INLAND ACID SULFATE SOIL SYSTEMS ACROSS AUSTRALIA

Hicks Warren and Rob Fitzpatrick. 2008. Greenhouse Emissions and Toxic Gas Emissions from Soil Organic Matter and Carbonates Associated with Acid Sulfate Soils. In Inland Acid Sulfate Soil Systems Across Australia (Eds. Rob. Fitzpatrick and Paul Shand). pp 137-148. CRC LEME Open File Report No. 249. (Thematic Volume) CRC LEME, Perth, Australia. CHAPTER 6

GREENHOUSE EMISSIONS AND TOXIC GAS EMISSIONS FROM SOIL ORGANIC MATTER AND CARBONATES ASSOCIATED WITH ACID SULFATE SOILS Warren Hicks1,3 and Rob Fitzpatrick2,3

1 CSIRO Land and Water, GPO Box 1666, Canberra ACT 2601 2 CSIRO Land and Water, Private Bag 2, Glen Osmond SA 5064 3 Cooperative Research Centre for Landscape Environment and Mineral Exploration (CRC LEME) GREENHOUSE EMISSIONS Soil organic matter in wetlands In their undisturbed state wetland soils are natural accumulators of carbon and sulfur. Jickells and Rae (1997) reported work by Wollast (1991) indicating that shelf sediments appear to be major sinks of organic carbon on a global scale. This review cites further work by Twilley et al (1992) in which wetlands are estimated to contain one third of continental shelf carbon with the bulk of this storage in mangrove systems. Chmura et al (2003) combined the literature data from tidal saline wetlands at 154 sites to produce a global data set of soil carbon density (i.e. concentration expressed as weight/volume). They found an average carbon concentration for mangrove swamps (5.5±0.04 % w/v) and an average salt marsh carbon concentration (3.9±0.3 % w/v). The formation and accumulation of pyrite to form an acid sulfate soil first requires the reduction of sulfate by bacteria. This reaction needs organic carbon to proceed and can be represented by the simple equation (Berner 1984): 2CH2O + SO4

2– → H2S + 2HCO3–

Pyrite formation is thus accompanied by the consumption of metabolisable carbon and the accumulation of residual organic carbon. Berner (1984) described the observed relationship between organic carbon and reduced sulfur in sediments and Morse and Berner (1995) showed that the relationship between this residual carbon and reduced sulfur in sediments could be represented by the equation:

where R is the mole ratio of (residual buried) organic carbon to total reduced sulfur, ¶M is the fraction of the total organic carbon deposited in the sediment metabolised, ¶CS is the fraction of metabolised carbon utilised for sulfate reduction and ¶SP is the fraction of reduced sulfur buried. In normal (non-euxinic) marine sediments the C:S mole ratio has been found to be 7.5 (±2.1) (Morse and Berner 1995). These ratios are routinely used to distinguish between marine and freshwater paleoenvironments. This buried residual carbon forms a pool of that has been removed from the active carbon cycle. Disturbance of ASS can return this carbon to the active carbon pool as carbon dioxide either through microbial oxidation or by burning. It may also be remobilised in waterways as dissolved and suspended particulate organic carbon.

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Hicks et al (1999) conducted a study of coastal wetlands drained for agricultural development in the 1970’s and found considerable loss of soil carbon. Two soils were examined, a Typic Sulfihemist and a Typic Sulfaquent. The average carbon concentrations to a depth of 1 m were 5.8% and 2.9% w/v respectively. Drainage of the Sulfihemist resulted in subsidence of more than 1 m and a profile carbon loss of 710 t C ha–1 and for the Sulfaquent, 0.29 m subsidence and a profile carbon loss of 220 t C ha–1. These represent a significant loss of soil carbon. The processes leading to loss and remobilisation are likely to be repeated in disturbance of any wetland where carbon has accumulated. Profile data, changes following drainage and processes for the Typic Sulfihemist are summarised in Figure 1.

Figure 1. Soil profile data and processes for the disturbance of a Typic Sulfihemist. Profile descriptions according

to Soil Survey Staff (1996).

