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Soil genesis on hypersaline tidal flats (apicum ecosystem) in a tropical semi-arid estuary (Ceara, Brazil).

Introduction

The terms salt flats (Ridd and Stieglitz 2002), supratidal flats, hypersaline tidal flats and unvegetated flats (Hadlich et al. 2009) are used to refer to coastal ecosystems on the periphery of fluviomarine plains in arid and semi-arid estuaries. These environments are common in many parts of the world (e.g. northern Australia, south-eastern Spain, western Gulf of Mexico; see Ridd and Stieglitz 2002; Alvarez-Rogel et al. 2007; Forbes et al. 2008) and in the north-eastern region of Brazil (Hadlich et al. 2009), where are they are called apicum (a native name meaning saltwater marsh originating from the Tupi-Guarani language, a Brazilian indigenous group). These environments are generally located in the supra-littoral fringes, in an intermediary position between mangroves and terrestrial environments (i.e. coastal tablelands). They generally occur in association in regions where the rainy season is concentrated in ~3 months of the year (Lebigre 2007).

These marginal environments of semi-arid tropical estuaries are marked by conditions in which water evaporation largely exceeds the precipitation rate, resulting in hypersaline environments during most of the year. According to Ridd and Stieglitz (2002), these environments cover areas that generally exceed those of mangroves in arid and semi-arid estuaries in Australia, occupying the great part of tropical semi-arid intertidal zones. Due to their physiographic position, these environments are flooded only twice a month during the spring tides, whereas mangroves are flooded twice each day (Vieillefon 1969; Zack and Roman-Mas 1988; Forbes et al. 2008). Under these conditions, the high evapotranspiration rates, associated with low tidal flushing of accumulated salts and negligible river discharges into estuaries, can cause high concentrations of salt in soils that may exceeding 5 times that of normal values of seawater (Hollins and Ridd 1997; Ridd and Stieglitz 2002).

According to Meireles et al. (2007), the formation of hypersaline tidal flats is related to the transportation of sedimentary material by waves, wind and fluvial systems, which favours the formation of sand banks in estuaries, causing the silting and the shifting of mangrove channels. The consequent decreased flooding frequency by the tides and the restricted entrance of fresh water would increase salt concentrations (Vieillefon 1969; Zack and Roman-Mas 1988) and promote the formation of hypersaline tidal flats.

Most plant species do not have the physiological ability to survive in tidal flat soils due, mostly, to the elevated levels of salinity. The vegetation in these environments is normally sparse and patchy, composed of succulent halophytes (c.g. Batis maritima and Portulaca oleracea) and Cyperaceae and Xyridaceae species, and, in some areas, remnant mangrove species can be also found (Vieillefon 1969; Nascimento 1993; LABOMAR/SEMACE 2005; Meireles 2005).

Despite the poor vegetation and the apparent absence of fauna, hypersaline tidal flat environments play an important ecological role in estuaries, since they shelter several mangrove-typical species as well as other animals that seek the coastal zones during a specific phase of their life cycles (Schaeffer-Novelli et al. 2000). The high density of the larval population of the crab Ucides cordatus, generally identified in Brazilian apicum tidal flats environments during the beginning of summer (rainy season), is one of the examples of the ecological role of these ecosystems (Nascimento 1993). Recent studies (Schmidt 2006; Meireles et al. 2007) emphasise the action of these ecosystems as true 'nurseries' for some species of great ecological importance (i.c. molluses, crustaceans and migratory birds).

Despite their ecological importance, hypersaline tidal flat environments have been greatly affected by anthropogenic activities on the north-eastern Brazilian coast. These impacts are main relate to urbanisation, deforestation and aquaculture (shrimp farming), which affect both direct and indirectly hypersalinc tidal flat biological and physico-chemical properties (Meireles et al. 2010). Brazilian environmental protection laws prohibit shrimp farming activity in mangrove areas but are permissive, with few restrictions, of the installation of this activity in apicum ecosystems (BRAZIL 2012). This has caused the degradation of many hypersaline tidal flat areas by both physical occupation and the discharge of shrimp ponds effluent-rich in nutrients (Lahman and Snedaker 1987; Coelho and Schaeffer-Novelli 2000).

Increasing degradation of hypersaline tidal flat environments in north-eastern Brazil has resulted in the need for basic information about their soils, essential to achieving a better understanding of the overall functioning of these ecosystems and to provide useful information for ecological and environmental studies. However, few studies have dealt directly with hypersaline tidal flat substrates using a pedological approach.

In rare edaphic studies of apicum environments from Brazil (Ruivo et al. 2005; Hadlich et al. 2009), its soils were classified as Arenosols and Fluvisols, due to the predominance of sand fraction and the irregular distribution of organic carbon contents with depth. In similar environments from a Mediterranean climate (region of Murcia, south-eastern Spain), Alvarez-Rogel et al. (2007) classified both Hypersalic Sodic Gleyic Solonchaks and Typic Aquisalids.

Thus, despite the necessity for soil information to understand fully the natural processes that take place in these environments, little is known about hypersaline tidal flats soils from Brazilian coastal areas (Schmidt 2006). The characterisation and classification of soils within these environments are essential to the conservation and sustainable management of these endangered ecosystems.

This paper contributes to the knowledge of hypersaline tidal flat (apicum) soils and their pedogenetic processes. The objectives of this study are to present physical, chemical, mineralogical and morphological data for representative hypersaline tidal flat soils (along with the adjacent coastal compartments--coastal tablelands and mangroves) in a semiarid estuary of the State of Ceara, to discuss the factors and processes that govern pedogenesis in apicum environments and to classify the soils according to the Soil Taxonomy system (Soil Survey Staff 2010).

