Assessment of the acidification risk of the acid sulfate soil materials in a tropical coastal peat bog: Muthurajawela Marsh, Sri Lanka
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Please do not remove this page Assessment of the acidification risk of the acid sulfate soil materials in a tropical coastal peat bog: Muthurajawela Marsh, Sri Lanka Vithana, Chamindra Lakmali; Ulapane, Prashani AK; Chandrajith, Rohana; Sullivan, Leigh A; et al. https://researchportal.scu.edu.au/discovery/delivery/61SCU_INST:ResearchRepository/1298707760002368?l#1398707750002368 Vithana, Ulapane, P. A., Chandrajith, R., Sullivan, L. A., Bundschuh, J., Toppler, N., Ward, N. J., & Senaratne, A. (2022). Assessment of the acidification risk of the acid sulfate soil materials in a tropical coastal peat bog: Muthurajawela Marsh, Sri Lanka. Catena, 216(Part A). https://doi.org/10.1016/j.catena.2022.106396 Published Version: https://doi.org/10.1016/j.catena.2022.106396 Southern Cross University Research Portal: https://researchportal.scu.edu.au/esploro/ ResearchPortal@scu.edu.au Downloaded On 2022/09/16 11:28:42 +1000 Please do not remove this page
Catena 216 (2022) 106396
Contents lists available at ScienceDirect
Catena
journal homepage: www.elsevier.com/locate/catena
Assessment of the acidification risk of the acid sulfate soil materials in a
tropical coastal peat bog:Muthurajawela Marsh, Sri Lanka
Chamindra L. Vithana a, *, Prashani A.K. Ulapane b, Rohana Chandrajith c, Leigh A. Sullivan d,
Jochen Bundschuh e, f, Nadia Toppler g, Nicholas J.Ward g, Atula Senaratne c
a
Faculty of Science and Engineering, Southern Cross University, Military Road, East Lismore, NSW 2480, Australia
b
Department of Geosciences, Stony Brook University, NY, USA
c
Department of Geology, Faculty of Science, University of Peradeniya, Peradeniya 20400, Sri Lanka
d
University of Canberra, 11 Kirinari St, Bruce ACT 2617, Australia
e
School of Civil Engineering and Surveying, Faculty of Health, Engineering and Sciences, University of Southern Queensland, West Street, Toowoomba, Queensland 4350,
Australia
f
Department of Earth and Environmental Sciences, National Chung Cheng University, 168 University Road, Min-Hsiung, Chiayi County, 62102, Taiwan
g
Environmental Analysis Laboratory, Southern Cross University, Military Road, East Lismore, NSW 2480, Australia
A R T I C L E I N F O A B S T R A C T
Keywords: Coastal regions in tropical countries encompass a diverse set of highly productive ecosystems with underlying
Pyrite acid sulfate (AS) soil materials. Muthurajawela Marsh is a tropical, saline, peat bog on the western coast of Sri
Sulfidic acidity Lanka and is known to contain AS soil materials. It is a critically important coastal wetland ecosystem and
Trace elements
provides a multitude of benefits and services to the surrounding environment and the people in the area. At
Jarosite
Marshy ecosystems
present, the AS soil materials in the marsh are at risk of exposure due to development activities in the sur
Retained acidity rounding areas. In this study, net acidity (NA) was quantified using an acid base accounting approach which
includes retained acidity (RA) in addition to actual and potential sulfidic acidity (AA and PSA). The NA and the
other soil characteristics were investigated in three soil profiles down to 1.5 m, along a north–south transect. All
sites contained hypersulfidic soil materials as confirmed by field pH (pHF) > 4, field oxidation pH (pHFOX) < 4
and sulfide content > 0.01%. Net acidity values ranged from 23 to 4000 mol H+ t-1which was above the rec
ommended management requirement value for peat and medium clay soils (i.e. 18 and 36 mol H+ t− 1). At the
northern site (S1), PSA was the main contributor to the NA, indicating future risk if the site were to become
exposed to air. Both AA and RA were major contributing fractions at the middle (S2) and southern (S3) sites, with
a possible imminent acidity discharge. All sites lack inherent buffering capacity, consequently, acidity can be
released from the oxidation of the AS soil materials leading to the potential impact on the marsh ecosystem. The
findings of this study indicate that human activities should be carefully managed to minimize the hazards that
can occur due to exposing AS soils in the marsh.
1. Introduction sulfuric horizon containing the products of RIS oxidation (secondary
iron minerals such as jarosite - RFe3(SO4)2(OH)6, R = Na+, K+, H3O+,
2+
Low-lying coastal areas in tropical regions are often underlain by NH+ 4 , Pb ) and subsequent hydrolysis of secondary iron minerals (Eq.
acid sulfate soil materials, which are characterized by substantial (2)) (Sullivan et al., 2012; Fanning et al., 2017).
amounts of potential and/or existing acidity (Andriesse and van Mens
voort, 2006; Sukitprapanon et al., 2018). The soil profiles of coastal acid FeS2+7/2O2 + H2O → Fe2++2H++2SO2-
4 (1)
sulfate soil (CASS) can consist of one or a combination of horizons, such KFe3(OH)6(SO4)2 + 3H2O → 3Fe(OH)3 + 2SO24 + 3H +K + +
(2)
as (a) a sulfidic horizon containing reduced inorganic sulfur (RIS)
(makinawite-FeS, pyrite, and marcasite-FeS2) that has the potential for The acidity in sulfidic horizon is present in latent form and is known
severe acidification through oxidation (Eq. (1)), and (b) an acidified as potential sulfidic acidity (PSA) (Ahern et al., 2004). The acidity in
* Corresponding author.
