A comparison of potential, active and post-active acid sulfate soils in Thailand

Geoderma Regional 7 (2016) 346–356

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A comparison of potential, active and post-active acid sulfate soils in Thailand Tanabhatsakorn Sukitprapanon a, Anchalee Suddhiprakarn a,⁎, Irb Kheoruenromne a, Somchai Anusontpornperm a, Robert J. Gilkes b a b

Department of Soil Science, Faculty of Agriculture, Kasetsart University, Bangkok 10900, Thailand School of Earth and Environment, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

a r t i c l e

i n f o

Article history: Received 23 August 2015 Received in revised form 10 August 2016 Accepted 12 August 2016 Available online 13 August 2016 Keywords: Potential, active and post-active acid sulfate soils Mineralogy Geochemistry Thailand

a b s t r a c t Acid sulfate soils in Thailand have been modified by drainage, liming, irrigated agriculture and paddy rice growing over a long period of time generating potential (PASS), active (AASS) and post-active (PAASS) acid sulfate soils which have different soil properties and extents of soil development. This research compares Thai PASS, AASS and PAASS using field observations, mineralogy and chemical properties. Eighteen acid sulfate soil profiles, representing PASS, AASS and PAASS located in the Lower Central Plain and the Southeast Coast regions, were investigated. All three types of acid sulfate soils (ASS) contain a reduced layer below the water table. The soils are dominated by quartz, feldspars, kaolinite and illite with the Southeast Coast ASS being more sandy than the Lower Central Plain ASS. The labile minerals pyrite, jarosite, goethite, hematite, gypsum and halite are present in variously colored redox concentrations and have resulted from variations in drainage and oxidation status induced by land management including addition of lime. Mineralogical and geochemical properties of PASS, AASS and PAASS mostly reflect the nature of parent materials, but the ASS can be discriminated by the presence of labile minerals and by the labile chemical properties pH, sulfur content, organic carbon, electrical conductivity, actual acidity and total acidity. This research has clearly identified the labile properties that reflect the transformation from potential through active and into post-active acid sulfate soils. In addition, the transformation from PASS to fully oxidized PAASS is not a simple one direction oxidation process in ASS which have experienced cyclical freshwater-reflooding for rice cultivation. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Acid sulfate soils (ASS) occupy approximately 17 million hectares (Andriesse and van Mensvoort, 2006) and are mostly situated in coastal plains in the Tropics and less commonly in temperate regions (Dent, 1986). ASS also occur in inland environments in lakes, wetlands and stream channels (Fitzpatrick and Shand, 2008). Thai ASS have developed in Holocene sediments in the Lower Central Plain with small areas located in the Southeast Coast and Peninsular regions (Sinsakul, 2000; Land Development Department, 2006; Janjirawuttikul et al., 2010). Janjirawuttikul et al. (2010) reported that ASS were distributed in the deltaic plain which is occupied by tidal flats and swamp sediments deposited in the middle-late Holocene. The whole area was originally occupied by swamps with potential acid sulfate soils, but the area has been developed over one hundred and forty years under management that includes drainage, liming,

irrigated agriculture and paddy rice growing (Attanandana and Vacharotayan, 1986) (Fig. 1). Acid sulfate soils in other countries have also been drained, but the land uses are often different from those in Thailand. For example, ASS are drained in order to lower the water table for pasture and sugarcane in Australia and pasture in Finland (Tighe et al., 2005; Boman et al., 2010; Yvanes-Giuliani et al., 2014), for oil palm in Malaysia (Auxtero and Shamshuddin, 1991) and for construction and pasture in the United States (Ross et al., 1988; Fanning et al., 2004). In contrast, paddy rice growing involves reflooding of the reclaimed ASS every year which induces particular changes in ASS properties. Under an undisturbed waterlogged anaerobic environment, soils affected by sulfidization and containing sulfide minerals near the soil surface are defined as potential acid sulfate soils (PASS) (Fanning, 2012). Pyrite is formed by the following reaction: Fe2 O3 þ 4SO4 2− þ 8CH2 O þ 1=2O2 →2FeS2 þ 8HCO3 − þ 4H2 O þ

2FeS2 þ 15=2O2 þ 7H2 O→2FeðOHÞ3 þ 8H þ 4SO4 ⁎ Corresponding author. E-mail address: [email protected] (A. Suddhiprakarn).

http://dx.doi.org/10.1016/j.geodrs.2016.08.001 2352-0094/© 2016 Elsevier B.V. All rights reserved.

ð1Þ ð2Þ

In general, iron sulfides in PASS comprise approximately 1–4% of dry weight (Gröger et al., 2011) and consist of pyrite (FeS2), mackinawite

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Fig. 1. Sampling locations for Thai acid sulfate soils in the Lower Central Plain and the Southeast Coast regions.

(FeS) and greigite (Fe3S4) (Fanning et al., 2010; Prakongkep et al., 2012) formed via reduction of iron oxides as a source of Fe (Eq. (1)) (Dent, 1986). Elemental sulfur may also be present (Burton et al., 2006a; Prakongkep et al., 2012). If these sulfur minerals are exposed to oxic conditions by natural or man-made drainage, sulfuric acid is produced (Eq. (2)) and soil pH will become ultralow (b 4) where there is insufficient neutralization, creating active acid sulfate soils (AASS) (Fanning, 2012). This extreme acidity has adverse effects on agriculture, aquaculture and environment in ASS landscapes (Ljung et al., 2009; Sullivan et al., 2012). When weathering and pedogenesis in AASS have advanced to a stage where sulfide minerals are no longer present near the surface of the soil and where pH has risen above 4, which is commonly due to liming and drainage, the soils are described as post-active acid sulfate soils (PAASS) (Fanning, 2012). At the time of sedimentation, PASS were uniform in their composition and properties. After PASS have experienced drainage, liming, fertilizers and paddy rice cultivation with periodic flooding they transform to AASS and eventually into PAASS which are considered as the mature stage. In the future, PAASS will become the dominant form of ASS in Thailand and elsewhere in the region due to the high productivity of these soils, particularly under irrigated agriculture. This publication identified changes in soil properties due to this particular management. Therefore, the research aim was to compare PASS, AASS and PAASS on the basis of field observations, mineralogical and chemical properties so as to identify affinities and differences among the three types of ASS.