Materials and methods

Environmental setting

The apicum environment under study is on the Acarau River estuary (Fig. 1), in the northern region of the State of Ceara (Brazil). The regional geology (at the Acarau Basin) is characterised by crystalline rocks with a predominance of gneisses, schists, phyllites, amphibolites and slates. All of these rocks are folded and metamorphosed, with the most important batholiths located at Jaibaras Group (the main structural feature in the region), from the Precambrian (Diniz et al. 2008; Hesp et al. 2009).

The fluctuations in sea level during the Quaternary have greatly influenced the evolution of the Brazilian coastal plains (Suguio et al. 1985). The sea level fluctuations and paleoclimatic changes, which occurred between 22 000 and 14 000 years BP (last transgressive periods), were relevant for the formation of an extensive coastal plain, which includes: estuaries, mangroves, dune fields, lagoons, ponds and the apicum hypersaline tidal flat (Meireles and Raventos 2002).

The surficial geology of the estuary is characterised by fluvial and recent aeolian littoral sediments, derived from sandy-clayish and sandy sediments of the Barreiras Formation. These sediments are characterised by reddish sand, solidly compacted, with conglomerate layers in a reddish sandy matrix (Diniz et al. 2008). The Serra Grande Group, which borders the Acarau Basin, consists of Phanerozoic acid rocks from the Silurian-Devonian periods and the basaltic, rhyolitic rocks of the Parapui Formation (Almeida and Andrade Filho 1999; Galvao 2002).

The climate in the region is tropical semi-arid (Fig. 1 a; Aw according to the Koppen classification), with precipitation from January to April (Fig. 2). There are two well-defined seasons: a long and dry season in winter and a short rainy and hot season in summer (Koppen and Geiger 1928). The main vegetation unit in the Acarau River and its tributaries is dominated by Caatinga (a seasonal, xerophilous thorn woodland/shrubland), and the area's climate is classified as tropical hot semi-arid (Maia et al. 2006). The average annual precipitation is 950 mm and the average annual evapotranspiration is 1600 mm (Lobato et al. 2008). Harsh droughts, occasionally associated with El Nino-Southern oscillation (ENSO), are common. Furthermore, an extremely high evaporative demand may cause evaporation rates > 10 mm day 1 (Folhes et al. 2009). Average annual maximum and minimum temperatures are 34 and 18[degrees]C, respectively.

The studied hypersaline tidal flat system is in the transition zone between the coastal tablelands (highlands, towards the inland areas) and the mangrove forests in the seaward position (Fig. 3). The vegetation fringing and forming patches within the Acarau River hypersaline tidal flat is dominated by the halophyte Batis maritima L. (Fig. 4f). The apicum environment under study is flooded twice a month during spring tides events, which allows sea water to evaporate between periods of inundation, whereas the mangroves are flooded daily. The area is a mesotidal environment, with a diurnal tide and a maximum range of 3 m (Jimenez et al. 1999).

Soil sampling

Two representative soil profiles from each coastal compartment (coastal tablelands, apicum and mangroves; Fig. 3) were selected based on their physiographic position, constituting a toposequencc. Soil profiles from coastal tablelands (PI and P2) were at the summit position (altitudes ranging between 18 and 29 m; Fig. 3). Apicum soil profiles (P3 and P4) were at central part of the toposequence (footslope positions). Profile P3 occupies a higher position within the hypersaline tidal flat (2 m) in close contact with highland vegetation (coastal tableland) and is subject to less frequent tidal flushing. Soil profile P4 (altitude ~1.5 m) is closer to the mangrove forest and thus marked by more frequent tidal flooding (Fig. 3). In the adjacent mangrove forest (toeslope position), two soil profiles (P5 and P6) were nearly at sea level. The soil profile P5 was close to the mangrove creek, under a Rhizophora spp. forest, whereas P6 was in a more seaward position, under an Avicennia spp. forest (Fig. 3).

Sampling was performed during the dry season of 2009. Soil pits were dug in coastal tableland (to a depth of ~2m) and apicum (to a depth of 1 m) and sampled by horizon. Mangrove soil profiles (P5 and P6) were sampled during the low tide with a special sampler for flooded soils, which consisted of a 0.9-mlong stainless-steel tube of 0.1 m diameter; samples were taken at 0-0.3, 0.3-0.6 and 0.6- 0.9 m depth.

For each soil, a detailed morphological description was carried out following the guidelines proposed by Schocneberger et al. (2002). Soil colours were obtained using the Munsell Color (2000) chart. The pH and redox potential (Eh) values of all apicum and mangrove samples were measured in the field, after equilibrating the cores and electrodes for ~2 min. Probes were inserted in the centre of each core to avoid contact with the atmosphere. The pH values of all samples were measured using a glass electrode calibrated using pH 4.0 and 7.0 standards. The Eh was measured in the field using a platinum electrode and the final readings were corrected by adding the potential (+244 mV) of a calomel reference electrode.

Analytical procedures

In the laboratory, subsamples from apicum and mangrove soils were air-dried, crushed and sieved, whereas part of the samples was frozen for posterior acid sulfate soil analysis (e.g. for the determination of sulfidic material and quantification of pyritic iron). All samples from coastal tableland soils were analysed after drying, crushing and sieving procedures.

The EC was measured in saturated extracts, using dried samples (Rhoades 1996), with a conductivity meter. Total organic carbon (TOC) was determined by dry combustion of dried samples, using a LECO-CNS Model 2000 (LECO Corp., St. Joseph, MI, USA), after treatment with 6 m HCl to remove inorganic C. Also, for samples from coastal tableland soils, a combined glass-calomel electrode was used to determine pH (1:2.5 solid/liquid ratio) values.