E-mail address: chamindra.vithana@scu.edu.au (C.L. Vithana).
https://doi.org/10.1016/j.catena.2022.106396
Received 20 December 2021; Received in revised form 8 May 2022; Accepted 15 May 2022
Available online 21 May 2022
0341-8162/© 2022 Elsevier B.V. All rights reserved.C.L. Vithana et al. Catena 216 (2022) 106396
sulfuric/thionic horizon already exists as retained acidity (RA) and ecosystems which are rich in biodiversity provide many services such as
actual acidity (AA) derived from jarosite/schwertmannite and soluble/ food, habitats and breeding grounds for wildlife, act as freshwater
exchangeable minerals respectively (Ahern et al. 2004). Consequently, storage, natural filter for toxic substances, protect shoreline, sequestrate
the acidity hazard is determined by quantifying the net acidity (NA) carbon and maintain hydrological balance (UNEP, 2006; Barbier et al.,
using an acid base accounting (ABA) approach (Ahern et al., 2004) (Eq. 2011). Direct economic benefits provided by coastal ecosystems include
(3)). the provision of food, timber, and employment along with eco-tourism.
Recreation, education, and research are the other important activities
Net acidity (NA) = Actual acidity (AA) + Retained acidity (RA) + Potential
associated with these ecosystems. At present, the coastal regions of the
sulfidic acidity (PSA) (3) tropics are being excavated, drained, or reclaimed to carry out numerous
However, in some studies, retained acidity has been excluded when anthropogenic activities such as coastal urbanization, infrastructural
quantifying the net acidity in acid sulfate soils thereby underestimating developments, constructions, agricultural activities (UNEP, 2006;
the potential acidity hazard (Grealish and Fitzpatrick, 2013; Sukipra Barbier et al., 2011). These anthropogenic activities may trigger the
panon, et al., 2015; Yau et al., 2016; Catalán et al., 2019). acidification and other hazards of acid sulfate soils including Al and Fe
Coastal acid sulfate soils occupy a diverse set of landforms such as toxicity, and the increased mobility of trace elements by driving the
inter-tidal swamps, marshes, wetlands, floodplains, backswamps, la oxidation of underlying sulfidic sediments and consequent acidification.
goons estuaries, and coastal embayments (Dent and Pons, 1995). These Such hazards can cause the death of vegetation, poor water quality, and
Fig. 1. Map showing the location of the Muthurajawela Marsh and sampling sites (Site 1-S1, Site 2-S2, and Site 3-S3).
2C.L. Vithana et al. Catena 216 (2022) 106396
harm aquatic animals thereby significantly impacting the stability and contribution towards the net acidity in (partially oxidized) acid sulfate
the functionality of coastal ecosystems and the services they provide to soils.
the environment and humans (Ljung et al., 2009).
In tropics, the majority of coastal acid sulfate soil materials are 2. Materials and methods
derived from Quaternary sediments deposited during the mid to late
Holocene period (10,000 to 6000 years BP) and distributed across the 2.1. Study area
coastal regions in northern Australia, the northern part of South Amer
ica, West Africa, South-and South-East Asia (Dent, 1986; Attanandana Muthurajawela Marsh is located on the west coast of Sri Lanka
and Vacharotayan, 1986). Their behaviour, impacts, and management (7◦ 01′ 27′′ N 79◦ 50′ 41′′ E), between the main commercial city, Colombo,
approaches have long been studied in all regions except South Asia (e.g. and the country’s main International airport (Fig. 1). The northern
Bloomfled and Coulter, 1974; van Breemen, 1982; Johnston et al., 2010; boundary of the marsh is the Negombo Lagoon, which together with
Fall et al., 2014; Shamshuddin et al., 2014; Combatt Caballero et al., Muthurajawela marsh forms an integrated coastal ecosystem. The
2019; Mendonca et al., 2021). For example, some of the modified CASS southern and eastern boundaries of the marsh are highly urbanized and
landscapes with improved soil and water management practices have industrialized areas. The western boundary of the marsh is the Hamilton
been used to cultivate certain crops such as rice, oil palm, cocoa, and Canal which separates the marsh from sea (GCEC, 1991).
sugarcane in Thailand, Vietnam, Indonesia, Malaysia, Brazil, and The marsh is perennially water-logged due to surface or near-surface
northern Australia (Kinsela and Melville, 2004; Shamshuddin et al., groundwater. The input of both saline and freshwater has generated
2004; Grealish and Fitzpatrick, 2013; Sukitprapanon et al., 2020; brackish water conditions that have resulted in the formation of highly
Mendonca et al., 2021). However, our understanding of acid sulfate soils diverse ecosystems. Saline conditions are developed by the inflow of
in the tropical regions is incomplete due to the limited contribution from seawater via the Hamilton Canal during the high tides and dry seasons.