2. Materials and methods 2.1. Soil sampling Eighteen study sites were examined, these represent potential, active and post-active ASS in the Central Plain and the Southeast Coast regions of Thailand. The soils have developed under tropical savanna and tropical monsoon climates, respectively (Fig. 1). PASS for this study were collected from the Southeast Coast as it was impossible to collect PASS in the Lower Central Plain because fish and shrimp ponds and paddy rice have replaced the ASS formerly occupying mangrove swamps on the Lower Central Plain. Large areas adjacent to the Chao Phraya River now support extensive rice cultivation with much drainage and agricultural development occurring between 1870 and 1889 (Fig. 1) (Attanandana and Vacharotayan, 1986). The PASS are waterlogged soils covered by swamp forest of mangrove and nipa (Table 1). The AASS and PAASS study sites have been drained for agricultural activities, particularly growth of paddy rice which involves several periods of flooding of soil profiles each year. The PAASS have been managed under this regime for more than a century whereas drainage of AASS has occurred more recently. Acid sulfate soils were collected in the dry season during October 2012 to January 2013. They were collect by hand auger to a depth of 2 m where reduced original sediments occurred in every case. The water table of AASS and PAASS was situated at approximately 80 to 100 cm depth.

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Table 1 Site descriptions for Thai acid sulfate soils investigated in this research. ASS typea

Locationb

Classification

Landform

Elevation (m) MSLc

Textured

Land use

PASS1 PASS2 PASS3 PASS4 PASS5 AASS1 AASS2 AASS3 AASS4 AASS5 AASS6 AASS7 AASS8 PAASS1 PAASS2 PAASS3 PAASS4 PAASS5

SC SC SC SC SC LCP LCP LCP LCP SC SC SC SC LCP LCP LCP LCP LCP

Typic Sulfaquent Typic Sulfaquent Typic Sulfaquent Typic Sulfaquent Typic Sulfaquent Hydraquentic Sulfaquept Hydraquentic Sulfaquept Hydraquentic Sulfaquept Hydraquentic Sulfaquept Hydraquentic Sulfaquept Hydraquentic Sulfaquept Hydraquentic Sulfaquept Hydraquentic Sulfaquept Sulfic Endoaquept Sulfic Endoaquept Sulfic Endoaquept Sulfic Endoaquept Sulfic Endoaquept

Swamp Swamp Swamp Delta Delta Floodplain Floodplain Floodplain Floodplain Floodplain Floodplain Floodplain Floodplain Floodplain Floodplain Floodplain Floodplain Floodplain

9 1 6 8 2 6 3 3 2 1 7 5 2 6 8 3 4 11

SCL C SCL SCL L C SiCL SiC C C C L SL C C SiCL C C

Swamp forest Swamp forest Swamp forest Mangrove, nipa Mangrove forest Paddy rice Paddy rice Paddy rice Weeds Paddy rice Swamp forest Paddy rice Paddy rice Paddy rice Paddy rice Paddy rice Paddy rice Paddy rice

a b c d

PASS = potential acid sulfate soils; AASS = active acid sulfate soils; PAASS = post-active acid sulfate soils. SC = the Southeast Coast; LCP = the Lower Central Plain. MSL = mean sea level. SL = sandy loam, SCL = sandy clay loam, SiCL = silty clay loam, SiC = silt clay, L = loam, C = clay.

Soil profiles were described and classified in Soil Taxonomy (Soil Survey Staff, 2014). Soil samples were packed in sealed plastic containers and immediately cooled to 4 °C in order to prevent the oxidation of sulfidic materials. Various types of redox concentration were handpicked from soil samples and a few drops of chloroform were added to suppress microbial activity. These hand-picked samples were kept at a low temperature until they were analyzed by X-ray diffraction (XRD). Based on field observations, mineralogical and chemical properties, these Thai ASS may be divided into 3 types: PASS, AASS and PAASS. The classification of these three types of soils in this research is based on the definition by Fanning (2012). PASS are anaerobic soils and containing sulfide minerals near the soil surface. AASS contain yellow redox concentrations of jarosite and sulfuric material (pH b 4) within 50 cm of the soil surface. PAASS are recognized when the pH has risen above 4 and red redox concentrations of hematite occur within 50 cm of the soil surface. The soil profiles could be divided into 3 key layers: topsoil, partly oxidized and waterlogged reduced layers with only topsoil and reduced layers occurring in PASS. This study uses the term partly oxidized layer because this layer contains residual peroxide-oxidizable sulfur (SPOS) (median 0.037 and 0.013% for AASS and PAASS, respectively) (Table 3). 2.2. Analytical methods 2.2.1. Physicochemical properties Physicochemical properties measured on field-moist soil samples are reported on an oven-dried (105 °C) basis. Particle size distribution was determined by the pipette method (Gee and Bauder, 1986). Soil color was determined on field-moist soil samples using a Munsell Soil Color Chart (Munsell Color, 2000). The redox potential (Eh) of PASS and the reduced layer of AASS and PAASS was measured with a calibrated electrode relative to a standard hydrogen electrode, the reading in mV was recorded when a steady state was obtained. The redox potential of topsoil and partly oxidized layers of AASS and PAASS was not measured because they had been partly oxidized by drainage. Soil pH and electrical conductivity (EC) of saturated paste were measured within 24 h of collection from the field. pH H2O was measured in water (1:1 soil to water) (National Soil Survey Center, 2004). pH H2O2 was determined in 30% hydrogen peroxide solution, adjusted to pH 5.5 (1:5 soil to solution) (Ahern et al., 2004). Total organic carbon (OC) was