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Pedogenic Carbonate Soils with pedogenic carbonate are estimated to cover about 50% (3.7 million km2) of the Australian continent and mainly occur across its southern and inland regions where the climate is arid, semi-arid or temperate. Their occurrence, physical and geochemical properties were extensively reviewed by Milnes and Hutton (1983). However limited information exists about the dynamics of this vast pool of soil inorganic carbon in relation to land use and management, and the magnitude of this pool in the wide range of ecosystems in Australia has not been assessed. Fitzpatrick and Merry (2000) examined the effect of ASS disturbance on pedogenic carbonate in three landscape units in South Australia:

1) Undulating Hills (Natrixeralfs with carbonate layers), 2) Plains (dunes and swales with Petrocalcic Xerochrepts), 3) Coastal (Petrocalcic Xerochrepts).

In summary, they found that calcareous soils are economically important to Australia because they predominate in the dryland cropping zone of southern and eastern Australia and the irrigated grain cropping, horticulture and cotton areas of the Murray-Darling Basin. This made them particularly subject to human alteration. Specifically the environments reported on involved the following controlling factors: • carbonate formation by rock weathering, which represents a net sink of CO2 from the atmosphere • carbonate dissolution, reprecipitation and removal from calcrete layers, which represents a transient

gain of CO2 to the atmosphere as co-released calcium and magnesium ions will eventually remove an equivalent amount of carbon dioxide

• carbonate dissolution in drained mangrove swamps, which leads similarly to a transient gain of CO2 to the atmosphere.

These processes of carbonate dissolution and formation are accelerated by soil degradation consequent upon human alteration of the ecosystem through the intensification of dryland agriculture and through urban development. Contemporary rising saline ground water tables (containing high levels of Ca and Mg), and acids produced in the nitrogen and carbon cycles of dryland agricultural systems and by oxidation of pyrite in drained sulfidic mangrove soils, for example, may advance the formation, dissolution, reprecipitation and removal of carbonate. Table 1 (from Fitzpatrick and Merry 2000) is a first approximation of the relative importance given to the main processes (parent material source of Ca and Mg, climate and hydrological leaching and acidification) providing weathering products that may control the carbonate pools in weathering environments in South Australia. The net losses and gains of CaCO3 pools are summarised and their relative importance is given. ASS occurred in all but the arid/semi-arid soil weathering environment. In the undulating hills (Natrixeralfs with carbonate layers) disturbance of ASS was likely to result in mobilisation and reprecipitation of carbonates. In the Plains and Coastal soil weathering environments, disturbance of the ASS was likely to result in the loss of pedogenic calcium carbonate; however there would be no net gain of carbon dioxide to the atmosphere as the calcium and magnesium released would eventually sequester an equivalent quantity of carbon dioxide elsewhere.

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Table 1 Estimates of the various processes, which control carbonate pools in four representative weathering environments in South Australia (from Fitzpatrick and Merry 2000) Processes providing weathering products

Soil Weathering Environments (Major calcareous soil type)

Undulating hills Plains (dunes and swales)

Coastal Arid/semi-arid

(Natrixeralfs with carbonate layers; Fig. 2)

(Petrocalcic Xerochrepts; Fig. 3)

(Petrocalcic Xerochrepts; Fig. 4)

(Calcids and Salids)

Climatic and hydrological leaching processes Rainfall, Soil temperature regime (downward flow)

550-700 mm Mesic High

300–500 mm Thermic Low

400 mm Thermic Low

< 250 mm Thermic/ Hyperthemic Very Low

Rising saline groundwater (upward flow)

High High No Low

Declining tidal groundwater No No Yes No Through flow (lateral flow)

High No No No

Parent material sources of Ca and Mg Non-carbonate rocks Mica schist None None Low Carbonate-rich deposits None Limestones Limestones Calcareous Aeolian deposits Minor Moderate Moderate Moderate Acidification processes providing weathering products Sulfuric acid from acid sulfate conditions

