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This study describes iron (Fe) pipestems formed around and within root channels during the development of a homogeneous unripe sulfidic sediment into a ripe soil where redox‐ or pH‐active elements segregate. Pipestems and adjacent soil material samples were characterised by chemical and mineralogical analyses and scanning electron microscopy (SEM) associated with energy dispersive spectroscopy (EDS). Pipestems occurred at 60–190 cm and at 30–130 cm in a young Finnish soil and a more mature Australian soil, respectively, while sulfidic materials were found below 170–180 cm. The pipestems consisted of mineral grains cemented together by Fe precipitate. There was an up to 24‐fold enrichment of Fe and an up to 13‐fold enrichment of sulfur (S) in the Finnish pipestems compared to the adjacent soil material, whereas the corresponding enrichment in the Australian soil was up to 27‐fold for Fe and up to 8‐fold for S. In the Australian pipestem matrices, the Fe concentration was as high as 40% compared to 14% in the Finnish ones. It was estimated that about 70 ton ha−1 S had been mobilised from the sulfidic material in the Finnish soil at the depth of 50–150 cm. Part of S has leached out but a substantial amount remained in the soil constituting the stock of retained acidity. Almost all pipestems contained a new solid phase precipitated within the former cortex cells of the plant roots. In the Finnish samples, this precipitate consisted of jarosite and schwertmannite. After oxidative exhaustion of sulfidic material in the surrounding soil, these metastable minerals are gradually hydrolysed, associated with leaching of S. In the mature Australian soil, most of these minerals had already been transformed to goethite. Pipestems formed after roots in unripe soil are sites for synthesis and hydrolysis of minerals and serve as routes for atmospheric oxygen into the reduced subsoil and for soluble reaction products to exit the soil. Pipestems have an important role in the ripening of the soil profile. [ABSTRACT FROM AUTHOR]

In Australia, the assessment of acidity hazard in acid sulfate soils requires the estimation of operationally defined acidity fractions such as actual acidity, potential sulfidic acidity, and retained acidity. Acid--base accounting approaches in Australia use these acidity fractions to estimate the net acidity of acid sulfate soils materials. Retained acidity is the acidity stored in the secondary Fe/Al hydroxy sulfate minerals, such as jarosite, natrojarosite, schwertmannite, and basaluminite. Retained acidity is usually measured as either net acid-soluble sulfur ([S.sub.NAS]) or residual acid soluble sulfur ([S.sub.RAS]). In the present study, contributions of schwertmannite and jarosite to the retained acidity, actual acidity, and potential sulfidic acidity fractions were systematically evaluated using [S.sub.NAS] and [S.sub.RAS] techniques. The data show that schwertmannite contributed considerably to the actual acidity fraction and that it does not contribute solely to the retained acidity fraction as has been previously conceptualised. As a consequence, [S.sub.NAS] values greatly underestimated the schwertmannite content. For soil samples in which jarosite is the only mineral present, a better estimate of the addedjarosite content can be obtained by using a correction factor of 2 to [S.sub.NAS] values to account for the observed 50-60% recovery. Further work on a broader range of jarosite samples is needed to determine whether this correction factor has broad applicability. The [S.sub.RAS] was unable to reliably quantify either the schwertmannite or the jarosite content and, therefore, is not suitable for quantification of the retained acidity fraction. Potential sulfidic acidity in acid sulfate soils is conceptually derived from reduced inorganic sulfur minerals and has been estimated by the peroxide oxidation approach, which is used to derive the [S.sub.RAS] values. However, both schwertmannite and jarosite contributed to the peroxide-oxidisable sulfur fraction, implying a major potential interference by those two minerals to the determination of potential sulfidic acidity in acid sulfate soils through the peroxide oxidation approach., Introduction Over 500000 [km.sup.2] of acid sulfate soils (ASS) are found globally, mainly in highly populated, coastal plains (Sullivan et al. 2012). In Australia, nearly 215 000 [km.sup.2] of land [...]