the South Asian region. The main freshwater sources are rainfall, surface runoff, and discharges
Sri Lanka, an island in South Asia, also contains low-lying coastal from a network of canals and streams in the north (Dandugam Oya) and
landforms with Quaternary sediments that developed during the mid- northeast (Ja-Ela) (Fig. 1). The marsh, being located in the wet climatic
Holocene (Weerakkody, 1992). However, acid sulfate soils are poorly zone of Sri Lanka, receives a high annual rainfall (2500 mm) while
recognized in Sri Lanka, except for two locations in the west and exposed to annual ambient temperatures of 21.5–31.5 ◦ C, with 80%
southern coastal areas. Muthurajawela Marsh on the west coast and relative humidity. The soils in Muthurajawela Marsh are organic
Nilawala River Basin located on the southern coast are two main loca dominant with “sulfidic” and “sulfuric” soil materials (IUSS Working
tions identified with acid sulfate soil materials (Fernando and Sur Group, WRB, 2014).
anganee, 2009). Muthurajawela Marsh is an important coastal
ecosystem, as it is the only saline peat bog and one of the twelve priority 2.2. Geological evolution of the marsh and peat formation
wetlands in Sri Lanka (Fig. 1) (Bambaradeniya et al., 2002). Similar to
other coastal ecosystems across the globe, Muthurajawela Marsh is rich Muthurajawela Marsh is believed to form during the mid to late
in biodiversity and provides a multitude of ecological services to the Holocene period on a Pleistocene landscape (Senaratne, 1987; Cooray
environment and economic benefits to the people living in the vicinity and Katupotha, 1991; Senaratne and Dissanayake, 1991). At higher sea
(Bambaradeniya et al., 2002; Emerton and Kekulandala, 2003; Vithana levels during the mid-Holocene period, the combined area of Muthur
et al., 2021). The soil in the marsh is classified as ‘bog soil’ with the ajawela Marsh and Negombo Lagoon would have been a large lagoon/
presence of pyrite in the sub-surface indicative of potential acid sulfate lake connected to the sea from the north and bordered by a sand spit on
soils (Seneratne and Dassanayake, 1999). Sulfate, required for pyrite the western margin (Figure S1). During the subsequent low sea-level
formation, is derived from saltwater intrusion via the Hamilton canal period around 6000 BP, marine and terrestrial sediments accumulated
(Fig. 1), while laterites on the eastern border of the marsh provide the at the southern end of the large lagoon forming mudflats, followed by
source of iron (Dissanayake, 1987; Senaratne, 1987; Pitawala et al., the growth of salt marsh vegetation (Senaratne and Dissanayake, 1991).
1994). The continuous marine regression and accumulation of terrestrial sedi
Soil physical and chemical characteristics of Muthurajawela Marsh ments gradually transformed the southern part of the lagoon into a
were comprehensively discussed elsewhere (Vithana et al., 2021). forest. Over time, the forest was flooded and large organic debris
Briefly, the field pH of the marsh varies from highly acidic to neutral accumulated forming peat overlying the pyritic Holocene sediments.
(1.6–7.0) and the organic matter content ranges from 6 to 41 % and the The sedimentation and peat formation continued at a slow place towards
soil occasionally contains high total sulfur content in the surface peat up the north confining the lagoon to the northern part and forming the
to 17% by mass (Pitawala et al., 1994; Idamekorala and Mapa, 2003). marsh at the southern end (Figure S1) (Senaratne and Dissanayake,
Pyrite oxidation due to natural and anthropogenic activities has resulted 1991).
in acidic and sulfate-rich interstitial water with pH 2.0–5.0 and sulfate
concentration about 3000 mg L-1 (Dhanayake et al, 1991; Pitawala et al., 2.3. Sampling
1994). At present, Muthurajawela Marsh is severely threatened by
anthropogenic activities such as land reclamation, fragmentation, ex Soil sampling, preservation, and storage were carried out according
cavations, constructions, illegal settlements, and municipal and clinical to the procedures described in Sullivan et al. (2018a). Three soil cores
garbage dumps. These activities can significantly impact the ecosystem down to 1.5 m depth were collected along a north to south transect (S1,
services and the economic value through acidification and mobilization S2, and S3) of the marsh using a Russian-D corer (Fig. 1). Prior to soil
of trace elements (Vithana et al., 2021). Therefore, it is critically sampling, the background characteristics of each site were recorded. The
important to gain a proper understanding of the acid sulfate soil mate soil cores were sectioned into layers based on colour differences.
rials present in the marsh, of their behviour, and of potential issues Approximately, two tablespoons of soil from each layer were kept aside
arising from their disturbance, in order to minimize the impact of the for immediate field analysis. Key physical characteristics of each layer
acid sulfate soils hazards on the ecosystem. The main objective of this are presented in Table 1. The remaining soil was divided into four
study was to systematically characterize the nature of acid sulfate soils portions and placed in plastic bags for laboratory analyses of moisture
materials and to assess the status of the acidity present in those materials content, trace elements, organic matter content, acidity, and X-ray
in the Muthurajawela Marsh. The quantification of retained acidity in diffractometric analyses. After expelling the air present, these plastic
acid sulfate soils in tropical regions is a novelty of this study and we bags were sealed and stored at 4 ◦ C until they were brought to the
aimed to highlight the importance of retained acidity fraction and its laboratory. For bulk density analysis, soil from each layer was collected
3C.L. Vithana et al. Catena 216 (2022) 106396
Table 1
Morphological and physical characteristics of soils at the three sites of Muturajawela Marsh, Sri Lanka.