Moderate Moderate High No

Acid from use of fertilisers & changes in C-N cycle

High Moderate No No

Overall impacts of processes Net gain or loss of pedogenic CaCO3

Gain Static in dunes/ loss in swales

Loss Static or passive

Areal significance of process Minor Major Minor Major TOXIC GAS EMISSIONS In areas with irrigated agriculture, excess drainage waters are often stored in disposal basins to avoid returning saline waters to rivers. An unexpected environmental concern associated with disposal basins is that some of them emit noxious odours when water levels are lowered or attempts are made to dry them. This phenomenon is commonly observed in saline wetlands and disposal basins of Lower River Murray floodplains, for example in Figure 2. The emission of noxious odours can result in the loss of aesthetic, recreational and tourism values associated with nearby areas and is thought to represent a potential health risk. The causes and mechanisms of noxious odour generation from disposal basins are not known but are almost certainly associated with the cycle of sulfur (S) in these environments. Sulfur is widespread in the environment and occurs as a very common salt (as sulfates, SO4

2–) in disposal basins and salinised floodplains. However, unlike other common salts such as sodium chloride, SO4

2– salts are biologically reactive and can be transformed into a variety of inorganic compounds (such as pyrite – FeS2) and organic compounds (S is a key building block of proteins). The raising of water levels in the River Murray and associated floodplain wetlands by the construction of weirs, as well as waterlogging caused by rising water tables has resulted in the widespread occurrence of accumulated reduced sulfur compounds (Lamontagne et al 2006; Fitzpatrick et al 2008) in the lower River Murray. This has produced a pool of reduced sulfur compounds vulnerable to disturbance through a variety of management interventions such as water level manipulations and salt interception schemes as well as natural phenomena such as drought, all of which are likely to be exacerbated by climate change.

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Figure 2. Noxious smell events can decrease the aesthetic values associated with wetlands and could also be a

community health concern (Barmera, South Australia). Murray Pioneer, 14 February 2003. What’s emitted and how badly can it smell? There is evidence from overseas studies that wetlands emit a range of sulfur gases and that they vary according to factors such as salinity, wetting–drying regime, soil type and diurnal cycles (Lomans et al 2002). Three main types of S gases can be emitted by wetlands: hydrogen sulfide, volatile organic sulfur compounds (VOSC) and sulfur dioxide (SO2). These differ in the way they are produced (Table 1). The human nose can detect some of these compounds at very low concentrations. Hydrogen sulfide: The rotten-egg smell. Under conditions of low oxygen, hydrogen sulfide is produced by microorganisms in the water columns and sediments of wetlands by the process of sulfate reduction (Figure 3). Most wetland sediments will be without oxygen at depth and will have some degree of sulfate reduction occurring. Another mechanism to produce H2S is also a familiar one – the decomposition of organic matter rich in organic S, like eggs. VOSCs: A wide variety of volatile organic sulfur gases occur in the environment and many give pleasant tastes and flavours to foods. However, several have very unpleasant odours. Some VOSC even play a role

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in climate regulation (VOSC emissions from oceans contribute to cloud formation and thus the albedo effect). VOSCs can be produced by a wide variety of mechanisms including: decomposition of organic matter; releases by algae; in marine, estuarine and salt marsh environments, the degradation of dimethylsulfoniopropionate (DMSP) an organic osmolyte (a substance that is part of an organism’s salt balance); and as detoxification by-products when organisms are exposed to H2S. Noxious smells ssociated with VOSCs are also a problem in waste treatment plants, pulp mills and aqueducts.

ed by olcanoes, and by fossil fuel burning and it is a key ingredient for the formation of smog in cities.

able 2. Some common sulfur gases.

a Sulfur dioxide: When previously anoxic sediments are exposed to oxygen, sulfur dioxide (SO2) can be produced during the oxidation of sulfides (e.g., pyrite) by oxygen (Figure 3.). This process can also be catalysed by another group of microorganisms, the “sulfide oxidisers”. SO2 has an acrid smell and can have adverse health effects following chronic exposure. Large quantities of SO2 are also emittv T Compound Main origin Hydrogen sulfide

Decomposition of sulfur-containing compounds in organic matter e.g. Reduction of sulfate

H2S H

SH

L-Cysteine NH2

HS OH

O

HS

NH2

OH

O

OH

O

NH2

S

L-Methione

nyl sulfide* COS Carbo

CO

S

Reaction of hydrogen sulfide with organic matter e.g.