determined by sample combustion on an Elementar CN analyzer (Elementar, Vario Macro). Titratable actual acidity (TAA), titratable peroxide acidity (TPA) and peroxide-oxidizable sulfur (SPOS) which is the sulfate produced from the oxidation of reduced-inorganic sulfur during the peroxide oxidation were determined by the complete suspension peroxide oxidation combined acidity and sulfur (SPOCAS) method (Ahern et al., 2004). The total elemental concentrations in bulk soil samples were determined by a combination of X-ray fluorescence spectrometry (XRF) of fused lithium metatetraborate discs for Al, Ca, Fe, K, Mg, Mn, Na, P, Si, Ti and Zr (Norrish and Hutton, 1969) and by inductively coupled plasma-optical emission spectrometry (ICP-OES, Perkin-Elmer Optima 7300 DV) of aqua regia digests (3:1 HCl:HNO3 at 130 °C for 1 h, APHA, 1998) for As, Ba, Be, Bi, Ce, Co, Cr, Cu, Gd, La, Mo, Nd, Ni, Pb, Rb, S, Sc, Sr, Th, V, Y and Zn. Data were assessed for accuracy and precision using a rigorous control system included reagent blanks and certified reference materials (STSD 3) for ICP-OES analysis (Lynch, 1999) and OREAS 43P for XRF analysis (Ore Research and Exploration Pty Ltd., 1997). The Fe and Al in poorly-crystalline and crystalline oxide minerals were determined using ammonium oxalate and dithionite citrate bicarbonate extractions, respectively (Rayment and Lyons, 2010). Sodium, Cl and SO4 in porewater were extracted with Milli-Q water (1:2.5 soil to solution) (Frankenberger et al., 1996). Soluble Na was determined by ICP-AES and soluble Cl and SO4 were determined by ion chromatography (Lachat instruments Model 8500 Quikchem series 2).

2.2.2. Mineralogical properties XRD analysis of hand-picked redox concentrations and efflorescences was performed on randomly oriented powder samples on a low background holder. Patterns were obtained between 3 and 70° 2θ with 0.02° 2θ step size and a scan speed of 0.02° 2θ per second using CuKα radiation. The 110 reflection of goethite was used for calculating aluminum substitution using the Vegard line (mol% Al = − 532.26d + 2224.8), where d is the spacing of the 110 reflection of goethite (Brindley and Brown, 1980). Substitution of Al in hematite was determined from the a dimension of the hematite unit cell obtained from the 110 reflection: mol% Al = 3109–617.1a (Schwertmann et al., 1979). Mean crystallite dimension was calculated from diffraction line broadening (Klug and Alexander, 1974).

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2.2.3. Statistical analysis Principal component analysis (PCA) was used to determine elements of similar geochemical behavior and to group soil samples on the basis of their geochemical affinity (Bellehumeur et al., 1994). In this study, all data were log transformed to meet the requirement of normality for regression analysis (P b 0.05) and PCA carried out using Statistica software version 8.0 for Windows (StatSoft Inc., 2007). 3. Results and discussion 3.1. Soil profile characteristics 3.1.1. Potential acid sulfate soils Potential acid sulfate soils occur within a few kilometers from the coast (Fig. 1), the elevation of these soils varies between 1 and 9 m above MSL (Table 1). Organic matter rich sediments have been inundated with estuarine water containing sulfate which has been reduced to sulfide (Eq. (1)) (Dent and Pons, 1995). The soils are Typic Sulfaquents in Soil Taxonomy (Soil Survey Staff, 2014) (Table 1). Two meter deep profiles were sampled by auger and consist of topsoil and a reduced subsoil layer. The PASS consist of dark greenish gray (Gley1 4/10 Y) sandy clay loam to dark grayish brown (2.5Y 4/2) clay (Tables 1, 2) with various amounts of organic carbon (median 39 and 23 g kg−1 for topsoil and reduced layers, respectively) (Table 3). In some PASS profiles, the 0–20 cm depth topsoil has been partly oxidized because of lowering of the water table, consequently, the soil is dark grayish brown (2.5Y 4/2) with reddish (2.5YR 3/6) and yellowish redox concentrations (2.5Y 6/8) present in root zones. Live and decayed roots are present in both layers. pH H2O values are near-neutral with lower values in the topsoil layer (range 3.5–7.0) (Table 3). pH H2O2 is much lower than pH H2O throughout the soil profile due to H2O2 oxidizing pyrite, other sulfur minerals and organic matter. PASS profiles generally have reducing conditions (Eh 100 to −100 mV) except for the topsoil layer (N100 mV) (Table 2). The median value of actual acidity (titratable actual acidity, TAA) of PASS is much lower in the topsoil layer (15 mol H+ t−1) than in the reduced layer (68 mol H+ t−1) (Table 3) due to downward diffusion and

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leaching of soluble products from pyrite oxidation and hydrolysis of jarosite by rain and tidal flooding (van Oploo, 2000) and/or the lower content of iron sulfides (SPOS) in the topsoil (median 0.082%) than in the reduced layer (median 1.9%) (Table 3). Total acidity (titratable peroxide acidity, TPA) ranges from 10 to 3852 mol H+ t−1, the higher values occurring in the reduced layer (Table 3). Electrical conductivity (EC) ranges from 1.1 to 74 dS m−1. Soluble Na and Cl in porewater are high throughout the PASS profiles because of residual seawater. The median SO4:Cl ratio values are 0.31 and 0.48 for the topsoil and reduced layers, respectively. Ratios are lower than 0.50 indicating that sulfate originates mostly from residual seawater (Mulvey, 1993).