Before chemical and physical analyses, the air-dried samples from apicum and mangrove soils were washed with 60% (v/v) aqueous ethanol to remove salts (Bower et al. 1952; Sumner and Miller 1996) until the silver nitrate test (AgN[O.sub.3] 0.05 N) indicated absence of chloride. The pipette method (Gee and Bauder 1986) was used for particle-size analysis, preceded by oxidation of organic matter with [H.sub.2][O.sub.2] (30%) by using a combination of physical (overnight shaking) and chemical (0.015m M [(NaP[O.sub.3]).sub.6] + 1.0M NaOH) dispersal methods. Potential acidity (H + Al) was determined by extraction with a 1 M Ca acetate solution at pH 7 (Quaggio et al. 1985). Cation exchange capacity (CEC) was calculated as the sum of exchangeable cations (Ca, Mg, K, Na, and H + Al; Sumner and Miller 1996).

Exchangeable K and Na were extracted with 0.05 N HCl + 0.025 N [H.sub.2]S[O.sub.4] (Mehlich I; Mehlich 1953), and exchangeable Ca, Mg and Al were extracted by shaking with 1m KCl (1:5, soil: solution), from which the cations were determined by flame photometry (K and Na), atomic absorption spectrophotometry (Ca and Mg), and by titration with a standard NaOH solution (Al). The calcium carbonate equivalent was obtained by AO AC method (AOAC 1970) and Metson (1956).

In order to test for the presence of sulfidic materials, the total potential acidity (TPA) was determined after oxidation of fresh soil (mangrove and apicum) samples with [H.sub.2][O.sub.2] (30% at pH 5.5; 1 soil: 5 solution), followed by pH measurement, according to Konsten et al. (1988).

Additionally, to quantify the pyritic fraction, a partitioning of solid-phase Fe was determined using the method proposed by Lord (1982) in fresh samples. Briefly, the method consists in the extraction of two operationally defined fractions: FI. iron oxyhydroxides: extracted with 20 mL of a solution of 0.25 M sodium citrate+ 0.11 m sodium bicarbonate, with 3g of sodium dithionite then samples were shaken for 30 min at 75[degrees]C and centrifuged at 10000 rpm (4[degrees]C) for 30 min; F2, pyrite iron: extracted with 10 mL of concentrated HN03, then samples were shaken for 2 h at room temperature and washed with 15 mL of ultrapure water.

Before the extraction of pyrite-Fe, samples were pre-treated with 10 M HF for 16 h under agitation to eliminate sheet silicate Fe, and concentrated [H.sub.2]S[O.sub.4] was then added to eliminate Fe associated with organic matter (Lord 1982; Huerta-Diaz and Morse 1990, 1992). Between each step of the extraction procedure, samples were washed with 20 mL of ultrapure water. The degree of pyritisation (DOP) was calculated as follows:

DOP(%) = [(pyritic Fe)/(oxyhydroxides Fe + pyritic Fe)] x 100

The DOP determines the percentage of reactive iron incorporated in the pyritic fraction (Berner 1970) and enables comparison between soils with different concentrations of reactive iron. The DOP was calculated by considering reactive iron (iron that can react with sulfide to form pyrite), as FI which mainly consists of iron oxyhydroxides (Berner 1970).

Mineralogical analysis was carried out on surface and subsurface samples from the studied profiles. The mineralogical composition of the clay minerals ([less than or equal to] 2[micro]m) was identified by X-ray diffraction (XRD). Clay samples were prepared according to the method of Jackson (1969) and analysed as oriented aggregates after being saturated with [Mg.sup.2+] and [K.sup.+]. The [K.sup.+]- saturated samples were analysed at 25[degrees]C and after heating to 350[degrees]C and 550[degrees]C for 2h (K25[degrees]C, K350[degrees]C and K550[degrees]C, respectively), whereas the [Mg.sup.2+]- saturated samples were processed like oriented aggregates (Mg) and were also glycerol saturate (Mg-gli). Ground sand samples were spread on a double-sided tape mounted on a glass coverslip and analysed as a random powder sample (Jackson 1969). The X-ray diffractograms were obtained with a diffractometer Shimadzu XRD 6000 (Shimadzu Corporation, Tokyo), operating at 40 kV and 20 mA using Cu-[K.sub.[alpha]] radiation, a graphite crystal monochromator, at 1.5 [degrees]2 [THETA] [min.sup.-1] in the 3-5 [degrees]2 [THETA] range (for oriented aggregates) and 5-70 [degrees]2 [THETA] range (for random powder).

The photomicrographs of pyrite were obtained from the dense soil fraction, which was separated with bromoform (CH[Br.sub.3] density 2.89 g m[L.sup.-1]). The dense sample was placed on an aluminum support for observation by scanning electron microscopy. Elemental analysis (EDS, energy-dispersive X-ray spectrum) was carried out with an Evo LS15 analyser (Carl Zeiss Microscopy GmbH, Jena, Germany). Microanalysis was carried out, after an internal calibration, using an Inca X-act Microprobe (Oxfor Instruments PL, Abingdon, UK) at 20 kV and a waiting time of 100 s.