Site Depth Colour Texture Bulk density (g/ Organic matter content pHF pHFOX Rate of pHFOX Rate of Fizz
(cm) description cm3) (%) reaction reaction
S1 13 Brownish black Silty clay loam 0.2 27 5.9 3.6 H N
33 Brownish black Silty clay loam 0.2 24 6.8 1.2 H N
46 Grayish black Silty clay loam 0.2 36 6.8 1.0 X N
70 Grayish black Clay 0.6 12 6.9 2.2 V S
78 Grayish black Clay 0.7 16 7.2 2.1 V S
90 Grayish black Clay 0.6 14 7.1 2.7 V M
123 Bluish black Clay 0.6 18 6.7 1.9 V S
150 Brownish gray Clay 0.4 29 7.0 1.4 V S
S2 30 Brownish black Clay loam 0.6 27 6.2 4.4 M NA
sandy
50 Grayish black Clay loam 0.6 24 6.1 1.0 V NA
sandy
74 Brownish black Clayey sand 0.5 16 6.0 1.1 V NA
82 Brownish black Clayey sand 0.5 17 6.0 1.1 V NA
100 Brownish black Clayey sand 1.0 9 5.9 0.9 V NA
115 Brownish black Clayey sand 0.6 20 6.1 1.0 V NA
134 Grayish black Clayey sand 0.8 9 6.0 0.8 V NA
150 Grayish black Clayey sand 1.3 6 6.0 0.8 V NA
S3 7 Grayish black Clayey sand 0.9 8 4.4 2.7 H NA
28 Dark grayish Clayey sand 0.6 26 4.2 3.0 V NA
black
48 Bluish black Clay 0.3 59 4.3 1.1 V NA
82 Bluish black Clayey sand 0.2 59 5.6 1.4 X NA
97 Bluish black Clay 0.2 47 6.6 3.7 V NA
150 Bluish black Clay 0.4 23 6.5 1.1 X NA
pHF- Field pH.
pHFOX- Field oxidation pH.
pHFOX reaction - M− Medium, H-High, V-Vigorous, X-Extreme.
Fizz test - N-Not calcareous, S-Slightly calcareous, M− Moderately calcareous, NA-Not Assessed.
in containers of pre-determined volume and subsequently transferred to After drying, large pieces of organic material, and shell fragments were
a plastic bag and sealed. For the incubation experiment (chip-tray in removed and the samples were sieved (C.L. Vithana et al. Catena 216 (2022) 106396
titratable actual acidity and retained acidity analyses by replicates and diffraction patterns were recorded from 5 to 80◦ θ in steps of 0.02◦ 2θ
blank analyses. Prior to the sulfate determination in titratable actual with a 2-second counting time.
acidity and retained acidity analyses, the extracts were centrifuged at
4500 rpm and filtered using 0.45 µm nylon filters. An appropriately 3. Results
diluted filtrate was used to determine the SHCl and SKCl.
Potential sulfidic acidity was determined by measuring the chro 3.1. Study area and regional setting
mium reducible sulfur (CRS) fraction, a measure of the reduced inor
ganic sulfur (RIS) content in the sample. The chromium reducible sulfur The S1 was in the northern segment of the marsh. It was located at
content was measured by following the method described in Burton the edge of a settlement area and covered with live marshy grasses such
et al. (2008). In brief, the RIS content in the powdered soil sample was as sedges and reeds. Similar to the S1, the S2 which was in the middle
treated with acidic chromous chloride (acidic CrCl2) solution in a sealed segment of the marsh was also covered with marshy grasses and located
extraction chamber (50 mL). After the addition of acidic CrCl2 solution, on the fringe of a shallow depression. The S3 at the southern segment of
the sealed extraction chamber was shaken at 150 rpm for 48 h to allow the marsh was located adjoining to a canal and surrounded by mangrove
all reduced inorganic sulfur in the soil sample to diffuse as H2S. The vegetation. In the S3, the surface was covered with dried leaves instead
diffused H2S was trapped as zinc sulfide (ZnS) in an alkaline zinc acetate of a live grass cover. The groundwater level was almost at the surface,
solution in a 10 mL tube placed inside the sealed chamber. After 48 h, saturating the soil profile at each of the three sites.
the tube with ZnS precipitate was carefully removed from the extraction
chamber and immediately analyzed for chromium reducible sulfur by
iodometric titration method. The chromium reducible sulfur and the 3.2. Soil physical characteristics
potential sulfidic acidity contents were then calculated.