3.1.2. Active acid sulfate soils Active acid sulfate soil profiles were collected from the Lower Central Plain (AASS1-4) and the Southeast Coast regions (AASS5-8) (Fig. 1). They contain all three layers and were classified as Hydraquentic Sulfaquepts (Table 1). These soils are located about 1 to 7 m above MSL (Table 1) and have been drained within relatively recent times into a network of canals. They have been used for irrigated agriculture, excluding AASS6 which is a former wetland drained by local people and is under lowland forest. Soil texture ranges from sandy loam to clay. The topsoil layer (0–20 cm thick) has a black color (2.5Y 2.5/1) with light olive brown redox concentrations (2.5Y 5/3). The underlying partly oxidized layer (20–170 cm) consists of olive yellow (2.5Y 6/8) and yellow (2.5Y 7/8) redox concentrations in a brown and gray matrix; i.e. very dark gray (2.5Y 3/1), light olive brown (2.5Y 5/3). A grayish (2.5Y 4/1) reduced layer exists below the partly oxidized layer and is below the water table. The median pH H2O value of the topsoil layer is 3.6 (range 3.2–5.5), pH H2O is similar in the underlying partly oxidized layers with a median value of 3.7 (range 3.2–4.5) and increases in the reduced layer with a median value of 4.7 (range 3.5–5.9). Values of pH H2O2 are much lower than pH H2O for all soil samples because of the oxidation in H2O2 of iron sulfides, sulfur and organic matter. The topsoil layer is richer in OC (median 27 g kg−1) than the partly oxidized (median

Table 2 Morphology of representative Thai potential, active and post-active acid sulfate soils. Typea

Layer (symbol)

Depth (cm)b

Boundary

Color (Munsell)

%

pH H2O

pH H2O2

Eh mV

PASS5

Topsoil Reduced

(Ag) (Cse)

Abrupt

(Ap)

0–20 20–50 50–82 82–200 0–20

2.6 2.0 1.7 1.7 1.9

256 −159 −154 −100 –

20–50

3.3

1.9

–

(Bjyg2)

50–75

3.3

1.9

–

(Bjyg3)

75–170

100 100 100 100 70 30 40 35 25 40 35 25 75 25 100 75 25 55 35 10 40 35 25 70 30 100

6.0 6.5 6.3 7.0 3.2

(Bjyg1)

2.5Y 4/2 (dark grayish brown) 2.5Y 4/2 (dark grayish brown) Gley1 4/10Y (dark greenish gray) Gley1 4/10Y (dark greenish gray) 2.5Y 2.5/1 (black) 2.5Y 5/3 (light olive brown) 2.5Y 3/1 (very dark gray) 2.5Y 6/2 (light brownish gray) 2.5Y 6/8 (olive yellow) 2.5Y 5/3 (light olive brown) 2.5Y 4/2 (dark grayish brown) 2.5Y 7/8 (yellow) 2.5Y 5/2 (grayish brown) 2.5Y 7/8 (yellow) 2.5Y 4/1 (dark gray) 10YR 3/2 (very dark grayish brown) 5YR 4/4 (reddish brown) 2.5Y 6/1 (gray) 10R 3/6 (dark red) 7.5YR 6/8 (reddish yellow) 2.5Y 5/1 (reddish gray) 10R 3/4 (dusky red) 10R 4/8 (red) 10R 4/2 (weak red) 2.5Y 8/8 (yellow) 2.5Y 4/1 (dark gray)

AASS2

Topsoil Partly oxidized

3.6

2.0

–

3.5 5.2

1.5 2.4

−203 –

4.2

2.9

–

3.8

2.4

–

3.8

2.4

–

4.1

1.7

−73

Abrupt

Abrupt PAASS3

Reduced Topsoil

(Cse) (Ap)

170–200 0–17

Partly oxidized

(Byg1)

17–50

(Byg2)

50–90

(Bjyg)

90–170

Abrupt

Abrupt Reduced a b

(Cse)

170–200

PASS = potential acid sulfate soil; AASS = active acid sulfate soil; PAASS = post-active acid sulfate soil. Reduced layers in PASS, AASS and PAASS are situated below water table which is about 80–100 cm.

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Table 3 Median and range (in parenthesis) of labile chemical properties of topsoil, partly oxidized and reduced layers of PASS, AASS and PAASS. Propertya

pH H2O pH H2O2 OC (g kg−1) EC (dS m−1) Soluble Na (mg kg−1) Soluble Cl (mg kg−1) Soluble SO4 (mg kg−1) SO4:Cl Total S (%) SPOS (%) TAA (mol H+ t−1) TPA (mol H+ t−1) a b c d

PASSb (n = 25)

AASSc (n = 40)

PAASSd (n = 25)

Topsoil

Reduced

Topsoil

Partly oxidized

Reduced

Topsoil

Partly oxidized

Reduced

n=5

n = 20

n=8

n = 24

n=8

n=5

n = 16

n=4

5.1 (3.6–6.0) 2.3 (1.9–2.6) 39 (18–45) 26 (1.7–74) 2581 (94–10,096) 1922 (885–4784) 841 (310–4218) 0.31 (0.17–2.2) 0.15 (0.14–0.33) 0.082 (0.064–0.16) 15 (4.2–18) 96 (10−133)

5.8 (3.5–7.0) 1.7 (1.1–2.5) 23 (8.3–66) 29 (1.1–59) 2693 (70–7504) 3493 (408–6183) 1484 (101–5560) 0.48 (0.15–5.9) 1.5 (0.12–6.9) 1.9 (0.089–5.7) 68 (0.97–350) 985 (27–3852)

3.6 (3.2–5.5) 2.1 (1.6–2.4) 27 (15–54) 4.8 (1.1–13) 568 (64–2646) 1117 (14–2712) 588 (234–1772) 1.6 (0.09–34) 0.15 (0.10–1.1) 0.062 (0.033–0.17) 53 (28–235) 157 (65–257)

3.7 (3.2–4.5) 2.3 (1.8–2.6) 7.5 (3.2–23) 7.5 (1.1–20) 711 (68–3463) 1022 (12−3003) 701 (315–4005) 1.4 (0.25–69) 0.20 (0.069–0.88) 0.037 (0.003–0.32) 79 (19–256) 101 (33−301)

4.7 (3.5–5.9) 2.3 (1.4–3.4) 9.1 (1.5–20) 12 (2.8–28) 1374 (115–4866) 1795 (32–3605) 1197 (404–6222) 1.3 (0.21–29) 0.45 (0.048–1.7) 0.43 (0.001–1.8) 39 (8.9–351) 283 (41–1536)

4.5 (4.0–5.7) 2.4 (2.1–2.7) 24 (13–27) 1.8 (0.99–4.3) 159 (72–424) 47 (21–279) 359 (187–530) 8.4 (1.5–13) 0.093 (0.069–0.11) 0.034 (0.017–0.061) 26 (20−100) 35 (4.0–144)

