Factors contributing to the acid sulfate soil scalding process in the coastal floodplains of New South Wales, Australia.
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) (Hashimoto and Roy 1996) and are often called 'potential acid sulfate soil' (PASS) (White and Melville 1993). PASS was laid down during recent sea level rise, which stabilised about 6000 years ago (Roy 1984). After that, the estuarine deposits were covered by various thicknesses of alluvial deposits, ranging from several metres deep at the main river levees to a very thin covering of fine-grained sediments in sheltered areas at distance from the rivers (Walker 1963; Ahern et al. 1998). The sheltered areas became backswamps that are typically waterlogged and covered by thick layers of peat.
Coastal floodplains in NSW have been drained for over 100 years, mainly for agricultural purposes (Robertson et al. 1998). Drainage has allowed air to penetrate many backswamp soil profiles, causing peat loss, shrinkage and drying (Wosten et al. 1997). It has also caused oxidation of PASS, and leads to the formation of actual acid sulfate soil (commonly referred to as ASS). It is now well understood that ASS formation contributes to fish kills and other negative environmental impacts along the NSW coast (Willett et al. 1992; Sammut et al. 1995; White and Sammut 1995).
Soil-water and leachate associated with ASS formation are known to contain toxic amounts of soluble sulfuric acid, and hydrolysable iron and aluminium (Hyne and Wilson 1997; Lawrie and Blunden 2000). Bare surfaces can be caused when the acid-buffering capacity of overlying soil is exceeded and phytotoxic amounts of acidity and soluble aluminium concentrate in the upper soil horizon (Kargbo et al. 1993; Mulvey 1993), killing off and chronically excluding vegetation. Salt crusts often form on these bare areas in dry periods. This phenomenon has been recognised in several countries around the world, including Senegal (Sadio and van Mensvoort 1992), Niger (Ducloux et al. 1994), and Guinea-Bissau (Fanning and Burch 2000) in Africa; and the USA (Keller et al. 1986). In Australia, these ASS scalds commonly occur in the NSW coastal floodplains (Rosicky et al. 2000).
In NSW, ASS scalds have been examined and sampled from the Richmond catchment in the north, to the Shoalhaven catchment in the south. Soil cores were collected from 13 ASS scalds along the NSW coast. All of the ASS scalds had pH values at or below 4 in the top 1 m of the soil profile. They had significant pyritic zones within 1 m of the soil surface, which were actively oxidising and producing acidity and salinity. The ASS scalds were also found to have extremely high levels of soluble and exchangeable acidity and aluminium (Rosicky et al. 2002). The scalds were all in areas influenced by deep (> 2 m) drainage, yet usually had shallow watertables (< 50 cm) during a time of slightly below-average rainfall during the previous 12 months (Rosicky et al. 2002). The purpose of this current study is to understand why particular areas become scalds while adjacent land remains vegetated.
Cores examined in this investigation were taken as part of a larger sampling exercise designed to characterise ASS scalds along the NSW coast (Rosicky et al. 2002). In this study, a scald/adjacent vegetated paddock pair from each of 4 catchments are compared: the Macleay (Site 1), Manning (Site 2), Hawkesbury (Site 3), and Shoalhaven (Site 4) catchments (see fig. 1, Rosicky et al. 2004). The suffixes S (scald) and V (vegetated) will henceforth be used after relevant site numbers to distinguish between different cores at the same site (e.g. 4S and 4V). Duplicate 2-m-deep cores, about 50 cm apart, were taken with a tapered gouge auger, in both the scalds and the adjacent vegetated areas. Cores were taken about 15-20 m into areas identified (with the help of government officers or landowners) as permanent scald or permanent vegetation. The extracted cores were shaved to avoid contamination, and individual samples were placed into plastic bags (0-10 cm, 10 20 cm, then every 20 cm). Air was manually expelled and the samples were stored on ice for transport. Samples were dried to 70[degrees]C. Soil properties pertinent to this study included pH and salinity (electrical conductivity, EC) from 1 : 5 soil-water suspensions, and reduced inorganic sulfur content (present mainly as pyrite) using the chromium-reducible sulfur ([S.sub.CR]) method of Sullivan et al. (1998). Duplicate core samples were tested separately to verify sampling consistency. Site locations were identified on 1 : 25 000 CMA topographical map sheets. Site descriptions, land use, and watertable depths were noted in the field. Landowners and government officers were interviewed regarding their observations and ideas about the dynamics of their ASS scald.