The organic matter content was determined by a loss on ignition All soil profiles were dark brown to greyish black without distin
method (Salehi et al., 2011). Briefly, a soil sample was oven-dried at guishable redoximorphic features (Table 1). Clay and sand were the
105 ◦ C for 2 h and fused at 360 ◦ C in a muffle furnace for another 2 h. dominant textural fractions in the profiles (Table 1). In the S1, surface
The weight loss between 105 ◦ C and 360 ◦ C representing the organic soil down to 50 cm was fine-textured (silty clay loam) and the clay
matter content was calculated. According to Salehi et al. (2011), most of content increased down the profile (Table 1 and Figure S2). The coarse
the organic matter in the soil sample is ignited by 360 ◦ C with minimum shell fragments in the bottom layers (>80 cm) of the S1 indicated a
loss of structural water and other inorganic components (e.g. carbon change in depositional conditions. The soil texture in the S2 became
ates, pyrites). coarser towards the bottom as indicated by the change in texture from
Soil samples of known volume were dried at 105 ◦ C until at constant clay loam sandy to clay sand towards the depth (Table 1and Figure S2).
weight and their bulk densities were determined as described in Sullivan In the S3, a coarser texture with more sand was noted down to 50 cm and
et al. (2018b). the texture became fine towards the middle, with a gradual increase in
The chip-tray incubation was carried out to identify the occurrence clay content towards the base of the profile (Table 1 and Figure S2). The
of potential acid sulfate soil (PASS) materials. Soil samples were subject majority of organic matter in the S1 and S2 soil profiles was visibly
to oxidation under ambient conditions and the samples with pHF ≥ 4.0 recognizable, whereas in the S3, except at the surface layers (30 cm),
were selected for the incubation experiment (Fig. 2). Homogenized soil organic matter was unrecognizable indicating its fully decomposed na
from each depth was added to a labeled compartment in the box and the ture. The recognizable organic matter was mostly in the form of roots
box was kept in dark under ambient conditions for 9 weeks. At the end of and twigs. Organic matter content was closely associated with clay
9 weeks, the soil pH (pHINC) was determined after moistening the soil by content being low in sandy layers compared with clayey layers (Table 1
adding a few drops of deionized water. If pHINC < 4.0, the sample was and Figure S2). In S1 and S2, the organic matter content was higher in
classified as ‘sulfidic’ while pHINC > 6.5, the soil was considered as ‘non- the first 50 cm compared with the underlying depths (Table 1 and
sulfidic’. However, if pHINC was > 4.0 but < 6.5 the sample was incu Figure S2). In contrast to the S1 and S2, the organic matter content in the
bated for another 10 weeks. After a total 19 weeks of incubation, the soil S3 increased towards the bottom, being highest in the middle (i.e. ~
sample was classified as ‘sulfidic’ or ‘non-sulfidic’ based on the criteria 60% between 50 and 100 cm) (Table 1 and Figure S2). The surface, sub-
described above (Sullivan et al. 2018b). surface and the very bottom depths (150 cm) in the S1 were porous and
In order to determine the soil mineralogy, XRD spectra were ob rich in organic matter (24–36%) as indicated by very low bulk densities
tained using the Bruker D8 Advanced Eco X-ray Diffraction system. The 0.2–0.4 g cm− 3 compared to the remaining depths (Table 1). In the S2,
higher bulk densities (0.6 –1.3 g cm− 3) were observed below the sub-
Fig. 2. Variation of the field pH (pHF), field peroxide oxidation pH (pHFOX), and incubation pH (pHINC) after 19 weeks in three soil profiles down to 1.5 m (a) S1 (b)
S2 and (c) S3.
5C.L. Vithana et al. Catena 216 (2022) 106396
surface (>82 cm) especially in the depths containing more sand, less 6.5 (Table 2). In the S2, the highest actual acidity (83 mol H+ t− 1) was
clay, and less organic matter (6–9%). In the S3, soils below the surface found at 50 cm. From 50 cm to bottom, the actual acidity varied little
were more porous as indicated by very low bulk densities (0.2–0.4 g and remained stable at an average of 46 mol H+ t− 1 (Fig. 3b). In the S3,
cm− 3) with abundant organic matter (23–59%) (Table 1). the actual acidity gradually increased from the surface to the bottom and
in the bottom, it was twice as high as the surface soil (Fig. 3c).
3.3. Field pH (pHF) and field peroxide oxidation pH (pHFOX)
3.5.2. Retained acidity (RA)
In the S1 site, the pHF throughout the soil profile was near-neutral Retained acidity was only observed in S2 and S3 and was present at
(5.9–7.2) (Fig. 2a). The pHF in the S2 was near-neutral (6.0–6.2) in all depths (i.e. pHKCl was < 4.5) (Table 2). Similar to the actual acidity,
the surface and the sub-surface depths (0–75 cm) and became moder the maximum amount of retained acidity was present in the sub-surface
ately acidic (75 cm) (Fig. 2b). In contrast (1656 mol H+ t− 1) and bottom of the profile (3013 mol H+ t− 1) at the S2
to both S1 and S2 the pHF in the surface and the sub-surface depths and S3, respectively (Fig. 3b and c). At the S3, the retained acidity in the
(down to 80 cm) in S3 was moderately acidic (4.2–5.6) and became bottom of the profile was nearly two times higher than that of the S2
near-neutral (6.5–6.6) below the sub-surface (>80 cm) (Fig. 2c). (Fig. 3c).