4.1 (3.1–6.4) 2.4 (2.1–5.1) 4.9 (2.1–14) 2.4 (0.92–4.9) 225 (89–652) 98 (17–353) 438 (186–1034) 4.5 (2.4–49) 0.11 (0.043–1.0) 0.013 (0.001–0.082) 75 (3.9–281) 92 (1.8–225)

5.0 (4.1–6.8) 1.8 (1.5–2.0) 14 (12–18) 9.2 (6.7–11) 621 (389–798) 111 (17–189) 904 (81–1859) 6.4 (2.1–76) 0.82 (0.70–1.4) 1.2 (0.5–1.4) 101 (5.1–264) 628 (28–1161)

OC = organic carbon; EC = electrical conductivity; SPOS = peroxide-oxidisable sulfur; TAA = titratable actual acidity; TPA = titratable peroxide acidity. PASS = potential acid sulfate soils. AASS = active acid sulfate soils. PAASS = post-active acid sulfate soils.

7.5 g kg−1) and in the reduced layers (median 9.1 g kg−1) due to inputs of agricultural residues to the topsoil. The TAA of the partly oxidized layer (79 mol H+ t−1) is higher than for the topsoil (53 mol H+ t−1) and reduced (39 mol H+ t−1) layers, because jarosite occurs in the partly oxidized layer. TPA values range between 33 and 1536 mol H+ t− 1 with the TPA of the underlying reduced layer being higher than in overlying layers. Total S and SPOS values are also higher in the reduced layer. The median value of total S in the reduced layer is 0.45% and for SPOS it is 0.43%. The median EC values of topsoil, partly oxidized and reduced layers of AASS are 4.8, 7.5 and 12 dS m−1, respectively. Soluble Na, Cl and SO4 values also generally increase downward through soil profiles, ranging between 64 and 4866, 12–3605 and 234–6222 mg kg−1, respectively. The maximum values of these variables occur in the reduced layer of AASS4 indicating the persistent influence of sea water (van Oploo et al., 2008). The median values of SO4:Cl ratio are 1.6, 1.4 and 1.3 for topsoil, partly oxidized and reduced layers, respectively. Values of this ratio are higher than 0.50 indicating that these AASS contain sulfate from both seawater and oxidation of iron sulfides (Mulvey, 1993). 3.1.3. Post-active acid sulfate soils Post-active acid sulfate soils contain all three layers and were classified as Sulfic Endoaquepts (Table 1). They are located about 3 to 11 m above MSL. PAASS are located at the marginal zone of the former delta (Fig. 1) (Hattori, 1972) and range in texture from silty clay to clay. The soils have a dark surface layer (very dark grayish brown (10YR 3/2)) with reddish brown redox concentrations (5YR 4/4) along root zones and cracks. In the partly oxidized layer, the redox concentrations are mainly reddish, e.g. dark red (10R 3/6), reddish yellow (7.5YR 6/8), dusky red (10R 3/4), red (10R 4/8) in a gray to reddish gray matrix extending downward to approximately 100 cm depth. Below 100 cm depth, yellowish redox concentrations (2.5Y 8/8) with red matrix (10R 4/2) occur but a grayish color (2.5Y 4/1) is dominant in the reduced waterlogged layer as for AASS.

pH H2O is higher than 4.0 within 50 cm of the soil surface (Table 2). This is partly due to liming and leaching over many years which has reduced the acidity derived from the oxidation of sulfide and the hydrolysis of jarosite. However, pH H2O of the partly oxidized layer below 50 cm decreases where yellow redox concentrations occur and the pH of the reduced layer increases (median 5.0). The median OC value is 24 g kg− 1 in the topsoil, it decreases considerately in the partly oxidized layer (4.9 g kg − 1 ) and increases in the reduced layer (14 g kg− 1). The median TAA values for the topsoil (26 mol H+ t−1), partly oxidized (75 mol H+ t−1) and reduced (101 mol H+ t−1) layers are lower than those for AASS, which is due to liming and long-term leaching of these soil profiles over more than a century. The TPA of PAASS is lower than that for AASS, except for the reduced layer because this layer still contains sulfidic materials. Although these PAASS have been drained and developed for a long time, the underlying reduced waterlogged layer is identical to the same layer in PASS and AASS. The amounts of total S and SPOS for PAASS range from 0.043 to 1.4% and 0.001 to 1.4%, respectively. The median SPOS value is much lower for the topsoil than the waterlogged reduced layer. The minimum SPOS (0.001%) occurs in the partly oxidized layer of PAASS1 and PAASS4. This indicates that oxidation of iron sulfide is nearly complete in this layer of PAASS. The median EC, soluble Na, Cl and SO4 values increase downward into the reduced layer. The median EC values of PAASS are about one third those of AASS. The median values of SO4:Cl ratio in PAASS are 8.4, 4.5 and 6.4 which are much greater than the critical value of 0.50 (Mulvey, 1993). The PAASS sites are located further inland than the PASS and AASS sites and may have been less affected by seawater at these PAASS locations. Alternatively, leaching of salts has taken place over a long period of time. The SO4 in PAASS is mostly from oxidation of sulfides rather than being soluble salt retained from seawater.

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Table 4 Concentrations of labile minerals in soil matrix and redox concentrations of Thai PASS, AASS and PAASS. ASS typea

Layer (symbol)

PASS

Topsoil Reduced

(Ag) (Cse)

AASS

Topsoil Partly oxidized

(Ap) (Bjyg1) (Bjyg2) (Bjyg3) (Cse) (Ap) (Byg1) (Byg2) (Bjyg) (Cse)

PAASS

Reduced Topsoil Partly oxidized

Reduced

Depth (cm)

Halite

Pyrite

Jarosite

Goethite

Hematite

Gypsum

0–20 20–50 50–82 +82 0–20 20–50 50–75 75–170 170–200 0–17 17–50 50–90 90–170 170–200

xx xx − − − − − − − − − − − −

− x xx xx − − − − xx − − − − xx

− − x − x xx xx xx − − − − xx −

x − − − x x x x − x xx xx xx −

− − − − − − − − − x xxx xxx xx −

− − − − x xxx xx xx − x xxx xx x −

xxx = moderate (20–40%), xx = small (5–20%) and x = trace (b5%). a PASS = potential acid sulfate soils; AASS = active acid sulfate soils; PAASS = post-active acid sulfate soils.