Results and discussion

Morphological properties

Morphological, chemical and analytical data of the representative soil profiles are presented in Tables 1 and 2. Both hypersaline tidal flats soil profiles (P3 and P4) presented low chroma and gley colours throughout all horizons (Table 1, Fig. 4a, b). These colours are characteristic of gleying process, the reduction of ferric compounds under waterlogged

conditions, with the production of bluish, greenish grey or whitish matrix colours (Munsell colours) (Schwertmann 1992). However, lower chroma (<2; Table 1) and the dominance of gley hues (10GY and 5GY; Table 1) in P4 indicate a more intense gleying process in this site, similar to those found in mangrove soils (Table 1, Fig. 4c), probably in response to the proximity to the sea, which favours longer periods of water stagnation and, thus, gleisation.

By contrast, profile P3, in an inner zone in the tidal flat (transition with coastal tablelands; Fig. 3), presented more mottling (Table 1, Fig. 4d), indicative of alternating periods of oxidation and reduction and thus associated with more frequent watertable fluctuations. Coastal tableland soils presented higher chroma colours (Table 1), evidencing better drainage conditions, due to their higher topographic position, sandy texture (Table 1) and, thus, higher permeability.

In the apicum soils, lithological discontinuities were identified from field observations of texture and soil colour and were corroborated by TOC depth profile (Table 2). Clear horizon boundaries (mostly clear and smooth, see Table 1) provided morphological evidence on the formation of these soils from material transported by fluvial and/or aeolian processes, which corroborates the hypothesis proposed by Meireles et al. (2007) for the formation of the apicum hypersaline tidal flat ecosystem. Additionally, the presence of buried mangrove plants in P4, associated to high concentrations of pyrite iron (see below), also appears to support this interpretation.

Furthermore, erosive processes in coastal tableland soils may also provide sediments to the apicum, establishing an interaction between both coastal compartments. The single-grained structure of coastal tableland soils (Table 1), mostly caused by sandy textures (sand >90%, Table 1) and low TOC contents (Table 2), may favour this erosive process.

With regard to soil structure, both mangrove and apicum soils did not present recognisable macroaggregates (massive soil structures; Table 1, Fig. 4a-c). Despite the reduced flooding frequency by the tides and the restricted entrance of fresh water in the tidal flat (Zack and Roman-Mas 1988), especially compared with mangrove forest soils, both hypersaline tidal flat soils showed weak pedality (Table 1, Fig. 4a, b). This morphological characteristic may be related to both the hydrological conditions and the plant cover. The absence of macroaggregates indicates that both P3 and P4 were subjected to an incipient physical ripening, which consists of dehydration and shrinkage, an increase of permeability, crack development, an increase in permeability and the development of soil structure (Ellis and Atherton 2003; Vermeulen et al. 2003), probably in response to regular flooding events. Additionally, since plant activity is a key factor for structure development and, thus, soil ripening (Ellis and Atherton 2003), the poor and patchy vegetation cover in hypersaline tidal flats (Fig. 4g) is probably another determinant factor for weakly structured surface horizons.

Physical properties

Soil physical properties varied widely depending on the pedon (Table 1). Sand, silt and clay contents of P3 were in the ranges 70-89, 5-13 and 6-19%, respectively, whereas in P4, contents were 40-97, 2-29 and 1-31%. A higher proportion of sand was observed in P3 (Table 1) than P4. This textural contrast between P3 and P4 may have been generated by the different physiographic positions of both profiles.

Because of its physiographic position, the sediment supply at the P3 location probably depends on both tidal action and erosion of upland (PI and P2) soils by pluvial discharge. Thus, the coarser texture of P3 is probably related to its location close to the transition between hypersalinc tidal flats and coastal tablelands (Fig. 3b), where soils are more subject to input of eroded material from surrounding upland soils (Marques 2010). The presence of soils derived from the sandy-clay sediments of the Barreiras Group in the coastal tablelands, such as PI and P2 (Typic Quartzipsamments, Table 1) and sandy clay loam Ultisols (Lima et al. 2006; Vieira et al. 2012), is in accordance with the ideas expressed above. This sediment input from upland soils may also explain the sandy-clay-loam texture in the mangrove soil profile P5, which is in closer contact with the apicum. By contrast, higher clay contents in P6 (Table 1) may evidence the dominance of an estuarine depositional environment (less energetic).

Another factor contributing to the sand dominance in profiles P3 and P4 is the theory of aeolian influence on apicum hypersaline tidal flat formation (Meireles et al. 2007). In fact, aeolian sediment transport and dune migration in Ceara coastal zones are known as key factors in the geomorphological dynamics of intertidal areas, especially because dunes play an important role in the coastal sedimentary budget (Jimenez et al. 1999).

The dominant winds and the rainfall regime of the Ceara coast are mostly ruled by the intertropical convergence zone (ITCZ). Thus, when the 1TCZ is in its northernmost position (usually between August and December), intense southeasterly winds (average velocities of 7.75ms ') and low rainfall dominate the coastal zone (Sauermann et al. 2003). Previous studies have reported average dune migration rates of 17.5m [year.sup.-1] and bulk sediment transport rates of 90-100 [m.sup.3] [m.sup.-1] year 1 (Jimenez et al. 1999). Therefore, the great wind intensity along the Ceara coast would promote changes in the morphodynamics of the estuarine system, causing high rates of dune migration, siltation of tidal channels and the supply of sediments to sand banks, which would evolve to apicum hypersaline tidal flats (IB AM A 2005; FUNCEME 2009).

