Nature of redox concentrations in a sequence of agriculturally developed acid sulfate soils in Thailand

Accepted Manuscript

Title: The Nature of Redox Concentrations in a Sequence of Agriculturally Developed Acid Sulfate Soils in Thailand

Author: Tanabhatsakorn SUKITPRAPANON, Anchalee SUDDHIPRAKARN, Irb KHEORUENROMNE, Somchai ANUSONTPORNPERM and Robert J. GILKES

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S1002-0160(17)60449-1 10.1016/S1002-0160(17)60449-1 NA

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Please cite this article as: Tanabhatsakorn SUKITPRAPANON, Anchalee SUDDHIPRAKARN, Irb KHEORUENROMNE, Somchai ANUSONTPORNPERM and Robert J. GILKES, The Nature of Redox Concentrations in a Sequence of Agriculturally Developed Acid Sulfate Soils in Thailand, Pedosphere (2017), 10.1016/S1002-0160(17)60449-1.

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ACCEPTED MANUSCRIPT PEDOSPHERE Pedosphere ISSN 1002-0160/CN 32-1315/P

doi:10.1016/S1002-0160(17)60449-1

The Nature of Redox Concentrations in a Sequence of Agriculturally Developed Acid Sulfate Soils in Thailand Tanabhatsakorn SUKITPRAPANON1, Anchalee SUDDHIPRAKARN1,*, Irb KHEORUENROMNE1, Somchai ANUSONTPORNPERM1 and Robert J. GILKES2 1

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) * Corresponding author. E-mail: [email protected].

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ABSTRACT Potential acid sulfate soils (PASS) were drained for agriculture resulting in the formation of active acid sulfate soils (AASS) which gradually evolved into post-active acid sulfate soils (PAASS). Various redox concentrations (precipitates, costings, mottles) occur in these soils as a result of pedogenic processes including biological activity and effects of land management. While several studies have determined the mineralogy and geochemistry of acid sulfate soils (ASS), the mineralogy and geochemistry of redox concentrations occurring in a sequence of ASS through PASS to PAASS have not been investigated. This study examined the mineralogy and geochemistry of redox concentrations and matrix within 18 Thai PASS, AASS and PAASS. The labile minerals are predominantly controlled by oxidation status and management inputs. The unoxidized layer of PASS, AASS and PAASS contains pyrite and mackinawite. The oxidation of iron sulfides causes acidification and accumulation of yellow redox concentrations of jarosite and iron (hydr)oxides within shallow depths. As the soils become well-developed, they are recognized as PAASS and the jarosite and goethite transform to hematite. As ASS are drained, Co, Mn, Ni and Zn move downward and are associated with iron sulfides and Mn oxides in the unoxided layer. Arsenic, Cu, Cr, Fe and V concentrations do not change with depth because these elements become associated with jarosite and iron (hydr)oxides in yellow and red redox concentrations including the root zone in the partly oxidized layer of AASS and PAASS. Arsenic is associated with pyrite under reducing conditions. Key Words: redox concentrations, mineralogy, geochemistry, trace metals, acid sulfate soils

INTRODUCTION Potential acid sulfate soils (PASS) disturbed by natural and man-made drainage experience the oxidation of iron sulfide minerals and sulfur that are stable under reducing conditions (Kraal et al., 2013). Consequently, the soils become acidic and release dissolved Fe, Al, Mn, Ni and other contaminants into groundwater if there is insufficient neutralization capacity (Åström, 2001; Österholm and Åström, 2002; Burton et al., 2008; Gröger et al., 2011). Potential acid sulfate soils (PASS) are waterlogged soils with highly reducing conditions and the soils are transformed by drainage to active acid sulfate soils (AASS) due to the oxidation of iron sulfides and sulfur. Eventually, AASS may be transformed by land management practices to post-active acid sulfate soils (PAASS) (Fanning, 2012). In Thailand,

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MATERIALS AND METHODS

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Soil sampling

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where drainage of ASS has been carried out for more than 140 years, many PASS have become PAASS but little is known of the effects of this transition on soil composition. Under the reducing condition of PASS, the dark gray color of organic-rich sulfidic sediments is evident, pyrite (FeS2) is the dominant sulfide comprising about 10 to 40 g kg-1 of the soil (Gröger et al., 2011). Oxidation of iron sulfides in PASS with little carbonate results in acidification and accumulation of iron minerals in forms of variously colored redox concentrations, which represent the zone of apparent accumulation of Fe-Mn oxides (Soil Survey Staff, 2014). The pale yellow minerals jarosite (KFe3(SO4)2(OH)6) and natrojarosite (NaFe3(SO4)2(OH)6), reddish-orange schwertmannite (Fe8O8(OH)4.6(SO4)1.7), lepidocrocite (γ-FeOOH) and goethite (α-FeOOH) (Sullivan and Bush, 2004; Fitzpatrick and Shand, 2008), pale yellowish green sideronatrite (Na2Fe(SO4)2.OH.3H2O) (Fitzpatrick et al., 2008) and red hematite (Fe2O3) (van Breemen and Harmsen, 1975) occur in these soils. In the Netherlands, sediments from the former Zuyder Sea were developed for agriculture in a similar manner to those in Thailand. The Dutch sediments contain more than 10 g kg-1 of reduced sulfur but they also contain relatively high amounts of carbonate which prevented acidification upon land reclamation (Pons and van Breemen, 1982), this is not the case in Thailand. Although a number of studies have determined the mineralogy and geochemistry in particular ASS environment such as coastal ASS (Deng et al., 1998; Åström, 1998; Sullivan and Bush, 2004; Boman et al., 2010; Vithana et al., 2015) and inland ASS (Fitzpatrick et al., 2008; Mosley et al., 2014; Virtanen et al., 2014; Creeper et al., 2015), the mineralogical and geochemical characteristics of redox concentrations occurring in a sequence of agriculturally developed ASS ranging from PASS through AASS into PAASS have not been investigated. Thus, the objective of this research was to determine the mineralogy and chemical compositions of various redox concentrations occurring in PASS, AASS and PAASS. This work will provide a better understanding of the changing nature of redox concentrations in acid sulfate soils as the soils are developed, particularly for agriculture.