High to vigorous reactions were observed when soil samples from
different depths in three sites were reacted with the 30% H2O2 (Table 1). 3.5.3. Potential sulfidic acidity (PSA)
The pHFOX below the surface of S1 and S2 were < 2.8 and ~ 1.0, Potential sulfidic acidity in the three soil profiles varied with the
respectively (Fig. 2a and b). In both S1 and S2, pHF below the surface maximum potential sulfidic acidity being observed at 90 cm, between 50
was decreased by 4.0 to 5.5 pH units when reacted with H2O2 (Fig. 2a and 100 cm depths, and at 150 cm in S1, S2, and S3, respectively (Fig. 3).
and b). In the S3, the pHFOX in all depths was below 3.7 (Fig. 2c). The surface layers (0–30 cm) at all sites contained less potential sulfidic
However, the pHF in the surface soils only dropped by 1.7 units on
addition of H2O2 compared with the depths below, which could be due
Table 2
to a lack of sulfidic soil materials. Although pHF in the surface of S3 was
pH of the KCl extract (pHKCl), net acid-soluble sulfur (SNAS), and chromium
decreased by < 2.0 units, it dropped by > 3.0 pH units below the surface
reducible sulfur (CRS) contents (%) in all depths at the three sites.
(Fig. 2c).
Site Depth (cm) pHKCl SNAS (%) stdev CRS stdev
(%)
3.4. pH after incubation (pHINC)
S1 13 5.5 0.0 0.0 0.0 0.0
33 6.3 0.0 0.0 0.5 0.3
After 9 weeks of incubation, the pHINC were 1.8–3.8 and 1.4–1.6 in 46 6.3 0.0 0.0 0.4 0.2
all depths in S1 and S2 excluding the surface (Fig. 2a and b). In both 70 5.6 0.0 0.0 1.7 0.2
sites, there was a drop of 3.5 to 5.0 pH units from the initial pHF. Further 78 5.6 0.0 0.0 2.4 0.2
90 7.8 0.0 0.0 3.8 0.2
incubation of 10 weeks (total of 19 weeks) resulted in a drop of pH in the 123 7.0 0.0 0.0 3.3 0.3
surface in both sites to < 4.0 and the pH of the other depths remained < 150 5.4 0.0 0.0 2.6 0.1
4.0 (Fig. 2a and b). In the S3, pHINC of all depths were < 4.0 except the
depths of 82 and 97 cm in which pHINC were 4.3 and 4.9, respectively
after 9 weeks of incubation (Fig. 2c). Over the subsequent weeks, the pH S2 30 5.7 0.3 0.0 0.0 0.0
50 4.4 1.7 0.3 1.9 0.4
in those two depths dropped to 4.0 (Fig. 2c). However, similar to the
74 4.4 0.7 0.3 2.0 0.2
other two sites, pHF in S3 also dropped by>0.5 pH units during the first 82 4.4 1.1 0.2 2.2 0.2
9 weeks of incubation indicating slow but continuous oxidation of sul 100 4.2 0.2 0.3 2.1 1.1
fidic soil materials. 115 4.5 1.3 0.0 1.4 0.2
134 4.0 0.4 0.3 2.1 0.5
150 3.7 0.6 0.2 2.0 0.5
3.5. Acidity fractions
3.5.1. Titratable actual acidity (TAA) S3 7 4.1 0.5 0.1 0.0 0.0
28 4.0 0.3 0.1 0.1 0.2
The S1 contained very little actual acidity with a maximum of 25 mol
48 3.8 1.1 0.6 1.0 0.1
H+ t− 1 at the surface depth (0–30 cm), while the S3 contained the 82 4.5 0.7 0.3 0.6 0.1
highest actual acidity with a maximum of 420 mol H+ t-1at the bottom of 97 4.5 1.5 0.1 0.8 0.1
the profile (100–150 cm) (Fig. 3a and c). Furthermore, in the S1, actual 150 2.7 5.0 0.0 2.0 0.1
acidity was absent from 90 cm to 123 cm depth due to the pHKCl being >
Fig. 3. Variation of the titratable actual acidity (TAA), retained acidity (RA), and potential sulfidic acidity (PSA) in three soil profiles down to 1.5 m (a) S1, (b) S2,
and (c) S3.
6C.L. Vithana et al. Catena 216 (2022) 106396
acidity (0–60 mol H+ t− 1) compared with the layers below (Table 2). In quartz was notable throughout the soil profile and pyrite and jarosite
the S1, chromium reducible sulfur content increased from 0.5 to 3.8% were the key sulfidic and sulfuric soil materials (Fig. 5c). There, pyrite
from 30 to 90 cm (Table 2), with corresponding increase in the potential was present from 48 cm downwards, however, the small peaks indicate
sulfidic acidity from 319 mol H+ t− 1 to 2389 mol H+ t− 1 (Fig. 3a). After lower pyrite content compared with S1 and S2 (Fig. 5c).