Chemical properties

In apicum and mangrove soils, the ratio of CEC to clay content, a commonly used proxy for clay activity, indicates the presence of high activity clays in representative profiles from both compartments, as judged by the values of >24 [cmol.sub.c][kg.sup.-1] clay (Table 2). In fact, the XRD data of the soils from P3, P4, P5 and P6 showed the presence of 2: 1 minerals (smectite, illite, vermiculite and interstratified smectite/illite; Fig. 5). Both hypersaline tidal flat soil profiles presented an exchange complex dominated by sodium ([Na.sup.+] > [Mg.sup.2+] > [Ca.sup.2+] > [K.sup.+]; Table 2), a condition also observed in mangrove soils. Considerably lower CEC and base saturation values in coastal tableland soils are related to their coarser textures and better drainage conditions.

The highest values of [Na.sup.+] (ESP >[greater than or equal to] 15%; Soil Survey Staff 2010), recorded in apicum soils, are related to the flooding events by sea water, the high evaporative environment and low relief (2 m above sea-level), which promotes the stagnation of water and thus accumulation of bases by preventing leaching.

The EC values in apicum soils ranged from 25 to 44 dS [m.sup.-1], with the highest values corresponding to the soil profile P4 (Table 2). The EC values showed a clear increase towards seaward positions where the hypersaline tidal flat soils come in closer contact with seawater, and the mangrove ecosystem. However, the EC values found in apicum and mangrove soil profiles are considerably higher than recorded in soils from other coastal ecosystem environments. In fact, the EC values in the present study were 140% higher than in mangrove soils from other parts of Brazil (Ferreira et al. 2010; Prada-Gamero et al. 2004).

These data are evidence of an intense salinisation process in hypersalinc tidal flat soils, probably due to an association of both its higher physiographic position in relation to other coastal environments (i.e. mangroves and salt marshes) and the high evapotranspiration rates. The more elevated position of hypersalinc tidal flat would promote a low flooding frequency (mostly related to spring tides events), while the high evapotranspiration rates (Lobato et al. 2008) would cause the evaporation of saline water (Zack and Roman-Mas 1988; Ridd and Stieglitz 2002) and the high salt concentration (Zack and Roman-Mas 1988). In fact, both hypersaline tidal flat soils presented white and wavy surface salt crusts (Fig. 4e) characteristic of an intense salinisation process (Hamdi-Aissa et al. 2004; Joeckel and Ang Clement 2005; Zenova et al. 2007).

Furthermore, the highest exchangeable sodium percentage (ESP) (>20%) in apicum soils indicates an intense solonisation/sodication process, which is probably favoured by the lower solubility of calcite compared with other salts that are commonly are found in coastal and arid environments, i.e. gypsum (CaS[O.sub.4].2[H.sub.2]O), anhydrite (CaS[O.sub.4]), halite (NaCl), sylvite (KCl), etc. In this case, the precipitation of calcium as CaC[O.sub.3] would remove dissolved and exchangeable [Ca.sup.2+] and promote its replacement by [Na.sup.+] (reaction 1; Langmuir 1997; van Breeman and Buurman 2002):

[Ca.sup.2+](ex) + 2[Na.sup.+.sub.(aq)] [right arrow] 2HC[O.sub.3] + CaC[O.sub.3] + [H.sub.2]O + 2[Na.sup.+.sub.(ex)] (1)

In fact, the near-neutral and alkaline conditions (pH 7.7-9.1) (see McBride 1994; Chesworth 2008) found in most horizons from both apicum soils (except the deeper horizons in P4) are evidence that the system is buffered by the dissolution and precipitation of CaC[O.sub.3] (reaction 2), which is consistent with the high calcium carbonate content found in these horizons (Table 2).

Under these conditions, practically all calcium is precipitated as calcite and the soil solution remains dominated by HC[O.sup.-sub.3] and [Na.sup.+], as the counter ion. As the calcite precipitation takes place, in response to the high evapotranspiration rates (especially in apicum, due to its higher topographic position when compared with mangrove), the concentration of C[O.sup.2.sub.3] strongly increases, leading to high pH values (pH >8.5) (for more detail see van Breeman and Buurman 2002; Chesworth 2008), and to an alkalisation process:

CaC[O.sub.3] + H+ [left and right arrow] [Ca.sup.2+] + HC[O.sub.3] (2)

A completely different situation was found in subsurface horizons (below 16 cm) from P4, where field pH values dropped to <5.5 (Table 2) and oxidation pH to <3. As previously mentioned, these deeper apicum soil horizons are probably related to a buried mangrove soil. The low oxidation pH values and the presence of pyrite in both studied mangrove soils (Table 2) support this hypothesis. In fact, previous work in this area (Marques 2010; Araujo Jr et al. 2012; Nobrega et al. 2013), as well as in other parts of Brazil (Ferreira et al. 2007), recorded high concentrations of iron sulfides, especially pyrite (FeS2) and meta-stable sulfides, i.e. AVS (FeS, [Fe.sub.3][S.sub.4]) in mangrove soils. The oxidation of these iron sulfides may have caused the lower field and oxidation pH in these subsurface horizons from apicum soils (Table 2). Thus, the watertable oscillation, evidenced by the presence of mottles (Table 1), indicates alternating redox conditions, which may be responsible for sulfide oxidation upon aeration during low watertable events (reactions 3, 4, 5 and 6) (Stumm and Morgan 1996).