[FIGURE 1 OMITTED]
Table 1 contains general locational and soil core information for the sites, including depth to watertables and pyrite zones (relative to the soil surface), surface pH, and surface pyrite and EC concentrations. Table 1 indicates the location of scalds in backswamps, at distance from the main river (see 'Ratio a : b'), and shows that surface acidity and salinity were generally higher in the scalds. Soil texture was found to be similar in scalded and vegetated cores at each site, except that a surface organic layer up to 30 cm thick was present in the vegetated cores and absent in the scalded cores. In each case, the cores consisted of dark blue or grey gel-like pyritic material overlain by fine- to medium-textured soil. Surface levels of the scalds were consistently lower than their surrounding vegetated areas. This elevation difference was observed at the scald-paddock interface as a step up from bare soil to vegetated paddock and was typically measured at 15-30 cm.
Comparisons of reduced inorganic sulfur (pyrite) concentrations in the top 2 m of the 4 paired scalded and adjacent non-scalded cores are shown in Fig. 1. The %[S.sub.CR] profiles of the vegetated cores showed essentially the same pattern as their scalded counterparts, although the corresponding results were always offset 20-40 cm further from the surface in the vegetated core profiles. The limit used to distinguish environmentally significant concentrations of [S.sub.CR] in medium- and fine-grained materials is 0.06% and 0.1%, respectively (Ahern et al. 1998). The scalded cores exceeded this limit at 50 cm depth at Sites 1, 2 and 4, and just below the soil surface at Site 3. At each site the surface soil samples from both scalded and vegetated cores exhibited a zone of increased pyrite concentration in the top 10-20 cm (Table 1).
Figure 2 shows similar patterns for the pH profiles in paired scalded and vegetated cores. Both scalded and vegetated cores had pH [less than or equal to] 4 in the top I m of the soil profiles. At Sites 1, 2, and 3 the pH was lower in the surface soil of the scalded cores than the vegetated cores, whereas Site 4 had identical pH values for both cores in the top 70 cm, including the surface layer (Table 1). The pH increased with depth in all cores. The paired cores showed similar pH results, offset in vegetated cores to slightly deeper levels.
[FIGURE 2 OMITTED]
Salinity ([EC.sub.1:5]) profiles in the paired cores again showed similar trends in both scalded and vegetated cores, particularly in the 1-2-m zone (Fig. 3). Three cores had [EC.sub.1:5] values of about 1-2 dS/m down to 2 m depth. Site 3 had much higher values, increasing to around 9-10 dS/m at 2 m depth. There was consistently less salinity in the top 1 m of the vegetated cores, especially in the top 50 cm. All scalded sites had higher surface-soil (top 10 cm) salinity concentrations than their vegetated counterparts (Table 1).
[FIGURE 3 OMITTED]
During field trips between late 1998 and early 2000, landowners and government officers were questioned on their knowledge concerning ASS scalds. The responses of these land managers and observations made by the authors have resulted in the identification of a range of factors that appear to have contributed to the ASS scalding process. In many cases, land was laid bare by some particular event or intervening factor and then remained chronically denuded. Some of these events or factors included fire, frost, prolonged inundation from major floods, flood scouring, deliberate topsoil removal, livestock trampling, excessive vehicular traffic, saline water intrusion into freshwater areas, saline water exclusion from saltmarsh and mangrove areas, dryland vegetation inundation, and wetland vegetation desiccation.
The comparisons of pyrite profiles ([S.sub.CR]) show that similar concentrations are found in scalded and adjacent paddock cores (Fig. 1). This is expected because all ASS scalds are within larger areas underlain by pyritic estuarine sediments. The consistent difference in depth occurrence for similar pyrite concentrations reflects the difference in surface elevation between scalded and adjacent vegetated surfaces. ASS scalds were visibly and consistently lower than the surrounding vegetated land due to a combination of factors (Fig. 4). ASS-scalded surfaces are susceptible to ongoing wind and water erosion. Scalded surfaces also lack the thick layer of mulch and living vegetation found on vegetated paddock surfaces, which can build up the surface layer over time and also trap wind- and water-borne sediment. The [S.sub.CR] data reflect this 20-40-cm difference; paddock cores were taken from a slightly higher elevation. Drainage affects a wide area and sulfidic zones are the same under both ASS scalds and adjacent vegetated paddock. There must be factors other than deep drainage and close proximity of the pyrite zone to the soil surface that influence which areas scald and which areas remain vegetated.
[FIGURE 4 OMITTED]
A smaller surface pyrite layer, found in each core examined for this study, is routinely present in both ASS scalds and surrounding paddocks examined along the NSW coast (Rosicky et al. 2004). Soluble sulfate and iron are being transported to the soil surface from underlying pyrite oxidation by capillary action of the shallow underlying watertables. There is often a greater concentration of pyrite in the surface layer under the adjacent vegetation (Table 1), probably because of increased availability of organic matter. Small zones of dead vegetation in paddocks on the periphery of the scalds can appear and quickly expand with the onset of dry weather conditions. These are most likely caused by oxidation of this surface pyrite. When air enters the surface-soil horizon, the surface pyrite can oxidise and release soluble acidity and other toxic solutes such as soluble iron and aluminium directly in the root-zone of paddock vegetation. This would contribute to vegetation decline.