90 cm, the chromium reducible sulfur content decreased gradually to
2.6% at 150 cm (Table 2). In the S2, the chromium reducible sulfur 4. Discussion
content remained stable at 2.0% up to 100 cm and below 134 cm
(Table 2 and Fig. 3b). The average potential sulfidic acidity in the cor 4.1. Soil properties at three sites
responding soil profile from 50 cm to 150 cm was 1230 mol H+ t− 1 ±
152. The chromium reducible sulfur content in the S3 from 30 to 100 cm As a result of progressive infilling and peat formation, variability in
was comparatively lower ( 0.01 % chromium reducible sulfur; Sullivan et al., 2010;
Jarosite, a secondary iron mineral formed from pyrite oxidation was IUSS Working Group, WRB 2014) and being within the range of those
noted only in soil profiles of S2 and S3 from sub-surface to bottom found in acid sulfate soil landscapes in other tropical countries (Table 3).
(Fig. 5b and c). In the S1, pyrite was present from 30 cm downwards and The laterite on the eastern border and the saltwater are the main sources
the concentration increased towards the bottom of the profile (Fig. 5a). of iron and sulfur for the formation of sulfidic soil materials (Dis
The first 50 cm of the S1 was rich in clay minerals (possibly kaolinite) sanayake, 1987; Senaratne, 1987; Pitawala et al., 1994). In all three
and gradually decreased towards the bottom of the profile, while a few sites, sulfidic soil materials (i.e. pyritic) were closely associated with a
calcite peaks were also observed in the bottom depth (Fig. 5a). Similar to clayey texture as shown in Figure S2.
the S1, pyrite was the only identified sulfidic mineral present in the S2 Soils below the surface in the S1 were unoxidized and consisted of
that was present below 30 cm with the highest concentration between ‘hyper-sulfidic’ soil materials (Table 1 and Fig. 2a) (IUSS Working
100 and 115 cm depth (Fig. 5b). Jarosite was the main sulfuric soil Group, WRB 2014; Sullivan et al., 2018b). The unoxidized nature of S1
material observed from 30 cm downwards (Fig. 5b). The sharp and can be further supported by the presence of pyrite peaks and the absence
abundant quartz peaks in the corresponding XRD spectra indicated of secondary iron mineral peaks in the corresponding XRD spectrum
higher quartz content compared to the S1 and S3 (Fig. 5b). In the S3, (Fig. 5a). With the increase in clay content towards the bottom and lack
Fig. 4. Net acidity (NA) in three soil profiles down to 1.5 m (a) S1, (b) S2, and (c) S3. Error bars are standard deviations of three replicates.
7C.L. Vithana et al. Catena 216 (2022) 106396
Fig. 5. Bulk mineralogy in three soil profiles down to 1.5 m (a) S1, (b) S2, and (c) S3 P-pyrite, Q-quartz, J-jarosite, K-kaolinite, G-gibbsite, F-feldspar, C-calcite.
Table 3
Sulfide and organic matter contents in acid sulfate soils in other tropical countries.
Location Soil profiles depth Landscape type Sulfide content Organic matter Reference
(m) (%) (%)
Sri Lanka 1.5 Saline peat bog 0.1–3.8 8.1–59.0 Present study
Central Kalimantan, Indonesia 1.4 Backswamp area of three rivers 2.6–5.2# 5.7–40.9 Anda et al., 2009
Negara Brunei Darussalam on north-west coast 2 Tidal rivers in mangroves and peat 3.44 1.7–41.4 Grealish and Fitzpatrick,
of Borneo areas 2013
North east part of Mekong Delta, Vietnam 3.5 and 7.0 Inland wetland in the Mekong 2.0–5.0 N.A Gröger et al., 2011
River Delta
East Trinity, Northern Australia 1.5 Degraded acid sulfate soil area 1.92 1.7–10.3 Johnston et al., 2009
Thailand-South East Coast 2 Swamp forest and delta 0.082–1.9* 6.7 Sukitprapanon et al., 2016
N.A. Not vailable.
*Pyrite was measured as peroxide oxidisable sulfur (SPOS) following SPOCAS method indicative of reduced sulfur.
#
Pyrite measured through the oxidation with HNO3 and extraction with HCl.
of soil structural development, the soil profile would have become less in the S2 were ‘hyper-sulfidic’ (Table 1 and Fig. 2b) (IUSS Working
permeable which resulted in limited oxygen diffusion down the soil Group, WRB 2014). In the S2, the partial oxidation of chromium
profile. Furthermore, the anoxic and reducing conditions throughout the reducible sulfur occurred from the surface to the bottom of the profile, as
soil profile were maintained by the thick grass cover on the surface and evident from the occurrence of net acid-soluble sulfur throughout the
the water-logged conditions. soil profile (Table 2 and Figure S2). However, the net acid-soluble sulfur
Both S2 and S3 contained partially oxidized soils indicating the content was lower than the chromium reducible sulfur in each depth and
occurrence of both sulfidic and sulfuric soil materials (IUSS Working was slightly decreased down the soil profile suggesting a lower rate of
Group, WRB 2014). Similar to the S1, the sulfidic soil materials present pyrite oxidation in corresponding depths compared to the surface. The
8C.L. Vithana et al. Catena 216 (2022) 106396
low oxygen diffusion towards the bottom was likely due to the lack of hydrolysis of iron hydroxysulfate minerals such as jarosite is likely to be
soil structural development and lack of organic matter (Table 1). discharged to the surrounding environment via runoff or canals or could
Soils in the S3 were mostly ‘hyper-sulfidic’ except the depth from 82 infiltrate into the groundwater.
