oxygen supply. However, previous research has shown that most submerged soils are practically devoid of oxygen within a few hours or days following submergence as a result of consumption by aerobic microorganisms (Ponnamperuma 1972). Any oxygen present in the materials in this study would have also been readily consumed by the process of sulfide oxidation, which is the governing oxygen consumption process in ASS materials (Bronswijk et al. 1993). The rapid consumption of oxygen has been shown by the measurement of oxygen concentration profiles for McLeods Creek B during oxidation (Ward et al. 2004b). The oxygen concentration in the partially oxidised McLeods Creek B ASS was less than the limit of detection (i.e. < 1% air saturation), with exception of the 0.2-mm-thick exposed surface after oxic incubation for 1 h. It is therefore expected that the role played by oxygen in the sulfide oxidation process would be minimal soon after being placed into a [N.sub.2] environment. The oxidation of sulfide for up to 120 days after being placed into the anoxic environment indicates oxidation by [Fe.sup.3+], which is the other major oxidant involved in the sulfide oxidation process.
Sulfide oxidation in the absence of oxygen will only continue as long as [Fe.sup.3+] is in solution (White et al. 1997), and although [Fe.sup.3+] is known to be an important pyrite oxidant over a wide pH range (e.g. Moses et al. 1987), it is not until the pH drops below ~3.5 that [Fe.sup.3+] becomes sufficiently stable in solution for rapid pyrite oxidation to take place (Evangelou 1995). Previous research has shown that while [Fe.sup.3+] may be an effective pyrite oxidant at circumneutral pH, the reaction cannot be sustained in the absence of oxygen (Moses and Herman 1991). It is also important to note that while the presence of oxygen is necessary for the regeneration of [Fe.sup.3+] from the bacterially catalysed oxidation of [Fe.sup.2+], the dissolution of iron(III) compounds upon further acidification may act as a further source of [Fe.sup.3+] for oxidation (van Breemen 1973).
While further sulfide oxidation in the absence of oxygen can be explained by [Fe.sup.3+]-mediated oxidation in the highly acidic ASS materials (i.e. pH <3.5), the McLeods Creek A ASS materials experienced sulfide oxidation (albeit at a reduced rate) when placed into the anoxic environment after 8 days of oxidation (Fig. 2a) despite having a near neutral pH (i.e. approx, pH 6.0). This continued oxidation in the McLeods Creek A ASS material could be attributed to a sufficiently low pH at the surface of some of the pyrite grains to allow further oxidation by [Fe.sup.3+] to take place; micro-niche effects are considered to be important in the initial stages of pyrite oxidation (Nordstrom 1982).
Minimal decreases in sulfide concentration were observed when both the Bungawalbin and Tuckean Swamp ASS samples were placed into the anoxic environment after 63 days of oxidation, despite both materials having a pH <3.5 (Fig. 1c, d). The sulfide content determined is a measure of the net sulfide content (i.e. resulting from both sulfide oxidation and formation), and therefore any sulfide oxidation that occurred would have been largely concealed by any simultaneous sulfide formation.
Factors controlling sulfide re-formation
The factors controlling pyrite formation are well known and include the presence of sulfate, iron-containing minerals, metabolisable organic matter, active sulfate-reducing bacteria (SRB), and anoxia alternating with limited aeration (Pons et al. 1982). While limited aeration is often required to oxidise the monosulfide to disulfide, under the conditions of this experiment, [Fe.sup.3+] may also act as the oxidising agent (Pons et al. 1982; van Breemen 1988). [H.sub.2]S may also be an important oxidising agent in the formation of pyrite under anoxic reduced conditions (Rickard and Luther 1997).
In some cases, though, the rate of sulfide formation may be extremely slow as other factors may be rate-limiting. Factors responsible for slow sulfate and iron reduction include low dissolved sulfate concentration, lack of easily reducible iron, low content of easily metabolisable organic matter, adverse effect of acidity on the activity of microbes, and soil nutrient status (Ponnamperuma et al. 1973; van Breemen 1976; Berner 1984). Soil pH may also limit the rate of pyrite formation (Wang and Morse 1995).