Fe[S.sub.2](s) + 7/2 [O.sub.2] + [H.sub.2]O) [left and right arrow] [Fe.sup.2+] + 2S[O.sub.4.sup.2-] + 2[H.sup.+] (3)

[Fe.sup.2+] + 1/4 [O.sub.2] + [H.sup.+] [left and right arrow] [Fe.sup.3+] + 1 /2[H.sub.2]0 (4)

[Fe.sup.3+] + 3[H.sub.2]O [left and right arrow] Fe[(OH).sub.3](s) + 3[H.sup.+] (5)

Fe[S.sub.2](s) + 14[Fe.sup.3+] + 8[H.sub.2]O [left and right arrow] 15[Fe.sup.2+] + 2S[O.sub.4.sup.2-] + 16[H.sup.+] (6)

For apicum soils, the oxidation pH values (Table 2) decreased to <4.0 only in P4 (Table 2), indicating the presence of sulfidic material, as established by Soil Survey Staff (2010). However, a pronounced drop in the pH was recorded in most apicum soil samples upon exposure to oxidising conditions (Table 2), which is clearly related to the presence of pyrite-Fe obtained from the sequential extraction method (Table 2; P3, 59 [micro]mol [g.sup.-1]; and P4, 151 [micro]mol [g.sup.-1]).

The presence of pyrite-Fe at the hypersaline tidal flats soils was evident mainly in the deepest layers of P3 and P4 (Table 2), probably related to the presence of an old buried mangrove, which is consistent with the formation of apicum hypersaline tidal flat from siltation of tidal channels (Meireles et al. 2007) and mangrove fringes. The presence of what appear to be degraded pyrite framboids in the deeper horizons of both apicum soils (Fig. 6) is consistent with this idea.

Previous studies on soils from another typical transitional ecosystem between mangroves and the highlands (locally called restinga forests) have recorded degraded pyrite framboids, which were attributed to a buried mangrove facies (Gomes et al. 2007a, 2007b). Additionally, in the subsurface layers the more reducing conditions may have favoured the preservation of the sulfidic material or even active sulfidisation. The high degree of iron pyritisation (DOP) obtained in these apicum deeper horizons (Table 2; 61 and 70.5% for P3 and P4, respectively), also support the latter hypothesis.

The morphological differences between pyrites from apicum and mangrove soils (Fig. 6) seem to be a result of more oxidising conditions in the former (Eh >400 mV, Table 2). In apicum soils, the crystallite edges of most pyrites are not sharp (Fig. 6a, b). By contrast, in mangrove soils, pyrites present well-defined crystal edges (Fig. 6d, e), probably due to less oxidising conditions in deeper layers (Eh <100 mV). These morphological differences may also be related to the formation of iron oxyhydroxide coatings on the surface of pyrite framboids from apicum soils, also in response to oxidation (see Belzile et al. 1997; Huminicki and Rimstidt 2009; Torrento et al. 2010). These coatings may also have prevented total pyrite oxidation, which would be expected under the oxic conditions found in apicum soils (Table 2).

On the other hand, the pyrite in surface layers of hypersaline tidal flats soils would have been almost completely oxidised (DOP <1%, Table 2) in response to the lower watertable levels established after the sedimentation events followed by a surface elevation (upbuilding). Because of this oxidation process, the produced dissolved Fe would precipitate as iron oxyhydroxides (see Ferreira et al. 2007). The highest concentrations of Fe-oxyhydroxides in both apicum profiles (Table 2; P3, 38-110 [micro]mol [g.sup.-1]; P4, 63-143[micro]mol [g.sup.-1]) and low DOP (Table 2; P3, 0.5-1.6%; P4, 0.5-0.6%) compared with those found in mangrove soils corroborate this hypothesis. In fact, the processes of vertical accretion and surface elevation have been pointed out as important factors for mangrove dieback in other estuarine areas of the world (Rogers et al. 2005).

Mean TOC contents varied from 3.9 to 11.8 g [kg.sup.-1] in P3 and P4, respectively. In both hypersaline tidal flat soil profiles, the TOC contents presented an irregular depth distribution pattern (Table 2), which corroborates the previously mentioned burial hypothesis. The relatively high TOC contents found in deeper layer in both apicum profiles (Table 2) are probably related to the existence of mangrove plant vestiges in subsurface horizons (i.e. P4, 3Czgjnk2; Fig. 4f). In fact, Meireles and Raventos (2002) identified ancient mangrove deposits (with ages of ~310 [+ or -] 45 years BP) covered by recent marine terraces on the eastern coast of Ceara (Brazil). This record of buried mangroves is consistent with the characteristics observed in P4, where buried plant material was found at 10-65 cm depth. Previous studies from other parts of Brazil (Nascimento 1993; Ucha et al. 2008; Hadlich et al. 2010) and the world (Marius 1985; Marchand et al. 2011) have also documented the existence of buried mangrove tissues in similar tidal flats (for further details, see Marques et al. 2013). It is noteworthy that these higher TOC contents in deeper layers would have favoured pyrite formation, since organic matter is a key factor for maintaining the activity of sulfate-reducing bacteria (Ferreira et al. 2007). This affirmation is consistent with the data of the sequential extraction (pyrite-Fe; Table 2).

The morphological, chemical and physical characteristics of the studied profiles were used to classify the soils taxonomically (Tables 1 and 2). According to the Soil Taxonomy (Soil Survey Staff 2010), the soils are classified as Entisols. The coastal tableland soils (P1 and P2) were classified as Typic Quartzipsamments due to their sandy composition (Table 1), mainly comprising quartz (Fig. 5c).