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The eighteen ASS sites representing PASS, AASS and PAASS investigated in this research are in the Lower Central Plain delta and the Southeast Coast of Thailand where estuarine sediments were deposited during the Holocene epoch (Sinsakul, 2000) (Fig. 1). Thailand is located in the tropical area between latitudes 5° 37' N to 20° 27' N and longitudes 97° 22' E to 100° 37' E. The climate is tropical savanna and tropical monsoonal climate with an average annual temperature of 27-30°C. The rainy season extends from mid-August to mid-October with an average rainfall of 903 and 1418 mm for the Central and Eastern regions, respectively (Meteorological Department, 2015). Soil materials are more sandy (clay-sandy clay loam) in the Southeast Coast area than in the Lower Central Plain (clay-silty clay loam) because the sediments in the Southeast Coast have been derived from granitic rocks (Charusiri et al., 1993), while those in the Lower Central Plain have originated in a complex alluvial and deltaic environment (Sinsakul, 2000). Field-moist soil samples were collected from topsoil, partly oxidized and unoxidized parent material layers in the dry season from October 2012 to February 2013. Soil samples were collected by hand auger to the maximum depth of 2 meters and packed in sealed plastic containers. They were described and classified in Soil Taxonomy (Soil Survey Staff, 2014) (Table I). Soil samples were split into two subsamples for bulk soil analysis and for analysis of soil matrix and redox concentrations. Fig. 1 Sampling locations for acid sulfate soils on the Lower Central Plain and the Southeast Coast of Thailand.

ACCEPTED MANUSCRIPT TABLE I Field data for Thai acid sulfate soils investigated in this researcha) ASS Typeb)

Locationc)

Classification

Landform

Elevation

Texturee)

Land Use

d)

(m) MSL Typic Sulfaquent

Swamp

9

SCL

Swamp forest

PASS2

SE

Typic Sulfaquent

Swamp

1

C

Swamp forest

PASS3

SE

Typic Sulfaquent

Swamp

6

SCL

Swamp forest

PASS4

SE

Typic Sulfaquent

Delta

8

SCL

Mangrove, Nipa

PASS5

SE

Typic Sulfaquent

Delta

2

L

Mangrove forest

AASS1

LCP

Hydraquentic Sulfaquept

Floodplain

6

C

Paddy rice

AASS2

LCP

Hydraquentic Sulfaquept

Floodplain

3

SiCL

Paddy rice

AASS3

LCP

Hydraquentic Sulfaquept

Floodplain

3

AASS4

LCP

Hydraquentic Sulfaquept

Floodplain

AASS5

SE

Hydraquentic Sulfaquept

Floodplain

AASS6

SE

Hydraquentic Sulfaquept

Floodplain

AASS7

SE

Hydraquentic Sulfaquept

Floodplain

AASS8

SE

Hydraquentic Sulfaquept

PAASS1

LCP

Sulfic Endoaquept

PAASS2

LCP

Sulfic Endoaquept

PAASS3

LCP

Sulfic Endoaquept

PAASS4

LCP

Sulfic Endoaquept

PAASS5

LCP

Sulfic Endoaquept

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SE

Paddy rice

2

C

Weeds

1

C

Paddy rice

7

C

Swamp forest

5

L

Paddy rice

Floodplain

2

SL

Paddy rice

Floodplain

6

C

Paddy rice

Floodplain

8

C

Paddy rice

Floodplain

3

SiCL

Paddy rice

Floodplain

4

C

Paddy rice

Floodplain

11

C

Paddy rice

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a)

PASS1

b)

Sampling in the dry season (October 2012 to January 2013); PASS = potential acid sulfate soils; AASS = active acid sulfate soils;

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PAASS = post-active acid sulfate soils; c)SE = the Southeast Coast, LCP = the Lower Central Plain; d)MSL = mean sea level; e)SL =

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sandy loam, SCL = sandy clay loam, SiCL = silty clay loam, SiC = silt clay, L = loam, C = clay.

Bulk soil sample analyses

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Physicochemical properties were determined on field-moist soil samples but are reported on an oven-dried (105°C) basis. The particle size distribution was determined by the pipette method (Gee and Bauder, 1986). Redox potential (Eh) of PASS was determined with a calibrated electrode. The Eh of AASS and PAASS was mostly not measured and is not listed in Table III because these layers had been partly and inhomogeneously oxidized by drainage. The Eh reading was recorded when a steady state was obtained. This occurred after approximately 5 to 10 minutes and values are reported in mV relative to the standard hydrogen electrode. Soil pH and electrical conductivity (EC) of saturated soil paste were measured in the laboratory within 24 hours of collection from the field. pH H2O was measured in water, using 1:1 soil to water (National Soil Survey Center, 2004). pH H2O2 was measured in 30 % hydrogen peroxide solution adjusted to pH 5.5, using 1:5 soil to solution (Ahern et al., 2004). Total organic carbon (OC) was determined using a CN analyzer (Elementar, Vario Macro). Peroxide-oxidizable sulfur (SPOS) which is the sulfate produced from oxidation of reduced inorganic sulfur during peroxide oxidation was determined by the complete suspension peroxide oxidation combined acidity and sulfur (SPOCAS) method (Ahern et al., 2004). Metal concentrations (As, Co, Cr, Cu, Fe, Mn, Ni, V and Zn) in bulk soil samples were determined by X-ray fluorescence spectrometry (XRF) of fused lithium metatetraborate discs (Norrish and Hutton, 1969) for Cr and V and by inductively coupled plasma-optical emission spectrometry (ICP-OES, Perin-Elmer Optima 7300 DV) of aqua regia digests (3:1 HCl:HNO3 at 130°C for 1 hr, APHA, 1998) for As, Co, Cu, Fe, Mn, Ni and Zn.