to 97 cm which was ‘hyposulfidic’ as the pHINC was between 4.0 and 6.5 The acidification risk in S1 is low as it is devoid of existing acidity
(pHINC was 4.03 and 4.63 respectively) by the end of 19 weeks incu except for a negligible amount of actual acidity (Fig. 3a). However, S1
bation (IUSS Working Group, WRB 2014). The surface and sub-surface contains potential acidity that could be mobilized and released in the
depths in S3 contained lower chromium reducible sulfur content event of disturbance of pyrite/sulfidic soil materials. During the titrat
(100 cm) which was mainly attributed able actual acidity analysis, the 78 to 123 cm depth showed an occur
to lower clay contents (Table 2 and Figure S2). However, towards the rence of buffering capacity as the pHKCl was > 6.5 (Table 2). This could
bottom the clay content increased (Table 1), and the chromium reduc be due to the effect of small shells and shell fragments (C.L. Vithana et al. Catena 216 (2022) 106396
a rainfall event as it was devoid of immediately available and stored Combatt Caballero, E., Jarma Orozco, A., Palencia Luna, M., 2019. Modeling the
Requirements of Agricultural Limestone in Acid Sulfate Soils of Brazil and Colombia.
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Brunei Darussalam: A case study to inform management decisions. Soil Use Manag.
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people living in these areas are unknown and need further study. The Gröger, J., Proske, U., Hanebuth, T.J.J., Hamer, K., 2011. Cycling of trace metals and
climate change-driven sea-level rise and severe weather events, such as rare earth elements (REE) in acid sulfate soils in the Plain of Reeds. Vietnam.
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The acidity in the marsh is likely to be buffered by seawater and this IUSS Working Group WRB., 2014. World reference base for soil resources 2014. World
process warrants further investigation. Furthermore, the impact of Soil Res. Report 106. FAO, Rome.
Johnston, S.G., Keene, A.F., Bush, R.T., Burton, E.D., Sullivan, L.A., Smith, D.,
seawater intrusion on soil and groundwater geochemistry of the marsh is McElnea, A.E., Martens, M.A., Wilbraham, S., 2009. Contemporary pedogenesis of
another aspect in need of study. severely degraded tropical acid sulfate soils after introduction of regular tidal
inundation. Geoderma 149 (3-4), 335–346.
Johnston, S.G., Burton, E.D., Bush, R.T., Keene, A.F., Sullivan, L.A., Smith, D.,
Declaration of Competing Interest McElnea, A.E., Ahern, C.R., Powell, B., 2010. Abundance and fractionation of Al, Fe
and trace metals following tidal inundation of a tropical acid sulfate soil. Appl.
Geochem. 25 (3), 323–335.
The authors declare the following financial interests/personal re Kinsela, A.S., Melville, M.D., 2004. Mechanisms of acid sulfate soil oxidation and
lationships which may be considered as potential competing interests: leaching under sugarcane cropping. Aust. J. Soil Res. 42 (6), 569–578.
CHAMINDRA VITHANA reports financial support was provided by Na Ljung, K., Maley, F., Cook, A., Weinstein, P., 2009. Acid sulfate soils and human health-A
millennium ecosystem assessment. Environmental International 35 (8), 1234–1242.
tional Science Foundation Sri Lanka. Mendonça, S.K.G., et al., 2021. Occurrence and pedogenesis of acid sulfate soils in
northeastern Brazil. CATENA 196, 104937.
Acknowledgement Mosley, L.M., Fitzpatrick, R.W., Palmer, D., Leyden, E., Shand, P., 2014. Changes in
acidity and metal geochemistry in soils, groundwater, drain and river water in the
Lower Murray River after a severe drought. Sci. Total Environ. 485–486, 281–291.
This project was funded by National Science Foundation (NSF), Sri Pitawala, H.M.T.G.A., Senaratne, A., Dahanayake, K., 1994. Sulphate-induced high
Lanka (Grant No. NSF/PDRS/2018/01). We thank Mr. Sachintha Sen acidity in a peatland-An example from Muthurajawela. Journal of National Science
Council, Sri Lanka 22 (4), 339–349.
arathne, Mr. Pasan Hewavitharana, Mr. Prasanna Wijerathna, and Mr. Salehi, M.H., Beni, O.H., Harchegani, H.B., Borujeni, I.E., Motaghian, H.R., 2011.
Chinthaka for their assistance in field sampling. Refining Soil Organic Matter Determination by Loss-on-Ignition. Pedosphere 21 (4),
473–482.
Schaetzl, R.A., Anderson, S., 2005. Soils: Genesis and Morphology. Cambridge University
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Senaratne, A., 1987. Geomorphological and geochemical evolution of the western coast
Supplementary data to this article can be found online at https://doi. of Sri Lanka with special reference to the Mannar lagoon and the Muthurajawela
swamp. University of Mainz, p. 206 pp.. Unpublished PhD Thesis.
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Senaratne, A., Dissanayake, C., 1991. Palaeo-environmental significance of some major
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