With regard to apicum and mangrove soils, profiles showed low chroma colours (gleyed horizons) and redoximorphic features (Fig. 4a, b, d) in response to the saturation and reduction, thus, indicating an aquic condition. The sulfidic material was applicable only for P4, P5 and P6, due to the insufficient pH decrease in P3 (after exposure to oxidising conditions) and due to the relatively high amounts of calcium carbonate equivalent (Soil Survey Staff 2010). However, it must be restated that considerable amounts of pyrite were found in the subsurface layers of all four soils (Table 2), along with pyrite framboids (Fig. 6a, b). According to Sullivan et al. (2010), P3 can be classified as a hyposulfidic soil since it did not present a strong acidification after the oxidative process (Table 2). The Sullivan et al. (2010) classification scheme seems adequate for hypersaline tidal flats soils with high CaC[O.sub.3] contents. By contrast, according to the same classification scheme, P4, P5 and P6 would be classified as hypersulfidic soils, indicating their strong acidification potential.

However, in Soil Taxonomy (Soil Survey Staff 2010), P3 would be classified as a Typic Fluvaquent due to the irregular decrease in content of organic carbon with depth (Table 2). Despite the fact that P4 presented the same organic carbon irregularly with increasing depth, this soil (and both mangrove soils, P5 and P6) would be classified as a Typic Sulfaqucnt due to the presence of sulfidic material evidenced by a sharp decrease in pH values ([less than or equal to] 4.0; Soil Survey Staff 2010). Although most soils were adequately classified, the authors propose the inclusion of the subgroup 'Fluventic in the Sulfaquent great group.

Mineralogical data

The XRD patterns for hypersaline tidal flats and mangrove soil profiles showed the same sand and clay mineralogy (Fig. 5a, b, d, e). In the sand samples from both hypersaline tidal flats and mangrove soil profiles, quartz appeared as the dominant mineral (d-spacing 0.334, 0.426 nm) followed by traces of feldspar (P3: 0.292 nm; P4: 0.296 nm; P5: 0.324 nm; P6: 0.321, 0.325, 0.330, 0.334, 0.653 nm). The dominance of quartz in both soils is probably related both to sand derived from dune migration and to the downhill erosion of the sandy-clay sediments (Lima et al. 2005; Sucupira et al. 2006) of the Barreiras Group (at upstream coastal tableland). In fact, the mineralogical composition of the coastal tableland soils is composed, essentially, of quartz (Fig. 5c), indicating that these soils are a potential source of sediments to the coastal plain.

The clay mineral composition of hypersaline tidal flat soils primarily consisted of kaolinite (0.357, 0.722 nm; which collapsed after heating at 550[degrees]C), interstratified smectite/illite (S/I; 1.132, 1.997nm), illite (1.00, 0.500, 0.334nm) and smectite (1.48 and 1.932nm, after glycerol solvation), the last especially at depth. This mineral assemblage is consistent with the assemblage of the mangrove soils (P5 and P6; Fig. 5e), and with the assemblage of other mangrove soils (Souza-Junior et al. 2008; Ferreira et al. 2010) from different sites on the Brazilian coast. This fact further corroborates the contribution of buried mangrove soils to the pedogenesis of apicum hypersaline tidal flat soils.

In the studied hypersaline tidal flat soils the kaolinite is probably related to an allochthonous origin, since soils from the coastal tablelands mainly comprise kaolinite and quartz (Correa et al. 2008; Giarola et al. 2009). On the other hand, smectites in these estuarine environments can be either detrital or authigenic. The authigenic origin may be related to the transformation of illite, due to the oscillating redox conditions (Velde and Church 1999), or the transformation of kaolinite into smectite in response to the marine influence and poor drainage (Vilhena et al. 2010). Additionally, the poor drainage associated with high concentrations of silica and basic cations ([Ca.sup.2+] and [Mg.sup.2+]) may favour the bisiallitisation process and, thus, smectite formation (Chamley 1989). The detrital origin may be related to the terrestrial soils around the studied estuarine environment, which are under a semi-arid climate and may contain 2: 1 minerals (both smectite and illite). The mixed-layer smectite/ illite in superficial layers may be related to a transformation process, which according to Velde and Church (1999) takes place in response to the under-saturation and hypersalinisation associated with the increased concentration of cations from sea water in the upper 10 cm, and also to the oscillating redox conditions (Velde and Church 1999).

Conclusions

The results show that hypersaline tidal flats soils are characterised by a sandy texture and quartz-dominated composition as a result of the coastal morphodynamic associated with the deposition of sandy material by wind action (e.g. dune deflation or migration) and burial of a former mangrove. However, despite the simple mineralogical composition, these ecosystems present a great diversity of processes, reflected by the contrasting acid-base conditions. The strong pH oscillation between soils and depths reflects the heterogeneity and complexity of the pedogenetic processes. The results show that in surface horizons, the most significant processes are the accumulation of sodium salts (solonisation) and the precipitation of calcium carbonates, which buffers the system pH. In addition, acidic pH values found in deeper layers correspond to the oxidation of sulfidic material probably associated with buried mangrove soils.

This burial process would have triggered an upbuilding (allochthonous superficial addition of mineral materials to the top of the soil), lowering the watertable and decreasing the flooding frequency by tides. The mineralogical data, characterised by detrital kaolinite and autochthonous pyrite, corroborate the post-burial pedogenesis theory. These new conditions associated with the high evapotranspiration rates would have intensified the accumulation of salts and, thus, salinisation and solonisation processes. Additionally, the still existent hydromorphism (especially in deeper layers), mainly related to periodic tidal flooding and the higher physiographic position in relation to the surrounding mangrove, would favour glcisation, the preservation of pyrite and, probably, the maintenance of sulfidisation process at minimum rates.