ACCEPTED MANUSCRIPT Organic matter, metal sulfides, clay minerals and some micas (mostly trioctahedral biotite) were almost completely dissolved in aqua regia, while sand-size quartz, feldspars, muscovite and amphiboles dissolved to a limited extent (Chao, 1984). Hereafter, the aqua regia digestion is referred to as the total element digestion. Soil matrix and redox concentrations analyses

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Hand-picked materials were separated from subsoil samples by hand under a dissecting microscope (Fig. 2). These materials consist of 43 soil matrices, 14 root zone materials, 13 red redox concentrations, 27 yellow redox concentrations and 20 unoxidized layer subsamples. A few drops of chloroform were added to the separated materials to suppress microbial activity. The color of redox concentration was measured in a field-moist condition using a Munsell Soil Color Chart (Munsell Color, 2000) (Table II). Concentrations of Al, As, Co, Cr, Cu, Fe, K, Mn, Ni, S, Si, V and Zn in hand-picked materials of soil matrix and variously colored redox concentrations were determined using ICP-OES on a digest obtained with aqua regia (3:1 HCl:HNO3) and a solid:solution ratio of 1:16 at 130°C for 1 hour (APHA, 1998). The analytical methods were assessed for accuracy and precision using a rigorous control system with reagent blanks and certified reference materials including stream sediment reference material STSD 3 (Lynch, 1999) and multi-element reference material OREAS 43P (Ore Research and Exploration Pty Ltd., 1997) for aqua regia digests and XRF, respectively. Fig. 2 Post-active acid sulfate soil (PAASS4) under paddy rice cultivation (a), soil profile of representative PAASS4 (b), topsoil with plant roots (c), red redox concentration in partly oxidized layer (d), yellow redox concentration in partly oxidized layer (e) and

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TABLE II

Morphology of representative Thai potential, active and post-active acid sulfate soils

PASS5

Topsoil

Topsoil

Partly oxidized

%

Texturec)

Structured)

moist

(Ag)

0-20

2.5Y 4/2 (Dark grayish brown)

100

L

1-SAB

(Cse)

20-50

2.5Y 4/2 (Dark grayish brown)

100

L

M

50-82

Gley1 4/10Y (Dark greenish gray)

100

L

M

82-200

Gley1 4/10Y (Dark greenish gray)

100

SiL

M

0-20

2.5Y 2.5/1 (Black)

70

SiCL

2-SAB

2.5Y 5/3 (Light olive brown)

30

2.5Y 3/1 (Very dark gray)

40

SiCL

2- AB

2.5Y 6/2 (Light brownish gray)

35

2.5Y 6/8 (Olive yellow)

25

2.5Y 5/3 (Light olive brown)

40

SiCL

1-AB

2.5Y 4/2 (Dark grayish brown)

35

2.5Y 7/8 (Yellow)

25

2.5Y 5/2 (Grayish brown)

75

SiC

1-AB

2.5Y 7/8 (Yellow)

25

(Ap)

(Bjyg1)

(Bjyg2)

(Bjyg3)

PAASS3

Munsell color

cm

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Unoxidized

AASS2

Depthb)

Layer (symbol)

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Typea)

20-50

50-75

75-170

Unoxidized

(Cse)

170-200

2.5Y 4/1 (Dark gray)

100

SiC

M

Topsoil

(Ap)

0-17

10YR 3/2 (Very dark grayish brown)

75

SiCL

2-SAB

5YR 4/4 (Reddish brown)

25

ACCEPTED MANUSCRIPT Partly oxidized

(Byg1)

(Byg2)

(Bjyg)

Unoxidized a)

(Cse)

17-50

50-90

90-170

170-200

2.5Y 6/1 (Gray)

55

10R 3/6 (Dark red)

35

7.5YR 6/8 (Reddish yellow)

10

2.5Y 5/1 (Reddish gray)

40

10R 3/4 (Dusky red)

35

10R 4/8 (Red)

25

10R 4/2 (Weak red)

70

2.5Y 8/8 (Yellow)

30

2.5Y 4/1 (Dark gray)

100

SiCL

2-AB

SiCL

1-AB

SiC

1-AB

SiC

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PASS = potential acid sulfate soil, AASS = active acid sulfate soil and PAASS = post-active acid sulfate soil; unoxidized layers in d)

PASS, AASS and PAASS are situated below the water table; c)L = loam, SiL = silt loam, SiCL = silty clay loam, SiC = silty clay; 1

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= weak, 2 = moderate, SAB = subangular blocky, AB = angular blocky, M = massive.

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Hand-picked materials were rapidly dried using ethanol and deposited onto a SiO2 single crystal low background holder for X-ray diffraction (XRD) using copper Kα radiation, scanning between 3 and 70° 2θ with 0.02° 2θ step size and scan speed of 0.02° 2θ per second. The morphology, chemical compositions and elemental mapping of hand-picked materials were determined by scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS; TESCAN VEGA3, 15kV) on samples prepared on aluminum stubs with a carbon coating. Statistical analyses

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Principal component analysis (PCA) of chemical analyses (Bellehumeur et al., 1994) was used to determine elements of similar geochemical behaviour and to group samples on the basis of their geochemical affinity. An analysis of variance (ANOVA) using Tukey’s HSD test for unequal sample sizes was used to determine significant differences in composition between the soil matrix of the partly oxidized layer, root zone material, red redox concentration, yellow redox concentration, and unoxidized layer. All data were log transformed to meet the requirement of normality for ANOVA analysis (P < 0.05). Log transformed data were standardized to zero mean and unit variance before doing PCA using Statistica software version 8.0 for Windows. RESULTS AND DISCUSSION

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Soil characteristics

Potential acid sulfate soils (PASS): The Thai PASS occurring in swamps or mangrove forests are classified as Typic Sulfaquents (Soil Survey Staff, 2014) (Table I). Potential acid sulfate soils are located up to several kilometers from the coast (Fig. 1). They consist of a layer of dark grayish brown (2.5Y 4/2) loam (0-20 cm), underlain by massive dark grayish brown to dark greenish gray (Gley1 4/10Y) silt loam (> 20 cm) materials (Table II), with various amounts of organic carbon (median 39 and 23 g kg-1 for topsoil and subsoil layers, respectively) (Table III).