The pH values of the soil cores should be an important determinant of plant health and survival. Below pH 4, major nutrients and trace elements are unavailable, soluble metals reach toxic concentrations, and both ammonium and hydrogen ions are detrimental to plant growth (Rorison 1972; Fenton and Helyar 2001). However, the paired scalded and vegetated cores in Fig. 2 show very similar, highly acidic pH values in the surface layers. The similar pH values in the soil profiles beneath both scalded and vegetated cores suggest that oxidation has occurred across a larger area than just below the bare scalded surface. This indicates that preferential subsurface acidification is not the primary cause of ASS scalding. Indeed, the pH differences at the surface of paired scalded and vegetated profiles are neither large nor consistent enough to explain the complete loss of vegetation in the scald and the persistence of vegetation nearby.
Salinity also has a major influence on plant growth and survival. Soils with plant-available salinity, or the EC of a saturated soil extract (ECe), > 4 dS/m are considered saline (Le Houerou 1993) and most agricultural crops and pastures require ECe < 10 dS/m (Maas 1993). The [EC.sub.1:5] can be converted into ECe using a conversion factor that relates to soil texture (Slavich and Petterson 1993). The conversion applicable to the soils examined ranges between 5.8 for heavy clay and 9.5 for loam. When these factors are applied to the surface [EC.sub.1:5] results in Table 1 and Fig. 3, it is clear that both surface and profile salinity are important limiting factors in freshwater plant growth and survival (particularly in the scalds). Whereas subsoil salinity is similar in both scalded and vegetated cores at all sites, and cannot be given as the primary cause of ASS scalding, surface salinity does vary more widely. The mulching effect of vegetation inhibits salt accumulation on paddock surfaces. The smaller differences at Site 1 and 4 were caused by prolonged inundation and consistent rain, respectively (observed by corresponding author). Salt crusts are common on the surface of ASS scalds in dry weather. However, this is more an effect of ASS scalding that would be intermittent between rainfall events, rather than an initial cause of vegetation loss.
The role of secondary factors
There were repeated observations of human-induced events that initially laid an area bare, leading to the development of ASS scalds. These observations give some insight into contributing factors that may instigate the process of surface deposition of toxic solutes. Drainage and pyrite oxidation have increased the concentration of toxic solutes in the soil-water. If surface soil is laid bare in areas underlain by drained and oxidising pyritic sediments, preferential evaporation from the bare surface during dry periods quickly concentrates acidity and salinity in the surface soil (e.g. Figs 2, 3), making plant germination and survival difficult. ASS-related conditions would quickly dominate, chronically discouraging vegetation. The observations made by landowners, government officers, and the authors are summarised into a number of factors that initially result in bare soil (Fig. 5).
[FIGURE 5 OMITTED]
Changes to hydrological regimes and associated vegetation dynamics
Most backswamps have been drained to facilitate grazing. Introduced dryland pasture grasses were planted, and these do not tolerate prolonged inundation. Efficient drainage is usually designed to clear surface water from paddocks in 5 days (Williams and Copeland 1996) rather than over several months as occurs in the absence of artificial drainage. However, very low areas (0-1 m AHD) are prone to inundation and waterlogging despite the presence of deep drains, and surface water can stand for many months. Introduced pasture species, which have grown well in drier times, cannot cope with waterlogging and die off from the lower-lying areas and shallow drains. If waterlogged conditions persist, wetland species often germinate and establish. When weather conditions become drier, wetland vegetation dies off, leaving surfaces devoid of vegetation again. Our observations indicate that major prolonged flooding can have a similar effect to that mentioned above, but over larger areas, killing introduced paddock vegetation and even water-tolerant species.
Changes to salinity regimes
Some ASS scalds occur where tidal estuarine water has been excluded from saltmarsh or mangroves, and freshwater regimes have been imposed. When these areas are moist and flushed with fresh water, introduced pasture grasses can thrive. As surface soils dry out, residual seawater salinity reaches concentrations that the introduced pasture grasses cannot tolerate. Conversely, deep drainage in low, flat, coastal floodplains allows the ingress of saline water to freshwater environments. Formerly waterlogged backswamps with thick peat layers are now at a lower elevation because of desiccation, peat loss, soil shrinkage, and erosion. They can be directly connected to the estuarine waters by deep drains. If a 1-way flap-gate is jammed open, saline water can cover low backswamp paddocks and has been observed to result in the death of vegetation adapted to freshwater conditions.
Loss of surface soil
Many different factors can lead to the loss of surface soil. Machinery damage and surface erosion, deliberate topsoil removal for use elsewhere, flood-scouring, and peat fires were some examples given in the observations. These events not only leave the soil bare, but also decrease the distance between the soil surface and the underlying pyrite layer.