3.2. Mineralogy of potential, active and post-active sulfate soils

3.3. Geochemistry of potential, active and post-active acid sulfate soils

XRD analysis shows that the matrix of all the ASS consists mainly of quartz, feldspars, kaolinite and illite, all these stable minerals have been inherited from the parent sediments. Concentrations of the labile minerals which are characteristic of ASS from the Lower Central Plain and the Southeast coast are listed in Table 4. Acid sulfate soils from the Southeast Coast region are more sandy than those from the Lower Central Plain and consequently contain more quartz and feldspars. These soils are located near the coast (Fig. 1) and contain halite which has originated in seawater. Pyrite occur in small to moderate amounts (5–20%) in PASS and reduced layers of AASS and PAASS. Pyrite has formed under reducing condition where organic matter and dissolved sulfate are abundant (Kraal et al., 2013). Jarosite occurs in trace amounts (b5%) in the reduced layer of PASS at 50–82 cm because of oxidation along root channels and it is present in small amounts in the partly oxidized horizon of AASS and PAASS. Jarosite is produced by oxidation of iron sulfides or may be precipitated directly if Eh, pH and the composition of soil solution are appropriate (Casas et al., 2007). In the topsoil and partly oxidized layers of AASS and PAASS where liming has been common practice, gypsum occurs in the soil matrix and in some yellow and red redox concentrations. Yellow redox concentrations and root zone mottles contain jarosite and goethite. Hematite is a characteristic mineral of red redox concentrations in PAASS, being present in moderate amounts (20–40%) with small amounts (5–20%) of goethite in redox concentrations in the topsoil and partly oxidized layers. Goethite can form from recrystallization of ferrihydrite or schwertmannite (Claff et al., 2011; Johnston et al., 2011) or from hydrolysis of jarosite (Boman et al., 2008). The recent study by Vithana et al. (2015) demonstrates that the transformation of jarosite to goethite is much slower than transformation of schwertmannite which is strongly affected by the hydrological condition in soils. The red redox concentration hematite in these soils is interpreted as having been formed by dehydration of goethite (van Breemen and Harmsen, 1975; Fanning et al., 2010; Gialanella et al., 2010) or by oxidation of iron in soil solution (Ocańa et al., 1995) during the long dry season under paddy rice cultivation under the tropical savanna climate. Consequently, hematite has accumulated and existed in the surface soil of ASS as shown in the well-developed PAASS (Table 2). This is different from the situation investigated by Costantini et al. (2006) where sediments had experienced uplift, weathering and subsequent burial so that the occurrence of hematite was related to the presence of paleosols. In our soils, hematite occurs at/near the soil surface and not in a buried paleosol.

The total elemental concentrations in these Thai ASS are shown in Table 5. The concentrations of heavy metals (As, Cr, Mn and Pb) are lower than the critical soil concentrations for environmental concern (Pollution Control Department, 2004) apart from As (median 9.3 mg kg−1, critical value 3.9 mg kg−1) (Table 5).

Table 5 Median, minimum, maximum values for total element concentrations for Thai acid sulfate soils (n = 90) and Thai critical soil concentrations for land use. Element (g kg Si Al Fe Ti Ca K Mg Na P S

−1

Median

Minimum

Maximum

Thai critical soil concentrationa

290 76 34 4.7 1.6 10 3.0 2.1 0.11 2.8

228 26 3.8 2.6 0.64 3.2 0.46 0.0001 0.004 0.43

408 107 147 9.1 16 19 9.0 15 3.8 69

– – – – – – – – – –

9.3 30 0.80 1.4 nd 32 5.7 85 19 2.3 15 79 2.8 14 11 19 32 7.6 24 8.7 101 9.2 28 218

2.2 4.0 0.065 0.26 nd 4.3 0.86 50 3.7 0.12 1.8 8.2 0.059 2.1 0.10 2.7 1.5 2.2 4.0 0.88 45 0.94 1.9 106

54 233 3.7 3.9 0.23 114 63 220 64 10 47 1214 70 49 65 49 79 17 162 16 284 35 117 468

3.9 – – – – – – 300 – – – 1800 – – – 400 – – – – – – – –

)

(mg kg−1) As Ba Be Bi Cd Ce Co Cr Cu Gd La Mn Mo Nd Ni Pb Rb Sc Sr Th V Y Zn Zr

a Critical concentration for residential and agricultural areas in Thailand (Pollution Control Department, 2004).

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Fig. 2. Principal component analysis based on chemical properties of bulk soil samples of Thai ASS, (a) distribution of chemical properties (variables), (b) distribution of partly oxidized and reduced layers of PASS, AASS and PAASS (cases). PASS = potential acid sulfate soils, AASS = active acid sulfate soils and PAASS = post-active acid sulfate soils. Partly oxidized layers include topsoil and partly oxidized subsoil layers, except for PASS which have only a partly oxidized topsoil layer.

Principal component analysis (PCA) of log total elemental concentration was used to identify affinity groups of elements including texture (Fig. 2). The first two components explain 53% of the variation in data. Attributes could be allocated into two main affinity groups (Figs. 2a, b). The first group consists of sand, Si and Zr and is particularly related to PASS and some AASS sites from the Southeast Coast region where sandy sediments that have been derived from granitic rocks and soils contain abundant sand size quartz, zircon and feldspars (Charusiri et al., 1993). The second group is diffuse consisting of clay, Al, As, Ba, Bi, Ca, Cd, Ce, Co, Cr, Cu, Fe, Gd, K, La, Mg, Mn, Mo, Na, Nd, Ni, P, Pb, Rb, S, Sc, Sr, Th, Ti, V, Y and Zn. Concentrations of the elements in this group are positively related to clay content (P b 0.05). Many of these elements are constituents of clay minerals and oxides, adsorbed by these minerals or present in pyrite (Deng et al., 1998; Toivonen and Österholm, 2011). The topsoil layer of AASS and PAASS contains elevated concentrations of rare earth elements (REEs) relative to other layers. These elements may have been introduced by management practice particularly inputs of phosphate fertilizer and lime which contain elevated level of REEs (Hu et al., 1998; Kabata-Pendias, 2011).