ACCEPTED MANUSCRIPT TABLE III Median and range (in parenthesis) of chemical properties of PASS, AASS and PAASS

Unoxidized

AASS

Topsoil

Partly oxidized

Unoxidized

PAASS

Topsoil

Partly oxidized

Unoxidized

Ehb)

OC

ECsat

SPOSc)

mV

g kg-1

dS m-1

g kg-1

5.1

2.3

149

39

26

0.82

(3.6-6.0)

(1.9-2.6)

(49-245)

(18-48)

(1.7-74)

(0.64-1.6)

5.8

1.7

-33

23

29

19

(3.5-7.0)

(1.1-2.5)

(-277-602)

(8.3-66)

(1.1-59)

(0.89-57)

3.6

2.1

-

27

4.8

0.62

(3.2-5.5)

(1.6-2.4)

(15-54)

(1.1-13)

(0.33-1.7)

3.7

2.3

7.5

7.5

0.37

(3.2-4.5)

(1.8-2.6)

4.7

2.3

(3.5-5.9)

(1.4-3.4)

4.5

2.4

(4.0-5.7)

(2.1-2.7)

4.1

2.4

(3.1-6.4)

(2.1-5.1)

5.0

1.8

(4.1-6.8)

(1.5-2.0)

-

(3.2-23)

-

-

(1.1-20)

(0.03-3.2)

9.1

12

4.3

(1.5-20)

(2.8-28)

(0.01-18)

24

1.8

0.34

(13-27)

(0.99-4.3)

(0.17-0.61)

-

4.9

2.4

0.13

(2.1-14)

(0.92-4.9)

(0.01-0.82)

-

14

9.2

12

(12-18)

(6.7-11)

(5.0-14)

PASS = potential acid sulfate soil; AASS = active acid sulfate soil; PAASS = post-active acid sulfate soil; b)Redox potential (Eh) in the AASS and

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pH H2O

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ASS typea)

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PAASS was not measured because these soils had been partly oxidized by drainage; c)SPOS = peroxide-oxidizable sulfur (g kg-1).

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pH H2O values are mostly near-neutral with some low values (Table III). pH H2O2 is much lower than pH H2O because of the oxidation of iron sulfides, sulfur and organic matter. The PASS subsoil mostly experiences a reducing condition (median Eh -33 mV) with the topsoil layer (median Eh 149 mV) (Table III) being oxidized. The EC values are similar for topsoil and subsoil layers ranging between 1.1 and 74 dS m-1 depending on the influence of seawater. SPOS contents range between 0.64 and 57 g kg-1. The median SPOS value of topsoil is much less (median 0.82 g kg-1) than for the unoxidized subsoil layer (median 19 g kg-1) (Table III) indicating that the unoxidized layer contains much more iron sulfide than the topsoil. Active acid sulfate soils (AASS): The active acid sulfate soils in this research are mostly used for paddy rice cultivation. The soils at AASS sites contain a 15 cm or more thick sulfuric horizon (pH < 4) with its upper horizon boundary within 50 cm of the soil surface. Median pH H2O values of the AASS are 3.6, 3.7 and 4.7 for topsoil, partly oxidized and unoxidized layers, respectively (Table III). The soils are classified as Hydraquentic Sulfaquepts in Soil Taxonomy (Soil Survey Staff, 2014) (Table I). Soil texture ranges from sandy loam to clay and three soil layers are recognized. The topsoil layer (0-20 cm) has a black matrix (2.5Y 2.5/1) with light olive brown redox concentrations (2.5Y 5/3) in the root channels. The underlying partly oxidized layer (20-170 cm) has an angular blocky structure and consists of olive yellow (2.5Y 6/8) to yellow (2.5Y 7/8) redox concentrations on the surface of peds of a very dark gray (2.5Y 3/1) and light olive brown (2.5Y 5/3) matrix. Beneath the partly oxidized layer is a massive dark grayish (2.5Y 4/1) unoxidized layer (Table II). The soil has been utilized for agricultural practices that retain crop residues, consequently the topsoil is richer in OC (median 27 g kg-1) than the partly oxidized layer (median 7.5 g kg-1) and unoxidized layer (median 9.1 g kg-1). The median EC value increases with depth and SPOS is much higher in the unoxidized layer where groundwater has excluded air thus preventing oxidation of Fe, S and organic matter.