Two-metre-deep soil profiles at 10 acid sulfate soil (ASS) scalds along the coast of New South Wales (NSW), Australia, were examined for salinity indicators. At 5 of the sites, permanently vegetated areas adjacent to the ASS-scalded land were also tested. Throughout the profiles, most sites had high soluble chloride ([Cl.sup.-]) concentrations (≤ 17 mg/g soil) and high soluble sulfate (S[O.sub.4.sup.2-]) concentrations (≤ 17 mg/g soil). Very low [Cl.sup.-] : S[O.sub.4.sup.2-] ratios (≤ 3) indicated active pyrite oxidation. Soil salinity (measured as electrical conductivity, EC) was extremely high in the top 2 m of most of the ASS scalds when related to the growth requirements of the typical introduced pasture species that were planted in these areas following drainage. This allows salinity, in addition to the extremely low pH of the surface soils, to contribute to land denudation, which can instigate or perpetuate pyrite oxidation and ASS-related land scalding. Although the sites had shallow watertables and soil-moisture content was high, the surface soil (top 0.10 m) of the scalds had consistently higher soluble [Cl.sup.-] and S[O.sub.4.sup.2-] concentrations and EC than adjacent vegetated areas. All coastal ASS areas investigated, typically freshwater backswamps used for cattle grazing, were underlain by estuarine-derived sediments containing saline ground water. The results demonstrate that revegetation of ASS scalds must include investigation and management of salinity, in addition to acidity, within the soil profile and at the soil surface. Additional keywords: Write oxidation, acid sulfate soil scald, acid sulfate soil salinity, backswamp management, backswamp drainage., Introduction The coastal floodplains in New South Wales (NSW), Australia are largely infilled barrier estuaries (Roy 1984). Extensive estuarine deposits, containing pyrite, were laid down approximately 10 000-6000 years ago [...]

This study provides a systematic analysis of the effect of common acid sulfate soil (ASS) sample pre-treatments (namely freezing, oven drying, and grinding) on chromium-reducible sulfur ([S.sub.CR]) and water-soluble sulfate determinations. The results show that oven drying and hand grinding of the samples prior to analysis resulted in a decrease in [S.sub.CR] (i.e. up to 20% compared to those of frozen samples). This lower [S.sub.CR] value was partly due to the oxidation of sulfides in the oven. For oven-dried ASS materials, more intensive grinding in a ring mill increased [S.sub.CR] values, most likely by abrading coatings from pyrite grains. For oven-dried mineral ASS materials the highest [S.sub.CR] values were obtained with 1 min of ring mill grinding, but for soils with appreciable organic matter (such as peat), 5 min of ring mill grinding gave the highest values. The results indicate that for some ASS materials, oven drying, regardless of the ensuing grinding procedure, results in underestimated [S.sub.CR] values. This study also demonstrates that an artifact of oven drying ASS materials can be greatly increased water-soluble sulfate contents. Additional keywords: chromium-reducible sulfur, water-soluble sulfate, grinding, freezing, drying, reduced inorganic sulfur., Introduction Acid sulfate soils (ASS) are a prominent feature of many low-lying coastal areas of Australia and around the world, with estimates of 20 million hectares of ASS worldwide (Dent [...]

Soil profiles in 10 persistently bare areas (i.e. scalds), mainly located in coastal backswamps of New South Wales, Australia, were examined for chromium-reducible sulfur content and selected chemical properties. At 5 of the sites, the adjacent paddocks with vegetation cover were also examined. All of the tested sites had been affected by the extensive drainage of the surrounding acid sulfate soil (ASS) landscapes and the consequent oxidation of pyrite. All sites had low pH values in the surface soil layers and these low pH values extended for up to 150 cm into the underlying unoxidised blue/grey pyritic estuarine gels. This can be attributed to the downward diffusion of acidity, either produced in the overlying oxidised zones of these soils or transported laterally across the landscape to these low-lying areas. Acidified unoxidised pyritic zones 120 cm thick can evidently form within several decades after drainage disturbance. At the scalded sites the depth from the soil surface to the main pyritic zone varied from the surface to >200 cm depth, indicating that this variable is not critical to ASS scald formation. For most of the sites examined, the chromium-reducible sulfur contents in the surface soil layers were appreciably higher than those in the immediately underlying soil layers. In most of the vegetated sites the chromium-reducible sulfur content in the surface layers was considerably higher than for the adjacent scalded site. The conditions necessary for pyrite formation (i.e. adequate supplies of organic matter, soluble iron, sulfate, and waterlogging) were found to exist at all sites, and the pyrite accumulations in these surface soil layers are considered to be neo-formed. The vegetated soil-profile pyrite and pH results were very similar to their scalded counterparts except that they had an extra 20-40 cm layer of vegetation and mulch that was missing from the scalded profiles. This indicates that there is considerable potential for more extensive scalding in these ASS areas. Additional keywords: subsurface pyrite, surface pyrite, pyrite oxidation, pyrite re-formation, acid sulfate soil pH, backswamp management, backswamp drainage., Introduction Acid sulfate soil (ASS) scalds are persistently bare areas of land associated with pyrite oxidation and ASS formation in drained coastal floodplains. In Australia, ASS scalds have not previously [...]