Surface pyrite oxidation
Pyrite forms on the surface of backswamp ASS scalds and surrounding paddocks in wet times (Rosicky et al. 2004). Surface pyrite oxidation can contribute to vegetation mortality by the production of toxic solutes within the root-zone during prolonged dry periods.
Behaviour of livestock
Unconstrained access of livestock can also delay or prevent revegetation on ASS scalds. In wet weather, livestock cause soil poaching and pugging of bare ground. Trampling and selective foraging of newly emergent seedlings also delays or prevents revegetation (Fig. 6).
[FIGURE 6 OMITTED]
Any factor that creates bare soil surfaces can cause ASS scalds in land underlain by a shallow, oxidising pyrite zone. Fires can burn off surface vegetation and leave bare ground. Excessive vehicular traffic can chronically denude an area. Severe frost events have been observed to kill paddock vegetation in the ASS landscapes of NSW (L. A. Sullivan, unpublished data). If weather conditions allow frost, followed by wind erosion (or fire) and on-going dry soil-moisture conditions, then revegetation becomes difficult as surface conditions become increasingly hostile.
This study has found that ASS scald formation is a result of a combination of direct and indirect factors, and suggests that ASS scald prevention and management needs to include practices that prevent prolonged vegetation denudation. Maintenance of higher watertables would keep pyrite layers in a reduced state for longer periods, inhibiting the constant production and delivery of ASS oxidation products to the surface. Fires would also be discouraged by wetter management regimes. Surface pyrite formation needs to be monitored and managed, by disallowing permanent surface inundation and employing the strategic use of lime. Stocking rates need to reflect the changing conditions in the backswamps, which would not be capable of constant production rates in wet v. dry conditions. Existing ASS scalds need to be fenced off from livestock. Inappropriate paddock grass species should be phased out in preference for grasses that can cope with waterlogging and generally wetter conditions. Machinery operation requires monitoring so as to prevent the creation of bare areas. Surface-soil flood scouring and topsoil removal needs to be prevented. Ploughing must be undertaken with caution and only when part of an active and ongoing management plan to re-grow suitable vegetative cover.
Table 1. Locational and soil core information for 4 NSW acid sulfate soil scalds S, Acid sulfate soil scalded; V, vegetated Site 1S 1V 2S 2V 3S 3V Catchment Macleay Manning Hawkesbury Nearest major Kempsey Taree Gosford town Distance to 1.50 3.75 0.25 foothill (km) (a) Distance to 6.75 3.75 4.75 main river (km) (b) Ratio a : b 0.22:1 1:1 0.05:1 Distance to 1.25 0.25 0.15 main drain (km) Size of scald 200.0 0.5 4.0 (ha) Depth to pyrite 70 110 70 110 15 layer (cm) Water level in +5 -20 -35 -70 -87 core hole (cm) (A) Surface pyrite 0.03 0.11 0.06 0.08 0.04 (%SCR) Surface pH (top 3.7 4.1 3.4 4.3 2.5 10 cm) Surface FC 0.97 0.03 2.67 0.13 8.19 (dS/m), (top 10 cm) Site 4S 4V Catchment Shoalhaven Nearest major Nowra town Distance to 0.75 foothill (km) (a) Distance to 4.12 main river (km) (b) Ratio a : b 0.16:1 Distance to 0.15 main drain (km) Size of scald 2.0 (ha) Depth to pyrite 70 90 layer (cm) Water level in -97 -110 core hole (cm) (A) Surface pyrite 0.02 0.04 (%SCR) Surface pH (top 3.3 3.3 10 cm) Surface FC 1.97 1.13 (dS/m), (top 10 cm) (A) Positive value indicates standing water; depths measured 20-30 min after core taken.
This research was funded by the Acid Sulfate Soil Program (ASSPRO), a NSW government initiative, and administered by the Acid Sulfate Soil Management Action Committee (ASSMAC). It forms part of a PhD research project, jointly supervised by Southern Cross University, Lismore, and Agriculture NSW, Wollongbar.
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Mark A. Rosicky (A,C) Leigh A. Sullivan (A), Peter G. Slavich (B), and Mike Hughes (B)
(A) Centre for Acid Sulfate Soil Research, Southern Cross University, Lismore, NSW 2480, Australia.
(B) New South Wales Agriculture, Wollongbar Agricultural Institute, Bruxner Highway, Wollongbar, NSW 2477, Australia.
(C) Corresponding author; email: firstname.lastname@example.org
Manuscript received 16 May 2003, accepted 10 May 2004
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|Author:||Rosicky, Mark A.; Sullivan, Leigh A.; Slavich, Peter G.; Hughes, Mike|
|Publication:||Australian Journal of Soil Research|
|Date:||Sep 1, 2004|
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