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Post-active acid sulfate soils (PAASS): The post-active acid sulfate soils are situated in the inland marginal zone of the delta where drainage and agriculture have been carried out for many years (Fig. 1). The PAASS contain the same sulfuric horizon (pH < 4) as the AASS, their sulfuric horizon occurs above 2 m depth but is too deep (> 50 cm of the soil surface) to affect most plants growing on these soils. These soils are classified as Sulfic Endoaquepts (Soil Survey Staff, 2014) (Table I). The PAASS range in texture from silty clay loam to clay. Three layers are recognized. The topsoil is dark grayish brown (10YR 3/2) with reddish brown redox concentrations (5YR 4/4) along root channels and cracks. The partly oxidized layer extends to approximately 100 cm depth with an angular blocky structure, redox concentrations are mainly dark red (10R 3/6), reddish yellow (7.5YR 6/8), dusky red (10R 3/4) and red (10R 4/8) occurring on the surface of structural units in a gray matrix (2.5Y 6/1). Yellow redox concentrations (2.5Y 8/8) in a weak red matrix (10R 4/2) occur below 100 cm, while a dark gray color (2.5Y 4/1) is dominant in the massive unoxidized layer (Table II). These data are consistent with the findings of van Breemen and Harmsen (1975), Öborn (1989), Janjirawuttikul et al. (2010), Johnston et al. (2011) and Sánchez-Maraňón et al. (2015), who demonstrated that yellowish brown (e.g. 10YR 5/6) and red redox concentrations (e.g. 7.5R 5/6, 10R 3/4) occurred in the upper layer and contained both pale yellow (e.g. 5Y 8/4) and yellowish brown (e.g. 2.5Y 5/2) redox concentrations at a greater depth in ASS profiles. These materials were underlain by a dark gray (e.g. 5Y 4/1) substratum. The soil color of ASS can change over a period of oxidation from very dark gray (4.3Y 3.3/1.5) to yellowish brown (9.8YR), this alteration becomes progressively deeper with development status of ASS (Sánchez-Maraňón et al., 2015). The PAASS contain high amounts of OC in the topsoil as a consequence of retention of harvest residues. The median pH H2O value is higher than 4.0 in the topsoil and partly oxidized layers (Table III) which partly reflects amelioration of acidity by liming and drainage. Median pH H2O2 is lower than pH H2O and in the same range as for PASS and AASS. The lower pH H2O2 value is caused not only by the oxidation of iron sulfides (Ahern et al., 2004; Soil Survey Staff, 2014) but also by the oxidation of soil organic matter. Douglas and Fiessinger (1971) stated that hydrogen peroxide (H2O2) causes the oxidation of organic matter and degradation of clay minerals, especially 2:1 clay minerals. Iron sulfides can be formed when ASS are reflooded by freshwater (Johnston et al., 2014). This research indicates that even though PAASS have been managed for a long period of time, the acidity in PAASS profiles can be due to oxidation of residual sulfidic materials and/or soil organic matter. The median EC value of PAASS is lower than for AASS due to leaching and EC increases with depth. The SPOS of the unoxidized layer is much higher than that of the partly oxidized layers. The low SPOS value of 0.01 g kg-1 (Table III) is for partly oxidized layers in PAASS1 and PAASS4 indicating that oxidation of iron sulfides is almost complete.

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Mineralogy of soil matrix and redox concentrations The soil matrix and variously colored redox concentrations in the ASS mainly consist of quartz, feldspars kaolinite and illite. Typical XRD patterns (Fig. 3) show the presence of minerals that are characteristic of the variously colored redox concentrations in topsoil, partly oxidized and unoxidized layers. The ASS located near the coast contain halite because they have been affected by seawater in the recent past. Pyrite and mackinawite occur in unoxidized layers of PASS, AASS and PAASS and in some partly oxidized layers of AASS and PAASS. Electron microscopy shows that both single crystal and framboidal forms of pyrite occur in the unoxidized layer (Fig. 4b, c). Fig. 3 Random powder X-ray diffraction patterns of hand-picked redox concentrations and matrix materials on a low background holder. Materials were selected from topsoil, partly oxidized and unoxidized layers in acid sulfate soils. Q = quartz, F = feldspar, I = illite, K = kaolinite, G = goethite, He = hematite, L = lepidocrocite, Py = pyrite, Ma = mackinawite, J = jarosite, Gyp = gypsum, Ha = halite, T = talc. Fig. 4 Backscattered electron micrographs with energy dispersive spectra showing (a) euhedral micron-size crystals of jarosite (K, Na, Fe, S) and associated clay minerals (Al, Si) in a yellow redox concentration in the partly oxidized layer of an active acid sulfate

ACCEPTED MANUSCRIPT soil, (b) euhedral, 5-10 µm crystals of pyrite (Fe, S, As) with minor quartz and clay minerals in the unoxidized layer of a potential acid sulfate soil and (c) framboidal pyrite (Fe, S) and minor clay minerals in the unoxidized layer of a potential acid sulfate soil.

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The AASS and PAASS, which are commonly limed by farmers, contain gypsum which is formed by the reaction of calcite and sulfuric acid. Jarosite, goethite and hematite, secondary iron minerals, present in the partly oxidized layer of ASS. Jarosite appears to be absent from within 50 cm of the soil surface of PAASS. Jarosite and small amount of goethite have been deposited in yellow redox concentrations coating the face of structural units/peds and in root channels within the gray matrix in the partly oxidized layer of AASS and PAASS (Fig. 3, 4a). The high redox conditions on the ped surfaces have resulted from oxygen transported in pores, channels and plant roots from above ground into unoxidized subsoil. The oxygen diffuses into the rhizosphere soil causing local oxidation (Ferreira et al., 2007). Jarosite is the product of oxidation of iron sulfides and it may have precipitated directly if Eh, pH and the composition of soil solution were appropriate (Fanning et al., 1993; Casas et al., 2007). Jarosite usually occurs in yellow redox concentrations, while goethite usually occurs in yellow to yellowish red redox concentrations (Fanning et al., 1993; Schwertmann, 1993). Small amounts of goethite in yellow redox concentrations observed in this study are probably due to the hydrolysis of jarosite near the surface of peds. Lepidocrocite occurs in red redox concentrations in the surface soil of PASS due to ferrous oxidation at near neutral pH (> 5) (Fitzpatrick et al., 2008; Johnston et al., 2011). Lepidocrocite does not commonly occur in red redox concentration, usually occuring in reddish yellow to strong brown redox concentrations (Schwertmann, 1993). The occurrence of lepidocrocite in red redox concentrations in the surface soil of Thai PASS is due to the soil where lepidrocrocite precipitates being colored by red hematite (Fig. 3). Goethite and hematite are present in yellowish orange and red redox concentrations in the topsoil and partly oxidized layers. The red redox concentrations of hematite are dominant in the upper part of partly oxidized layers of PAASS (Table II) which are located at high elevations ranging from 3 to 11 m above mean sea level (Table I). Although, there is little information about the stability of hematite under aquic moisture condition, the study by van Breemen and Harmsen (1975) reported that red redox concentration of hematite in the soil surface of ASS was associated with elevation and good drainage. In addition, this research indicates that the red redox concentration of hematite exist in Thai ASS, especially in upper soil profile of the well-developed PAASS. These may have formed by dehydration of iron (hydr)oxides, alteration of jarosite or by the oxidation of iron in soil solution during the long dry season of the tropical savanna climate in Thailand. The SEM backscattered electron micrograph of representative root zone material from a partly oxidized layer, shows iron oxides with quartz and clay minerals (Fig. 5a). The EDS spectrum shows that iron oxide, which can be either goethite (α-FeOOH) or lepidocrocite (γ-FeOOH), is a dominant mineral coating root material under oxidizing condition (Fig. 5b). XRD results for root zone materials (Fig. 3) indicate that it is goethite (Fig. 5b). The elemental mapping and associated ternary diagram of the chemical composition of matrix shows iron oxides with an admixture of Si, Al and other elements from clay minerals in the matrix (Fig. 5). Black root zone materials under a reducing condition contain framboidal and single crystal pyrite, mackinawite, elemental sulfur and iron oxides together with matrix materials (Fig. 6a). The chemical compositions of mackinawite and pyrite occurring in black root zone materials have Fe/S ratios consistent with the ideal compositions of FeS and FeS2, respectively (Fig. 6f). Elemental sulfur is also present (Fig. 6b, e) due to incomplete oxidation of pore water sulfides by O2, Fe3+ and Mn4+ (Aller and Rude, 1988) or the partial oxidation of iron monosulfides such as mackinawite (Burton et al., 2006). Fig. 5 Yellow root zone material from the partly oxidized layer in an active acid sulfate soil, (a) backscattered electron micrograph showing plant root, iron oxides, quartz and clay minerals with (b) x-ray spectra for goethite (FeOOH) and (c) ternary diagram of the chemical composition of the yellow root zone material showing that it is mostly a mixture of matrix minerals and iron oxide with no elemental sulfur or pyrite.