RN C

+ H2X R

HN

CSH

X

C

S

X R

NH2+S

RO

C

S

XH R OH + C

S

X X=S, O Carbon disulfide*

CS2

CS

S

Reaction of hydrogen sulfide/decomposition of organic matter (see above). o compounds in the soil and atmosphere: *Interconversion of these tw

COS + H2O CO2 + H2S CS2 + OH

– COS + HS

–

Methanethiol (MT)

CH3SH H3C

SH

Reaction of hydrogen sulfide with organic matter. Decomposition of sulfur-containing compounds in organic matter

Dimethyl sulfide

(CH ) S (DMS)

3 2

H3C

SCH3

Reaction of hydrogen sulfide with organic matter Decomposition of sulfur-containing compounds in organic matter e.g.:

S+O

OH CH3

SH

CH3

SCH3

OR

DMSP Dimethyl disulfide

(CH ) S(DMDS)

3 2 2

S

H3CS

CH3

Oxidation of Methanethiol

H3C

SH

H3C

CH3SH oxidation SS

reduction H3C taining compounds in organic matter Decomposition of sulfur-con

Oxidation of pyrite (FeS2) Sulfur dioxide

O2 O

SO

S

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Key biogeochemical processes It has been observed that disposal basins emit noxious smells mainly when they are drying. In the following, we review in more details the main chemical reactions that occur when saline wetlands are “wet” and the ones that may occur when they are “drying” (Figure 3). As few studies have been conducted on the emission of S gases in Australian wetlands – the mechanisms proposed here are hypotheses that will require confirmation using laboratory and field studies.

ds, including: The iurnal wetland O cycle, density stratification, pH, and ebullition (bubble formation).

ht when low O2 concentration in the water column would decrease ater column H2S oxidation rates.

y, it is possible that a pulse of H2S from bottom waters could be pidly released to the atmosphere.

oportion of the H2S produced in wetlands will escape to the tmosphere when wetlands are acidic.

an effective mechanism to emit sulfur gases produced in sediments to the atmosphere some wetlands.

itions, there is tendency for organic matter nd organic-S) to slowly accumulate in wetland sediments.

Under wet conditions – Inorganic S cycle: There are two key features of the sulfur cycle under wet conditions: 1) the accumulation of sulfides and organic-S in the sediments and 2) the trapping of S gases emitted from the sediments by the water column. In most wetlands, the decomposition of organic matter in the sediments consumes much oxygen, and anoxic conditions prevail within a few millimetres below the sediment-water interface. Under such anoxic conditions, dissimilatory sulfate reduction occurs and H2S is produced as an end-product (Figure 3). Much of the H2S produced in the sediments will react with Fe and other metals to form sulfide minerals, but some H2S can also escape to the water column by diffusion from the sediments. However, most of the H2S diffusing from sediments to the water column will rapidly react with O2 (eventually producing sulfate) and thus not reach the atmosphere (Figure 3). A few other factors could influence whether or not H2S will be emitted from wetland 2 Diurnal O2 cycle: It is common for small Australian wetlands to have large fluctuations in O2 concentration over a daily cycle. Oxygen is added to the water column of wetlands by diffusion from the atmosphere and by photosynthesis by plants, algae and some bacteria. In return, O2 is consumed in wetlands by the decomposition of organic matter and aerobic respiration. Because photosynthesis does not occur at night but respiration and decomposition do, there is a tendency for small wetlands to have lower O2 concentrations at night, especially in summer when warm temperatures increase the rates of decomposition and respiration. It can be hypothesised that H2S produced in sediments will be more likely to escape to the atmosphere at nigw Density stratification: Many wetlands have periods when the water column is stratified into layers with different densities because of differences in temperature and salinity. When stratified, there is a tendency for bottom waters to become anoxic and accumulate H2S because O2 consumption rates in sediments can be rapid relative to the rate of diffusion of O2 from the overlaying water layer. In shallow wetlands, such stratification is seldom long lasting when it occurs because wind can efficiently mix the water column. However, when wetlands destratify rapidlra pH: H2S is a “weak acid” and a proportion present in the water column will tend to dissociate into either HS– or S2– at different pH values. In general, at neutral and alkaline pH, the preferred forms in water will be HS– and S2– (which are not volatile) while at more acid pH values, H2S becomes favoured. Thus, it should be expected that a higher pra Ebullition: Diffusion is not the only process that can transfer H2S produced in sediments to the water column. When water becomes supersaturated with a given gas (or set of gases), bubbles will tend to form. Bubble formation in sediments is especially common in summer, when decomposition rates (and the production of CO2, N2, methane and other gases) are higher. The formation and release of bubbles in sediments could bein Under wet conditions – Organic S cycle: The organic sulfur cycle in wetlands is not as well known as its inorganic counterpart. Sulfur can be integrated in organic matter by the process of assimilatory SO4