Substantial marcasite and pyrite were recently identified in the upper-most unoxidised Holocene clay-peat sediments at Bungawalbin Swamp, a coastal backswamp on the Richmond River floodplain, north-east New South Wales, Australia. Marcasite (Fe[S.sub.2]), the mineral dimorph of pyrite, is considered a rare secondary mineral in estuarine sediments and its abundance at Bungawalbin Swamp is highly unusual. The morphology and distribution of marcasite in the sulfidic sedimentary profile was examined and compared with the palynological record. Marcasite, recognised by its distinctive platy morphology and disulfide composition, occurred exclusively within organic remnants in only the upper most 1 m of the sulfidic sediment layer. Pyrite occurred throughout the sulfidic profile. A dramatic change in the sedimentary conditions at Bungawalbin Swamp from a marine environment, characterised by the presence of avicenniaceae (i.e. mangrove), to a fresh-brackish environment correlates directly with the occurrence of marcasite. The occurrence of substantial marcasite may provide a valuable environmental proxy of fresh-brackish depositional swamp environments. Additional keywords: iron sulfide, pyrite, acid sulfate soil, sulfidic., Introduction Marcasite is the mineral dimorph of pyrite, sharing the same chemical composition of Fe[S.sub.2]. Diagenic pyrite is common in marine and estuarine sediments (Berner 1970), whereas diagenic marcasite has [...]

The observations presented in this paper illustrate that significant amounts of monosulfidic black oozes (MBO) were eroded from flood mitigation drainage canals and redistributed across a coastal floodplain during a flood event associated with extreme deoxygenation and a massive fish kill. MBO are organic materials enriched in iron monosulfides and thick layers can accumulate in drains affected by acid sulfate soils. Laboratory studies have demonstrated that MBO can react rapidly when brought into suspension to completely consume dissolved oxygen. The abundance of MBO in flood mitigation drains and their extreme reactivity implicated MBO in the acute deoxygenation of the Richmond River, north-eastern New South Wales, Australia, following a major flood in February 2001. The field observations of MBO redistribution provide valuable evidence to help explain how these materials may interact and contribute to the deoxygenation of floodwaters. Additional keywords: iron sulfides, pyrite, monosulfides, acid volatile sulfur, deoxygenation, MBO., Introduction Monosulfidic black oozes (MBO) are organic materials enriched in iron monosulfides. Thick layers of MBO accumulate in many canals that drain acid sulfate soil (ASS) landscapes (Bush et al. [...]

Organic oozes enriched in iron monosulfides are called monosulfidic black ooze (MBO). The occurrence and abundance of MBO were quantified in natural tributaries and man-made drainage canals on a typical coastal river floodplain. Thick deposits of MBO occurred in drains and the protected upper reaches of tributaries in acid sulfate soil (ASS) areas. Acid-tolerant lilies grew prolifically in these areas, contributing large amounts of decaying organic debris. The MBO contained up to 6.3% acid-volatile sulfur (equivalent to 18% iron monosulfide), an order of magnitude greater than reported previously for natural sediments. It also contained up to 6% pyritic sulfur (equivalent to approximately 11% pyrite). Sulfate and iron from surrounding ASS combined with abundant organic matter and protected flow in ASS drains provide excellent conditions for sulfate reduction, leading to iron sulfide precipitation and the accumulation of organic detritus. When mixed with water, the iron monosulfide in MBO can react within minutes to completely consume dissolved oxygen. There are thousands of drains in ASS areas that may have thick deposits of MBO. The MBO has the potential to cause rapid and severe effects on water quality. The challenge is to develop management practices that reduce the formation and accumulation of MBO in acid sulfate landscapes. Additional keywords: pyrite, iron sulfides, de-oxygenation, water quality., Introduction Organic oozes enriched in iron monosulfides are called monosulfidic black ooze (MBO). The reduction of organic matter by sulfate-reducing bacteria in these sediments produces hydrogen sulfide (H2S), which reacts [...]