ACCEPTED MANUSCRIPT Fig. 6 Root zone materials from the unoxidized layer of an active acid sulfate soil, (a) backscattered electron micrograph showing elemental sulfur, mackinawite, framboidal and single grain pyrite and iron oxides with X-ray spectra of (b) elemental sulfur and (c) framboidal pyrite. Maps of (d) iron and (e) sulfur and (f) a ternary diagram of the chemical composition of the various materials in the root zone material including iron oxides, mackinawite, pyrite and elemental sulfur in various admixtures and mixed with matrix minerals.

Geochemistry of soil matrix and redox concentrations

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Median concentrations of Al, As, Co, Cr, Cu, Fe, K, Mn, Ni, S, Si, V and Zn in five classes of separated soil matrix and redox concentrations from PASS, AASS and PAASS from the Lower Central Plain and the Southeast Coast are given in Table IV. Concentrations of Ca and Mg are not shown as they mostly reflect the various impacts of dolomitic limestone applied to soils. Similarly amounts of Na and Cl are not shown as they reflect the residual effect of sea water. The elemental compositions of the separated materials from soils of the Lower Central Plain are similar to those from the Southeast Coast. The amounts of Al, Cu and Si in the Lower Central Plain and the Southeast Coast soils are not systematically different for the five materials. Arsenic, Cr, Fe, K, S and V are more abundant in colored redox concentrations, while the concentrations of Co, Mn, Ni, S and Zn are higher in the unoxidized layer. Principal component analysis (PCA) of standardized log total concentrations of Al, As, Co, Cr, Cu, Fe, K, Mn, Ni, S, Si, V and Zn for the five ASS materials shows that the first two components explain 50% of the variation indicating that these materials are highly diverse in chemical composition which partly reflects the considerable differences in composition of the parent materials and various pedoenvironment conditions of formation (Fig. 7a, b).

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Fig. 7 Principal component analysis based on the chemical composition of hand-picked soil matrix and redox concentrations from Thai acid sulfate soils, (a) distribution of elements (variables) and (b) distribution of samples (cases).

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Elements are mostly allocated into two major groups, the first group consist of Co, Mn, Ni and Zn which are associated with unoxidized layers and are situated along the factor 2 axis. Under reduced conditions, Co, Mn, Ni and Zn are likely to be associated with iron sulfides and Mn oxides (Åström, 1998; Morse and Luther III, 1999; Hong-Bin et al., 2006; Claff et al., 2011; Morgan et al., 2012). However, the PCA data show that Co, Mn and Zn are separated from Ni in the unoxidized layer along the factor 1 axis which indicates that Co and Zn are more closely associated with Mn oxides than with iron sulfides in the unoxidized layer. Mn oxides have a great affinity for Co and Zn but have a poor affinity for Ni in natural environments (Tani et al., 2004). Reducing and acidic conditions of soils play important roles in the solubilization of Mn oxides (Kabata-Pendias, 2011). Much Mn and associated Zn, Co and Ni can occur in sediments in very fine-grained Mn oxides. After reduction, Mn and associated elements (Co, Zn and Ni) are released and incorporated into iron sulfides in PASS. These element are removed under acidic conditions (Morse and Luther III, 1999). Nickel is more strongly associated with iron sulfides than with Mn oxides in the unoxidized layer, although in this study Fe is not associated with Ni in the unoxidized layer (Fig. 7a). The PCA data show that Fe makes little contribution to the factor 2 axis (close to 0.0) because Fe is a major element in both colored redox concentrations and the unoxidized layer in the form of diverse redox sensitive Fe minerals, such as jarosite, lepidocrocite, goethite, hematite and iron sulfides, as well as occurring in silicate clay minerals such as illite (Fig. 3). Our data show that Fe has a close association with yellow and red redox concentrations (Table IV). As a consequence, Fe is more associated with As, Cr, K, S and V in colored redox concentrations than with Ni in the unoxidized layer as shown in Fig. 7a. According to Kabata-Pendias (2011), Ni can be present in sulfides in sediments rich in organic matter and under reducing conditions. X-ray spectra in this study demonstrate that Ni is present in framboidal pyrite in the waterlogged unoxidized layer (Fig. 6c).