2– reduction (Figure 3). This occurs when algae and bacteria consume sulfate and convert it to protein-sulfur within their cells. Organic sulfur accumulates on the bottom wetlands when plants and animals die. Because decomposition rates are slower under anoxic cond(a

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VOSCs will be produced in wetland water and sediments when wet, particularly by releases from algae and decomposition in the sediments. Which specific compounds are produced and at what rates is not known for Australian inland wetlands. Some of the VOSCs produced will be consumed by bacteria in the water column. How much of the remaining VOSCs will be released to the atmosphere and the factors controlling the rates of production are not well known. In addition, VOCS emission rates are related to other factors such as temperature, diurnal cycles, degree of sediment wetness and sediment texture. To our knowledge, our recent exploratory sampling of ambient air in the Loveday Basin has been the only study of VOSC emissions from Australian inland wetlands. Change in S cycling when wetlands are drying: Two key factors could influence the S gas emission rates when wetlands are drying: 1) the exposure of previously anoxic sediments to oxygen, and 2) the loss of the overlying water column as a “trap” for S gases emitted from the sediments. The proposed consequences for the changes in these two environmental factors on the inorganic and organic S cycle are outlined below. Inorganic S gas emission: When wetland sediments are exposed to the atmosphere, they will gradually oxidise, as oxygen can now penetrate more easily in the sediments. A key inorganic S process under these conditions will be the tendency to oxidise sulfides stored in sediments, producing both sulfuric acid and sulfur dioxide (Figure 3). The rates at which sulfide oxidation will proceed is dependent on the soil texture (lower in clayey, compared to sandy, sediments) and the frequency of desiccation features (which increase the rates of oxygen diffusion in the sediments), residual moisture, and ambient pH. Thus SO2 as opposed to H2S is the main S gas emitted by the inorganic S cycle when oxygen is present. There are potentially two mechanisms by which H2S could be emitted from drying wetland sediments. Firstly, because the oxidation of sediments occurs gradually from the sediment surface (Figure 3) the anoxic conditions suitable for dissimilatory S reduction (and H2S generation) could persist for significant lengths of time at depth. Some of the H2S produced within the anoxic zone could then be released to the atmosphere by diffusion or advection (movement of air through pores induced by winds, temperature contrasts, etc), processes that could be aided by the desiccation features that often develop when wetland sediments dry. Likewise, H2S produced during organic matter decomposition could more easily escape to the atmosphere in the absence of a water cover. In addition, organic matter decomposition rates could increase once sediments are exposed to the atmosphere. Regardless of whether H2S or SO2 is produced, or the rates of gas production within sediments change, it is likely that gaseous S emissions increase from drying wetland sediments because some of the gases produced are no longer trapped within the wetland by the water column. VOSCs: VOSCs are produced by various processes in both oxic and anoxic wetland sediments (Figure 3). Here again, there is limited information in the literature to determine whether VOSC emission rates to the atmosphere should increase in drying sediments. It can be speculated that VOSC emission rates will tend to increase in drying wetland sediments because of more rapid decomposition rates of sediment organic matter under oxic conditions and the decreased interception of some VOSCs by the water column. There is also evidence that more VOSCs are produced in sulfide rich environments. This could occur, for example, when microorganisms produce VOSCs as a detoxification mechanism against H2S or by chemical reactions between H2S and organic matter.