The soils examined here had abundant sulfate. The rate of sulfate reduction is generally only limited by sulfate concentrations less than about 5 mM (~500 mg S[O.sub.4.sup.2-]/L) (Berner 1984). An estimate of the dissolved sulfate concentration in these ASS materials was determined from the measured water-soluble sulfate concentrations and moisture contents. After 8 days of oxidation the ASS materials had dissolved sulfate concentrations of >2500 mg S[O.sub.4.sup.2-]/L, and such concentrations would not limit the rate of sulfate reduction. The measured porewater sulfate concentrations in McLeods Creek A ASS material of ~1500 mg S[O.sub.4.sup.2]-/L (van Oploo et al. 1998) also indicate abundant sulfate for sulfide formation. In addition, it is not expected that a low reducible iron content would limit the rate of sulfide re-formation in these materials, which prior to oxidation ranged between 0.54 and 1.59% (Table 1); further Feed also became available from the oxidation of sulfides (Fig. 3). The adverse effect of low pH on the activity of microbes has been suggested as a factor resulting in slow sulfate reduction. Previous research has indicated that SRB are only active in the pH range ~5.5-9.0 (Bass Becking et al. 1960; Connell and Patrick 1968). However, despite these findings, sulfate reduction is often observed in highly acidic ASS (e.g. Konsten et al. 1994), and it has been suggested that as SRB have been isolated from sites with pH 3-4, these bacteria must exist within microsites in the ASS matrix where the pH is more hospitable (Rabenhorst and James 1992). It is therefore expected that when sulfide formation is limited in the ASS materials examined in this study, it is most likely due to the lack of metabolisable organic matter, an unfavourable pH, a poor nutrient status, or a combination of these factors, rather than a lack of dissolved sulfate or easily reducible iron.
A steady increase in sulfide concentration over the time of this study was only observed with the Bungawalbin ASS material when placed into an anoxic environment following 63 days of oxidation (Fig. 2c). However, despite substantial sulfide formation, only a slight rise in pH was observed (Fig. 1c). Previous research has shown that the presence of organic matter may help to buffer soils at low pH, as it can provide a source of acidity on the strongly acidic carboxyl groups (Magdoff and Bartlett 1985). It has also been noted that ASS may have a high buffering capacity due to relatively large amounts of exchangeable and dissolved aluminium, basic sulfates of aluminium and iron, and adsorbed sulfate (van Breemen and Pons 1978). While the Bungawalbin ASS material showed substantial sulfide re-formation when placed into an anoxic environment after 63 days of oxidation, sulfide oxidation largely continued when placed into the same environment after both 8 and 126 days of oxidation. It is unclear from the data why sulfide re-formation was not observed in the Bungawalbin ASS material when placed into anoxic environments on these occasions.
The Tuckean Swamp ASS material showed a similar response with minimal change in sulfide concentration occurring when placed into an anoxic environment after 63 days of oxidation, and substantial sulfide oxidation when placed into the same environment after 8 days of oxidation. Both the Bungawalbin ASS material after 126 days of oxidation and the Tuckean Swamp ASS material after 135 days of oxidation gained considerable sulfide within 2 days of being placed in the anoxic environment. This was, however, followed by a similarly rapid period of sulfide oxidation.
McLeods Creek A and B ASS materials have relatively low contents of organic matter, and the lack of metabolisable organic matter in these samples is likely to have limited sulfide formation in the anoxic conditions.
The findings of this study have clearly shown that placing partially oxidised ASS materials into an anoxic environment, as could be gained by using management options such as flooding or watertable management, results in a substantial decrease in the rate of both sulfide oxidation and acidification. Further sulfide oxidation and acidification was generally observed when the ASS materials were placed into an oxygen-free environment regardless of the previous degree of sulfide oxidation and acidification.
Rapid sulfide formation was particularly evident with the peat ASS material, which showed a 0.47% [S.sub.CR] increase over 60 days of oxygen exclusion. The McLeods Creek ASS materials showed minimal sulfide re-formation, probably due to the lack of metabolisable organic matter. The results of this study have shown that it is difficult to predict the occurrence of sulfide formation in partially oxidised ASS materials, due to the complex interaction of the many factors involved.