ACCEPTED MANUSCRIPT TABLE IV Median with range in parenthesis of the total concentration of elements in various types of materials including redox concentrations in acid sulfate soils from the Lower Central Plain and the Southeast Coast of Thailand a) Element

Unoxidized layer

Soil matrix,

Root zone,

Red RCb),

Yellow RCb),

partly oxidized layer

all 3 layers

all 3 layers

all 3 layers

-1

--------------------------------------------------------- mg kg ----------------------------------------------------The Lower Central Plain

Al

(n = 8)

(n = 29)

(n = 6)

(n = 10)

(n = 20)

A

A

A

A

23672A

24504

25147

(20128-38716)

(17591-31930)

(16317-36032)

5.3B

2.6B

8.0AB

27A

29A

(0.042-9.3)

(nd-25)

(1.6-90)

(13-80)

(4.1-82) 27

26

(23-31) 11

Fe

A

10

(nd-26) B

1910

AB

(1241-2224) 177

A

(86-1037) 25A

Ni

(7548-84250) 1322

B

(818-4183) 55

B

S

9929

A

(424-13827) Si

1003

A

(744-1223)

Ac

27

B

(20-38) 56

A

A

B

(2.4-11) 57

A

(39-82) 6.9

A

(0.37-26)

B

A

(13950-75707) 1038

B

(833-1635) 105

B

114071

(32534-173881) 1119

B

(933-3115) 51

(3.9-99) 4.5B (2.2-7.0) 48A (23-88) 9.1A

(1.7-17) 30491

B

(nd-34) 115446A (16056-148199) 1909A (883-28620) 72B

(24-293)

(44-147)

(29-250)

(30-287)

9.1B

12B

3.4BC

0.38C

(5.9-18)

(5.9-17)

(nd-6.3)

(nd-13)

ce pt

(11-98)

16452

B

(25-40) 11

B

(13002-30238)

Mn

A

(6.2-23) 26474

K

28

(16-38)

4.9

(2.4-10)

B

M

Cu

(1.3-10)

ed

Cr

B

4.0

B

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3.6

B

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22

A

t

(13052-42227)

Co

Zn

24725

(21405-36457) As

V

26122

611

B

(183-29657) 927

A

(507-1477) 23

B

(10-61) 21

B

910

AB

(438-2572) 805

A

(713-963) 35

AB

(25-193) 28

AB

1793

AB

(731-15965) 935

A

(747-1151) 82

A

(54-178) 33

AB

4219A (448-62439) 951A (222-2251) 88A (20-304) 29B

(25-107)

(12-49)

(19-46)

(19-49)

(15-57)

(n = 12)

(n = 14)

(n = 8)

(n = 3)

(n = 7)

15597A

19898A

15991A

21951A

17254A

(10685-20647)

(10961-32353)

(8760-21822)

(10183-36542)

(7448-30987)

The Southeast Coast

Al

As

10

AB

(1.0-35) Co

12

A

(1.0-27) Cr

25

A

6.5

B

(1.0-12) 2.7

A

(0.78-24) 34

A

12

AB

(3.6-367) 3.4

A

(1.2-914) 30

A

44

A

(12-102) 7.1

A

(1.5-10) 49

A

32A (7.4-271) 6.2A (2.0-14) 49A

ACCEPTED MANUSCRIPT 14

(2.7-9.1) Fe

32852

B

(2155-45212) K

682

A

Mn

120

A

Ni

11

A

14083

A

(3838-38847) Si

673

V

Zn

A

710

24

(6.1-290)

4.2

(4.0-25)

6662

(293-4877)

A

(715-119503)

A

868

A

(378-1617)

33B

35B

51AB

(23-51)

(12-188)

(31-218)

37

14

A

17

(3.8-105)

A

(5.0-65)

(nd-42)

A

113607A (21245-157290)

A

(395-2729)

581A (444-14531)

A

44A

(34-61)

AB

(nd-1923)

B

623

60

(11-330)

A

21A

(58097-187417)

A

(295-848)

A

133825

A

(430-6735)

A

(20-87)

A

(nd-6.8)

(10329-192659)

(478-1306)

645

0.85

AB

(312-981)

(4.0-60) a)

51011

A

892

(20-96)

A

(nd-26)

B

(3276-46592)

7.3

(nd-20) S

12098

24

(8.3-235)

8.9

(3.2-41)

684

(509-1184)

(9.2-72)

A

nd

(9.2-270)

B

1.5AB

(nd-nd)

(nd-12)

AB

1300AB

1229

t

7.0

(19-138)

(839-8053) 713

A

(558-35689) 662A

(589-738)

(528-923)

187A

157A

(41-430)

(34-703)

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Cu

A

cr ip

(21-32)

20

A

(6.7-47)

26A (12-83)

Significant differences in the concentration of an element between unoxidized layer, soil matrix, root zone material, red redox

concentration and yellow redox concentration are indicated by different letters (A, B, C) at P < 0.05 (Tukey’s test); b)RC = redox

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The second group of elements in the PCA diagram consists of As, Cr, Fe, K, S and V which are associated with yellow and red redox concentrations and some root zone materials. The yellow redox concentrations are grouped towards higher Fe, S, K, As, Cr and V in the diagram, but show no separation from red redox concentrations because these elements are constituents of jarosite and iron(hydr)oxides, mostly occurring in yellow and red redox concentrations (Burton et al., 2008; Johnston et al., 2010; Gröger et al., 2011; Ketrot et al., 2014). The soil matrix is separated from other redox sensitive materials because it mostly contains smaller amounts of As, Co, Cr, Fe, K, Mn, S, V and Zn than are present in other materials (Fig. 7a, b).

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Distribution of trace metals in the soil profiles

The vertical distributions of trace metals (As, Co, Cr, Cu, Fe, Mn, Ni, V and Zn) for bulk soil samples from representative ASS profiles are shown in Fig. 8. The concentrations of As, Cr, Cu and V show no systematic change with depth. The higher amounts of Fe in the partly oxidized layer of PAASS indicates that some Fe has been transported by diffusion to the upper partly oxidized layer, by leaching from the upper layer or has moved upward via evaporation transport during the dry season. The behaviour of Fe in Thai ASS under a tropical climate is similar to that observed in other climatic zones such as the boreal (e.g. Finland) and subtropical (e.g. Australia) zones (Åström, 1998; Boman et al., 2010; Claff et al., 2011; Creeper et al., 2015). Harmsen and van Breemen (1975) considered that the enrichment of Fe in the partly oxidized layer of Thai ASS is caused by repeated flooding and redox cycles under paddy rice cultivation. Dissolved Fe2+, controlled by the reduction of Fe3+ minerals during soil flooding for paddy rice cultivation and also during natural flooding, can migrate toward the partly oxidized layer and oxidize during the dry season to form iron (hydr)oxides such as schwertmannite and goethite (Gialanella et al., 2010; Creeper et al., 2015). These minerals can transform to hematite in red redox concentrations at higher elevations and in well drained soils (van Breemen and Harmsen, 1975) as well as during the long dry period in a seasonal tropical climate.