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Figure 3. Sulfur cycling in wetlands under wet and drying conditions showing major pathways for sulfur

transformations. Knowledge needs to better manage the noxious odour problem. Hicks et al (unpublished data) undertook exploratory sampling of ambient air at the Loveday Disposal Basin (LDB). As far as we are aware this is the only study to analyse for a comprehensive suite of S-gases. These ambient air measurements revealed the presence of a range of gases in unexpected concentrations. Concentrations were at levels previously thought to be associated only with residues from their use as pesticides and fumigants for soil, grain silos and fruit. The spatial and temporal variation in the occurrence and concentration of these gases warrants further investigation as there is the potential for human health effects. The concentrations of a range of S gases in air samples taken mid-afternoon (~15:00) were at values close to their toxic limit values (Table 3). Early morning samples (~ 06:00) had lower levels of all gases apart from carbonyl sulfide.

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Table 3. Ambient air samples where at least one gas was at a concentration above a recommended value. Results are bolded and colour coded to match the relevant exposure value. No value for a particular exposure standard means that it is not explicitly covered by that standard.

Hydrogen sulfide

Carbonyl sulfide

Carbon disulfide

Dimethyl sulfide

Dimethyl disulfide

Dimethyl sulfoxide

H2S COS CS2 DMS DMDS DMSO threshold limit value (tlv)/time weighted avg (twa) for occ exp 10† 10† 10†; 2* 2† – –

threshold no effect value‡ 10 1.25 10 50 0.0035 – Chronic Ref Exp Level (California)¶ 0.008 – 0.3 – – – ppm, v/v ppm, v/v ppm, v/v ppm, v/v ppm, v/v ppm, v/v Loveday Basin, SA 25-Jun-07 14:30 10cm above water max <0.0001 0.02 0.005 0.08 <0.0001 <0.0001 mean <0.0001 0.008 0.003 0.028 <0.0001 <0.0001 sd 0.008 0.001 0.035 n 4 4 4 4 4 4 14:45 10cm above soil 1m from waterline

<0.0001 0.04 0.005 0.09 <0.0001 <0.0001 max mean <0.0001 0.020 0.003 0.058 <0.0001 <0.0001 sd 0.014 0.001 0.035 n 4 4 4 4 4 14:55 10cm above disturbed sediment max 0.06 9.4 0.800 8.6 0.800 0.060 mean 0.03 6.2 0.600 7.6 0.500 0.043 sd 0.022 2.6 0.216 1.308 0.216 0.015 n 4 4 4 4 4 4 Mussell Lagoon, SA 25-Jun-07 15:35 10cm above soil 3m from waterline max 0.007 0.004 0.001 0.002 <0.0001 <0.0001 mean 0.0055 0.0035 0.0015 <0.0001 <0.0001 n 2 2 2 2 2 2 *http://www.oehha.ca.gov/air/acute_rels/allAcRELs.html; http://www.oehha.ca.gov/air/chronic_rels/AllChrels.html † Chemwatch 2007; ‡ Chemwatch 2006 There have been no studies on the environmental controls on S gas emissions from inland Australian wetlands. Therefore, it is not possible to propose scientifically-defensible management guidelines to minimise noxious odour problems during water level management operations in wetlands and disposal basins. A list of critical knowledge needs to better understand the noxious odour problem includes: • Determination of which inorganic and organic sulfur gases are emitted from a range of wetlands and

disposal basins • Understanding of the environmental factors controlling the rates of emission for the most common

noxious-smelling gases. Based on the literature, these factors would include sediment texture, organic matter content, sulfide content, water content, salinity, pH, time of day, temperature and the presence or absence of a water column

• Determination of the relative significance of dissimilatory sulfate reduction and organic matter decomposition as sources of H2S emission during wetland drying events.