The ASS materials examined in this study showed little acidity consumed as a result of oxygen exclusion. The treatment of existing acidity by neutralisation or containment would therefore need to accompany management strategies that rely on oxygen exclusion to reduce acid generation. The incorporation of lime and organic matter onto the surface of ASS months prior to flooding has been recommended to enhance the benefits of excluding oxygen (Ahern et al. 1998). While the incorporation of lime would neutralise any further acidity produced, care must be taken, as the addition of insufficient lime may accelerate sulfide oxidation with some ASS materials (Ward et al. 2002a). The addition of readily available organic matter may reduce the rate of sulfide oxidation through the consumption of oxygen; however, the movement of the oxidation products to the surface upon flooding may encourage the formation of sulfides near the surface. This phenomenon has been observed on ASS scalds in Australia (Rosicky et al. 2002). In addition it is often impractical to maintain high watertables in subtropical areas with a pronounced dry season (Wilson et al. 1999), and therefore seasonal oxidation of sulfides formed at the soil surface and consequent release of acidity is likely to occur.
This study has shown that the response of a wide variety of partially oxidised ASS materials to the exclusion of oxygen was variable. The rate of sulfide oxidation and acidification and the production of soluble oxidation products such as sulfate, iron, and aluminium usually decreased markedly when placed in the anoxic environment. However, especially in highly acidic ASS materials (i.e. pH <3.5), further sulfide oxidation and acidification occurred (albeit at much slower rates) in the absence of oxygen, most probably due to oxidation by [Fe.sup.3+]. Sulfide re-formation occurred in the peat ASS material that had been oxidised for 60 days, with 0.47% SCR formed after 60 days of being placed in anoxic conditions. This substantial sulfide re-formation was accompanied by only a slight increase in pH. Minimal sulfide formation occurred in 2 of the oxidised ASS materials when placed in anoxic conditions, most likely due to a lack of readily available organic matter. The results show that the imposition of anoxic conditions on ASS materials, whether neutral or alkaline, or acidic as a result of previous sulfide oxidation, is generally effective in greatly decreasing the rates of further sulfide oxidation, acidification, and the production of soluble sulfide oxidation products. Although sulfide formation was observed in some ASS materials when placed in the anoxic environment, the amount of sulfide formation was minimal compared with the amount of previous sulfide oxidation. Sulfide formation was a process that was ineffective in reversing acidification under the conditions of this experiment. These results indicate that the treatment of sites containing actual ASS materials by management strategies that rely on oxygen exclusion, especially in severely acidified ASS materials, needs to be accompanied by strategies that include acid neutralisation or containment in order to reduce acid export from the site.
Table 1. Characteristics of the ASS materials examined prior to oxidation Location Depth Texture pH EC SCR (m) (dS/m) (%) McLeods Creek A 1.10-1.25 Light clay 6.4 1.57 1.96 McLeods Creek B 1.90-2.40 Light clay 8.2 2.37 2.91 Bungawalbin 1.00-2.40 None (i.e. 3.8 1.96 3.83 peat) Tuckean Swamp 1.00-1.40 Light clay 3.2 3.18 5.44 Location Total C S[O.sub.4.sup.2]- [Fe.sub.CD] (%) (mg/g) (%) McLeods Creek A 1.7 3.11 1.59 McLeods Creek B 1.7 3.06 0.61 Bungawalbin 11.8 5.89 0.54 Tuckean Swamp 2.2 10.21 1.46
This research was undertaken as part of Project 1.4 'Coastal soil processes and their management for sustainable tourism development' funded by the Cooperative Research Centre (CRC) for Sustainable Tourism.
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Nicholas J. Ward (A,B), Leigh A. Sullivan (A), and Richard T. Bush (A)
(A) Centre for Acid Sulfate Soil Research, Southern Cross University, Lismore, NSW 2480, Australia.
(B) Corresponding author; email: firstname.lastname@example.org
Manuscript received 11 July 2003, accepted 3 May 2004
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|Author:||Ward, Nicholas J.; Sullivan, Leigh A.; Bush, Richard T.|
|Publication:||Australian Journal of Soil Research|
|Date:||Sep 1, 2004|
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