ACCEPTED MANUSCRIPT Fig. 8 Vertical distribution of metals (As, Co, Cr, Cu, Fe, Mn, Ni, V and Zn) for bulk soil samples in representative soil profiles (a) active acid sulfate soil (AASS8) and (b) post-active acid sulfate soil (PAASS3).

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Arsenic and Cu can be retained in the jarosite and iron (hydr)oxides occurring in yellow and red redox concentrations in ASS under oxidizing conditions (Acero et al., 2006; Gräfe et al., 2008; Miretzky and Cirelli, 2010; Gröger et al., 2011; Freitas et al., 2015) and are present in pyrite under reducing conditions (Mango and Ryan, 2015). Consequently, As and Cu show no systematic change in concentration with depth. A backscattered electron micrograph with energy dispersive spectrum confirms that some As is present in single crystal pyrite (Fig. 4b). The constant vertical distribution of Cr and V in soil profiles indicates that these elements are mostly present in redox resistant minerals as was suggested by other studies (Richard and Bourg, 1991; Sohlenius and Öborn, 2004). The concentrations of Co, Mn, Ni and Zn are lower in the topsoil and partly oxidized layers of the representative PAASS4, but these elements are enriched at the boundary between the partly oxidized and unoxidized layers (Fig. 8, 9) as has been observed elsewhere, such as in the boreal zone (Åström and Deng, 2003). The boreal and tropical PASS have developed in Holocene marine sediments containing iron sulfides, so that the soils have a high potential to generate acidity when oxidized. Fig. 9 Principal component analysis based on iron and trace elements in bulk samples of Thai ASS, (a) distribution of elements (variables) and (b) distribution of samples (cases).

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The PCA diagram based on standardized log total concentrations of As, Co, Cr, Cu, Fe, Mn, Ni, V and Zn for the bulk soil samples shows that the unoxidized layer of PASS, AASS and PAASS is separated from the partly oxidized layers along the factor 1 axis due to greater concentrations of Co, Mn, Ni and Zn. This indicates that Co, Mn, Ni and Zn may mobilize downward through the soil profiles of AASS and PAASS to be present in redox sensitive minerals such as sulfides and Mn-oxides in the unoxidized layer. These metals may leach from the topsoil and partly oxidized layers to the unoxidized layer and to groundwater and into adjacent drains (Åström and Deng, 2003; Morse and Luther III, 1999; Burton et al., 2008; Boman et al., 2010; Claff et al., 2011). During the oxidation of ASS, the Southeast Coast ASS which have a sandy texture might be considered to be at a higher risk of acidification and mobilization of trace elements, especially Co, Mn, Ni and Zn, than the Lower Central Plain ASS which have a more clayey texture and higher pH buffering capacity. Creeper et al. (2015) suggested that management strategies should consider soil texture in the rehabilitation of sandy and clayey ASS. CONCLUSIONS

The various types of materials occurring in Thai potential, active and post-active acid sulfate soils indicate that these materials have diverse mineralogical and chemical compositions which are due to the different compositions of parent materials and to changes in pedoenvironment during the development of the soils for agriculture. We observed that the soils consist mostly of a matrix of quartz, feldspar, kaolinite and illite. Labile minerals occur in various materials depending on oxidation and drainage status. Pyrite and mackinawite are dominant sulfide minerals in unoxidized soil layer of PASS, AASS and PAASS. Under oxidizing conditions, jarosite, lepidocrocite, goethite and hematite are the secondary iron minerals in the partly oxidized layers of AASS and PAASS and in the topsoils of PASS, occurring in yellow and red redox concentrations. In the PAASS, red redox concentrations of hematite dominate in the upper part of soil profile (> 50 cm of the soil surface), while yellow redox concentrations of jarosite are present in the sulfuric horizon (pH < 4) occurring deeper than in AASS. Reclamation of Thai ASS for agricultural purposes has caused the oxidation of iron sulfides with acidification and mobilization of Co, Mn, Ni and Zn. Cobalt, Mn and Zn coprecipitated with iron sulfides and Mn oxides, while Ni substitutes in iron sulfides in the unoxidized

ACCEPTED MANUSCRIPT layer. Concentrations of As, Cr, Cu, Fe and V do not change with depth during ASS development because these elements are adsorbed or coprecipitated during the dry season within redox concentrations of jarosite and iron (hydr)oxides in the partly oxidized layer. Our results indicate that variations in the abundance and types of redox concentrations within ASS profiles can be used as indicators of soil development and reclamation status. Further research is required to understand the stability of jarosite, goethite and hematite in ASS under the wetting and drying cycles of paddy rice cultivation, as these minerals can control plant uptake of some elements (e.g. Fe, Cu, As) and affect the quality of water and sediments in acid sulfate soil environments. ACKNOWLEDGEMENTS

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The authors gratefully acknowledge the Royal Golden Jubilee Ph.D. Program under Thailand Research Fund and Kasetsart University for financial support (No. PHD/0150/2552), we gratefully acknowledge assistance from Dr. Nattaporn Prakongkep from Land Development Department, Rathanon Jaroenchasri, Rachan Leotphayakkarat from the Department of Soil Science, Kasetsart University, Michael Smirk, Kim Duffecy from the School of Earth and Environment, the University of Western Australia (UWA) and staff from the Centre for Microscopy, Characterisation and Analysis, UWA. REFERENCES

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