A number of technical challenges may have to be tackled before some of these questions can be addressed. Instrumentation to measure the concentration of some S gases (H2S and SO2) is readily available. However, many noxious odour-generating VOSC cannot be readily measured in the field because they occur at very low concentrations (but can still be detected by the human nose). In addition, measuring the rates of emission (“how much is produced per unit area of sediments per unit time”) is logistically more complicated than just measuring the ambient concentration of sulfur gases. In overseas studies, the most widely used method to measure S emission rates from exposed sediments is with incubation chambers. These are either coupled to measuring devices (for gases occurring at higher

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concentrations) or are used to trap S gases on columns or other media for later analysis in the laboratory. The analytical techniques required to measure S gases at low concentrations are challenging. From our review of the literature, only a handful of laboratories around the world appear to have developed the necessary techniques to measure S gas emissions from wetlands. CONCLUSION The S cycle and, in particular, the gaseous components of the S cycle have received limited attention in Australian inland wetlands. Studies overseas should be applied with caution to the Australian context because the environment and management issues associated with S are often different there. As Australian wetland managers must learn to “live with salt”, getting a better understanding of sulfur cycling in inland wetlands should be viewed as a priority. Ambient air measurements indicate that H2S may not be the main gas responsible for the foul odours. The observed ambient air concentrations represent the “best case” i.e. the lowest concentrations, as they were made in winter when the maximum amount of the gas is dissolved and microbiological activity is lowest. The highest concentrations occurred 10cm above disturbed sediment, with concentrations above the water and shoreline 1–2 orders of magnitude lower; there was also some diurnal variation with some gas concentrations higher for a mid-afternoon sample and others for a dawn sample. Unexpected concentrations of these gases were also observed at Mussel Lagoon, a control site for LDB studies. It is possible the source is the LDB but observations 150 m distant from the basin on Trussell Terrace found much lower gas levels, with many below the detection limit. This site is elevated with respect to the basin, which may account for this; however it may be that these gases occur more generally as a result of floodplain waterlogging and salinisation. Sulfur gases also represent a potential threat to the ecology and agricultural industries by affecting the chemical ecology of the environment especially in the riparian zone. They have the potential to affect populations of pollinators and also grape/wine quality. It is also worth noting that the concentrations of hydrogen sulfide observed were relatively low and this gas appears to be a minor component of the gas cocktail. This has implications for monitoring and abatement strategies. REFERENCES Chmura GL, Anisfeld SC, Cahoon DR and Lynch JC, 2003. Global carbon sequestration in tidal, saline wetland

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Fitzpatrick RW and Merry RH, 2000. Pedogenic carbonate pools and climate change in Australia. In: "Global Climate Change and Pedogenic Carbonates'. R Lal, JM Kimble, H Eswaran and BA Steward, Eds. Lewis Publishers, Boca Raton, Fl., pp.105-119.

Jickells T and Rae JE, 1997. Biogeochemistry of intertidal sediments. In: ‘Biogeochemistry of intertidal sediments’. TD Jickells and JE Rae, Eds. Cambridge University Press, New York, pp. 1–15.

Lamontagne S Hicks WS, Fitzpatrick RW and Rogers S, 2006. Sulfidic materials in dryland river wetlands. Marine and Freshwater Research 57, 775–788.

Lomans BP, van der Drift C, Pol A and Op den Camp HJM, 2002. Microbial cycling of volatile organic sulfur compounds. Cellular and Molecular Life Sciences 59, 575–588.

Milnes AR and Hutton JT, 1983. Calcretes in Australia. In ‘Soils: an Australian viewpoint’. Division of Soils, CSIRO: Melbourne/Academic Press: London, pp.119–162.

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Twilley RR, Chen RH and Hargis T, 1992. Carbon sinks in mangroves and their implications to carbon budget of tropical ecosystems. Water Air and Soil Pollution 64, 265–88.

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Wollast R, 1991. The coastal organic carbon cycle: fluxes, sources and sinks. In Ocean Margin Processes in Global Change. RFC Mantoura et al Eds, Wiley, Chichester, pp. 365–381.

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