Acid sulfate soil (ASS) scalds are persistently bare areas of land, occurring in the coastal backswamps of New South Wales (NSW), Australia. This study aims to understand why particular areas become ASS scalds, while adjacent areas remain vegetated. Some important soil parameters are compared and field observations are summarised. Soil core sampling in both ASS-scalded land and surrounding areas of permanently vegetated paddocks has demonstrated similar pyrite concentrations and depth occurrence, soil salinity, and soil acidity (pH). As conditions are similar beneath both vegetated and non-vegetated land, there must be some additional factors influencing which areas become denuded. Several disparate (usually human-induced) events were found to cause initial loss of vegetative cover. Once the soil is bare, surface evaporation causes toxic solutes to build up quickly at the soil surface and ASS scalding is perpetuated. Some of the intervening events include fire, flood, flood-scouting, deliberate topsoil removal, surface pyrite oxidation, saltwater inundation of freshwater paddocks, saltwater exclusion from saltmarsh or mangroves, changes to the vegetation regimes, excessive vehicular traffic, and over-grazing. Backswamp management needs to ensure that land underlain by shallow pyritic layers (or with soil-water that is enriched with the toxic by-products of pyrite oxidation) is not laid bare by accident or design. Similar soil chemical conditions underlying both ASS scalds and the surrounding permanently vegetated paddocks suggest that much larger areas are potentially at risk of ASS scalding. Additional keywords: pyrite oxidation, acid sulfate soil acidity, acid sulfate soil salinity, surface pyrite, subsurface pyrite, backswamp management, backswamp drainage., Introduction Coastal floodplains in New South Wales (NSW), Australia, are underlain by formerly estuarine sediments (Roy 1984). These estuary basin and intertidal muds contain large amounts of sulfides (mainly pyrite) [...]

Four acid sulfate soil (ASS) materials were subjected to anoxia after varying periods of oxidation to determine the geochemical response of these types of soils to flooding. The response of the partially oxidised ASS materials to the exclusion of oxygen was variable. The rate of sulfide oxidation, acidification, and the production of soluble oxidation products such as sulfate, iron, and aluminium generally decreased markedly when subjected to anoxia. However, especially in the highly acidic ASS materials (i.e. pH Additional keywords: soil acidification, pyrite oxidation, pyrite re-formation, chromium reducible sulfur, reduction, soil incubation., Introduction 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. [...]

The potential environmental hazard of acid sulfate soil (ASS) materials is directly related to both the net acidity and the rate that actual acidity is released from these soil materials into the environment. While current environmental hazard assessment techniques for ASS materials are able to quantify the net acidity, they do not take account of differences in the rate of sulfide oxidation (the dominant source of actual acidity) and differences in the rate of acidification. In this study the rate of sulfide oxidation during incubation was examined for 4 ASS materials. The effect of pH and oxygen availability on the rate of sulfide oxidation was assessed. The ASS materials were incubated in: (i) gauze where oxygen diffusion was not restricted, and (ii) sealed 100-µm-thick plastic bags which greatly limited oxygen diffusion. When oxygen diffusion was not restricted, an accelerated oxidation of sulfide occurred when the pH decreased below pH 4.0. The accelerated rate of sulfide oxidation at such low pH did not occur when oxygen diffusion was limited. This study indicates that the initial pH of an ASS material is a useful additional indicator of the potential environmental hazard of an ASS material when oxygen is expected to be non-limiting, such as when ASS materials are excavated and stockpiled. The recommended action criteria need to be reassessed, as the data indicate that the current criteria are conservative for alkaline and neutral ASS materials, but should be lowered for all acidic ASS materials (i.e. pH Additional keywords: pyrite oxidation, acidification, chromium reducible sulfur, soil incubation., Introduction The potential environmental hazard associated with acid sulfate soil (ASS) materials often results from the interaction between the net acidity, largely derived from sulfide oxidation, and the rate this [...]