The findings of this study indicate that the major geochemical characteristics of PASS, AASS and PAASS are not affected by oxidation status because the major minerals in these soils (clay minerals, quartz and feldspars) are not affected by redox process. The segregation of samples in Fig. 2b mostly reflects differences in parent materials. A principal component analysis based on labile properties indicates that oxidation and land management, particularly paddy rice cultivation, have affected total S, Fe, Ca, Na, SPOS, pH H2O, pH H2O2, OC, EC, TAA, TPA and labile minerals. The first two components of the PCA explain only 46% of the variation in data because these soils have diverse textures and compositions (Figs. 3a, b). The partly oxidized layer of AASS and PAASS is enriched in hematite, goethite, jarosite, gypsum, total Fe and Ca, while the reduced layer of all three types of soil has elevated amounts of SPOS, total S, TPA, OC, EC, pH H2O, total Na and halite. The topsoil of PASS contains considerable amounts of soluble salt and halite because of the influence of saltwater. The PASS and waterlogged reduced layers of AASS and PAASS have similar labile properties which mainly comprise pyrite, total S, SPOS and OC and are in single group in Fig. 3b. The increase of actual acidity (TAA) and the occurrence of

Fig. 3. Principal component analysis based on labile chemical and mineralogical properties of bulk soil samples of Thai ASS, (a) distribution of chemical and mineralogical properties (variables), (b) distribution of partly oxidized and reduced layers of PASS, AASS and PAASS (cases). PASS = potential acid sulfate soils, AASS = active acid sulfate soils and PAASS = post-active acid sulfate soils. Partly oxidized layers include topsoil and partly oxidized subsoil layers, except for PASS which have only a partly oxidized topsoil layer.

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Fig. 4. Vertical distributions for proportions of total Fe (a) and Al (b) extracted by NH4-oxalate and DCB solutions for representative potential acid sulfate soil (PASS5), active acid sulfate soil (AASS2) and post-active acid sulfate soil (PAASS3).

jarosite in the partly oxidized horizon of AASS and in the deeper part of the partly oxidized zone of PAASS soil profiles indicate that these soil materials are quite similar (Fig. 3b). The PAASS profiles are partly separated from AASS along the factor 2 axis. Separation along the factor 1 axis is due to higher concentration of goethite and hematite in PAASS which reflects greater soil maturity. 3.4. Extractable iron and aluminum in acid sulfate soils 3.4.1. Iron The proportions of total Fe present as NH4 oxalate extractable Fe (Feo) and DCB extractable Fe (Fed) are small and similar in the PASS and waterlogged reduced layers of AASS and PAASS because much free Fe is present in pyrite which is not soluble in DCB solution (Claff et al., 2011) (Figs. 4a, 5a). The proportion of Feo is higher than that of Fed in the deeper part of reduced layer approximately 150 cm from the soil surface of PASS because Fe is present as both poorly-crystalline Fe oxides and oxalate soluble minerals that contain Fe2+ in the reduced layer (Johnston et al., 2009; Claff et al., 2011). The proportion of Fed is

much higher than Feo in the partly oxidized layer in AASS and PAASS, being near 100% indicating that most Fe in this layer is present in the crystalline iron minerals jarosite, goethite and hematite. The topsoil of all three types of soil has only slightly more Fed than Feo. The DCB solution dissolves both poorly-crystalline and crystalline iron minerals, whereas oxalate solution dissolves poorly-crystalline iron minerals (Fig. 4a) (Claff et al., 2010), so that most of the free iron present at the soil surface can be present in poorly-ordered minerals such as ferrihydrite and schwertmannite which were not detected by XRD (Sullivan and Bush, 2004; Burton et al., 2006b). The enrichment of Fe in this horizon in AASS and PAASS is caused by the oxidation-reduction cycling due to soil flooding during paddy rice production. During flooding, Fe2+ is released by the reductive dissolution of Fe oxide minerals in soils, the released Fe2+ diffuses upward toward the topsoil where it is oxidized (Johnston et al., 2011). Also as the soil dries, dissolved Fe2+ is oxidized and precipitated as Fe (hydr)oxides (Sullivan and Bush, 2004; Vithana et al., 2015). Long term management under irrigated paddy rice production has caused cyclical redox transformations of Fe in AASS and PAASS (Thompson et al., 2006).

Fig. 5. Bivariate relationships between (a) total Fe and Fed and (b) Fed-Feo and Ald-Alo for topsoil, partly oxidized and reduced layers of acid sulfate soils.

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Table 6 XRD measurements of Al substitution and mean crystallite dimension in goethite and hematite in representative Thai acid sulfate soils together with a chemical measurement of Al substitution. Typea

Depth (cm)

Al*/ (Al* + Fe*)b

Mottle color

(mol% Al) PASS5 AASS2

PAASS3

a b c

0–20 0–21 21–45 45–75 75–115 17–48

0 0 1 1 0 5

48–65

4

Goethite Al substitution

Yellow Brown Yellow Yellow Yellow Yellow Red Yellow Red

Hematite MCD (110)

(mol% Al)

(nm)

2 2 0 0 0 0 – 0 –

nd nd 9 nd nd 6 – 9 –

c

Al substitution

MCD (104)

(mol% Al)

(nm)c

– – – – – – 4 – 1

– – – – – – nd – 11

PASS = potential acid sulfate soil; AASS = active acid sulfate soil; PAASS = post-active acid sulfate soil. Al* = mol Al calculated from (Ald-Alo)/26.982; Fe⁎ = mol Fe calculated from (Fed-Feo)/55.845. nd = not detected.

3.4.2. Aluminum A small proportion of total Al was dissolved by NH4 oxalate and DCB solutions (Fig. 4b). This result indicates that poorly-crystalline and crystalline Fe oxide minerals are not the major forms of the Al in these soils. Aluminosilicate minerals such as clay minerals and feldspars are the major forms of Al in Thai ASS as discussed in Section 3.2 and these minerals are not soluble in these selective extractants. The proportion of NH4 oxalate extractable Al (Alo) is higher than DCB extractable Al (Ald) in PASS and surface soil horizon and in some of the partly oxidized

layer of PAASS. This result indicates that most of the reactive Al is in poorly-crystalline minerals. The amount of ΔAl (Ald-Alo) has a positive relationship with ΔFe (Fed-Feo) for the partly oxidized layer (R2 = 0.74) (Fig. 5b) which arises through Al substitution in crystalline iron oxides which has been reported elsewhere (Bazilevskaya et al., 2011; Yvanes-Giuliani et al., 2014). The extent of Al substitution in Fe oxide minerals calculated from ΔAl and ΔFe is small being approximately 0–5 mol% Al which is consistent with values of 0–2 and 1–4 mol% Al calculated by XRD line displacement

Fig. 6. Bivariate relationship of peroxide-oxidizable sulfur (SPOS) in relation to organic carbon (OC), total sulfur (total S) and DCB extractable iron (Fed) at (a) (b) (c) potential acid sulfate soils (PASS), (d) (e) (f) active acid sulfate soils (AASS) and (g) (h) (i) post-active acid sulfate soils (PAASS).

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for goethite and hematite (Table 6). These result are much smaller than values up to 30 mol% Al substitution in natural and synthetic goethite (Bazilevskaya et al., 2011) and 2 to 20 mol% Al in iron oxides in ASS from Australia (Yvanes-Giuliani et al., 2014). 3.5. Environmental consequences of potential, active and post-active acid sulfate soils Acid sulfate soils in the Lower Central Plain and the Southeast Coast with a tropical climate have developed in the Holocene epoch as in others countries, but the Thai soils experience a different land use from ASS in Europe, Australia and USA where waterlogging is avoided (Åström, 1998; Tighe et al., 2005; Boman et al., 2010; Claff et al., 2011; Yvanes-Giuliani et al., 2014). ASS on the Lower Central Plain and the Southeast Coast have been drained for agricultural production, especially for paddy rice grown under waterlogged conditions. To reduce acidity and aluminum toxicity, ASS under paddy cultivation of rice have been managed for more than a century with drainage and liming (Attanandana and Vacharotayan, 1986). PASS have passed through an AASS stage and eventually into PAASS which represents a mature or nearly mature stage. Boman et al. (2010) suggests that drained pyrite-rich sediments require 50 to 100 years for the upper 90 cm to become completely oxidized in a temperate climate. However, in a tropical climate under a regime of regular flooding, our data shows that complete oxidation does not occur. The associations between peroxide-oxidizable sulfur (SPOS) and OC and between total S and DCB-extractable Fe (Fed) (reactive Fe), which are considered to be fundamental requirements for iron sulfide formation, can provide some clues regarding controls on iron sulfide formation at these three stages of ASS under paddy rice cultivation (Fig. 6). The relationships show that Fed is weakly negatively related to SPOS for PASS, AASS and PAASS (R2 = 0.12, 0.16 and 0.18, respectively). In contrast, total S is closely related to SPOS in PASS, AASS and PAASS (R2 = 0.87, 0.45 and 0.40, respectively) indicating that the availability of S in soils is more likely to be the major factor affecting iron sulfide accumulation. In addition, SPOS and OC are related in AASS and PAASS (R2 = 0.21 and 0.27, respectively) but are not related in PASS (R2 = 0.023) indicating that a sufficient supply of organic matter electron donor, which originates from organic residues added via agricultural practice on AASS and PAASS, is a crucial control on sulfate reduction and the formation of iron sulfides especially in near surface soil, when the soils are flooded. The study by Johnston et al. (2014) found that the formation of iron sulfides occurred when ASS are reflooded by freshwater. Therefore, the formation of iron sulfides can take place when Thai ASS are reflooded by freshwater during paddy rice cultivation and/or natural flooding. Consequently, the transformation from PASS to fully oxidized PAASS is not a simple one directional oxidation process in ASS which experience cyclical freshwater-reflooding. The chemical composition of these PASS, AASS and PAASS strongly reflects differences in the texture and mineralogy of parent materials which have not been affected by management. However, labile properties including pyrite, iron (hydr)oxides, jarosite, peroxide-oxidizable sulfur and total sulfur clearly identify the effects of drainage on Thai ASS. Iron has been mobilized in these soil profiles, being enriched under oxidizing conditions. The majority of iron accumulates as several crystalline oxide minerals under oxidizing conditions especially in PAASS, while Fe is mostly present in pyrite under reducing conditions. This study concludes that the different development status of PASS, AASS and PAASS is due to changes in labile properties which have been induced by drainage and management practices. Acknowledgments The authors would like to acknowledge the Royal Golden Jubilee Ph.D. Program under the Thailand Research Fund and Kasetsart University for research funding (No. PHD/0150/2552). We are grateful for

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advice and assistances from Dr. Nattaporn Prakongkep of Land Development Department, Rathanon Jaroenchasri and Rachan Leotphayakkarat from the Department of Soil Science, Kasetsart University and Michael Smirk and Kim Duffecy from the School of Earth and Environment, the University of Western Australia. Map. KML file containing the Google map of the most important areas described in this article. Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at doi: http://dx.doi.org/10.1016/j.geodrs.2016.08.001. These data include the Google map of the most important areas described in this article. References Ahern, C.R., McElnea, A.E., Sullivan, L.A., 2004. Acid Sulfate Soils Laboratory Methods Guidelines. Queensland Department of Natural Resources, Mines and Energy, Indooroopilly, Queensland, Australia. Andriesse, W., van Mensvoort, M.E.F., 2006. Acid sulfate soils: distribution and extent. In: Lal, R. 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