The pedogenetic evidence presented here is fundamental to a better understanding of the general functioning of hypersaline tidal flat ecosystems and may also provide useful information for the environmental studies focussed on conservation and sustainable management of these ecosystems in Brazil. Thus, the diversity of pedogenetic processes presented in this paper seems to be an important contribution to the knowledge apicum soils, an ecosystem that is still poorly studied (especially from a pedological point of view) and heavily threatened, despite the its great importance for conservation of coastal natural resources in north-eastern Brazil. Future pedological studies should aim to provide further information on the connections between hypersaline tidal flats and mangrove ecosystems in order to improve environmental policies and conservation laws, which should aim for the protection of these equally important ecosystems.

http://dx.doi.org/10.1071/SR13179

Acknowledgements

The first author benefited from a scholarship from Coordenayao de Aperfeicoamento de Pessoal de Nivel Superior, CAPES. The present study was financed by the Brazilian Government (CNPq).

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A G. B. M. Albuquerque (A), T. O. Ferreira (B,F), G. N. Nobrega (B), R. E. Romero (A), V. S. Souza Junior (C), A. J. A. Meireles (D), X. L. Otero (E)

(A) Departamento de Ciencias do Solo, Universidade Federal do Ceara, UFC, M.B.12168, Fortaleza, Ceara, Brazil.

(B) Departamento de Ciencia do Solo, Escola Superior de Agricultura Luiz de Queiroz, Universidade de Sao Paulo, ESALQ/USP, Piracicaba, Sao Paulo, Brazil.

(C) Departamento de Agronomia, Universidade Federal Rural de Pernambuco, UFRPE, Recife-Pernambuco, Brazil.

(D) Departamento de Geografia, Universidade Federal do Ceara, UFC, Fortaleza, Ceara, Brazil.

(E) Departamento Edafoloxia y Quimica Agricola, Facultade de Bioloxia, Universidade de Santiago de Compostela, Santiago de Compostela, Spain.

(F) Corresponding author. Email: toferreira@usp.br

Table 1. Morphological and physical properties of the
studied representative soil pedons Mottle quantity: few (f),
common (c), many (m); contrast: faint (F), distinct (D),
prominent (P). Structure: massive (m), blocky structure
which crumbles into single grains (sg), Boundary
distinctness: clear (c); Topography: smooth (s)

                      Matrix
           Depth      colour                   Mottle
Horizon    (cm)       (Munsell)    Colour     Quantity    Contrast

P1, Typic Quartzipsamment,
coastal tableland

c          0-12       2,5YR 3/2      sg          --          --
Cn1        12-50      10YR 4/2       sg          --          --
Cn2        50-107     10YR 5/3       sg          --          --
Cn3        107 156    10YR 5/3       sg          --          --
Cn4        156-200+   10YR 5/4       sg          --          --

P2, Typic Quartzipsamment,
coastal tableland

Ap         0 12       10YR 3/2       sg          --          --
ACn        12- 24     10YR 4/2       sg          --          --
Cn1        24-46      10YR 5/3       sg          --          --
C2         46-97      10YR 5/3       sg          --          --
C3         97-139     10YR 6/4       sg          --          --
C4         139-182    2,5YR 6/6      sg          --          --
C5         182-200+   2,5YR 6/4      sg          --          --

P3, Typic Fluvaquent (hyposulfidic),
apicum salt flat

Azgnk      0-6        5Y 6/2      10YR 6/4        f          P
2Czgnk1    6-15       5Y 6/2      10YR 5/6        c          P
3Czgn2     15-23      5Y 6/2      10YR 5/6        m          P
4Czgjn3    23-30      5Y 7/1      10YR 5/6        m          P
5Czgjn4    30-90+     5Y 6/1      10YR 5/6        m          P

P4, Typic Sulfaquent (hypersulfidic),
apicum salt flat

Azgnk      0-6        5Y 4/1         --          --          --
2Czgnk1    10-16      10GY 5/1    2,5YR 4/4       m          P
3Czgjnk2   16-65      5GY 4/1        --          --          --
4Czgn3     65-92+     10Y 3/1        --          --          --

P5. Typic Sulfaauent (hypersulfidic),
mangrove

Cgjn1      0-30       N 4/           --          --          --
Cgjn2      30-60      10Y 3/1        --          --          --
Cgjn3      60 90      5GY 3/1        --          --          --

P6, Typic Sulfaquent (hypersulfidic),
mangrove

Cgjnk1     0-30       10Y 3/1        --          --          --
Cgjnk2     30-60      10Y 3/1        --          --          --
Cgnk3      60-90      5GY 4/1        --          --          --

                                Silt
Horizon    Structure    Sand     (%)    Clay    Texture

P1, Typic Quartzipsamment,
coastal tableland

c              --        95       4       1     Sand
Cn1            --        98       1       1     Sand
Cn2            --        98       1       1     Sand
Cn3            --        98       1       1     Sand
Cn4            --        97       2       1     Sand

P2, Typic Quartzipsamment,
coastal tableland

Ap             --        97       2       1     Sand
ACn            --        96       2       2     Sand
Cn1            --        96       1       3     Sand
C2             --        96       1       3     Sand
C3             --        94       3       3     Sand
C4             --        93       3       4     Sand
C5             --        93       2       5     Sand

P3, Typic Fluvaquent (hyposulfidic),
apicum salt flat

Azgnk          m         81      11     8       Loamy sand
2Czgnk1        m         72      13     15      Sandy loam
3Czgn2         m         70      11     19      Sandy loam
4Czgjn3        m         77       9     14      Sandy loam
5Czgjn4        m         89       5     6       Sand

P4, Typic Sulfaquent (hypersulfidic),
apicum salt flat

Azgnk          m         40      29     31      Clay loam
2Czgnk1        m         53      26     22      Sandy clay loam
3Czgjnk2       m         62      16     21      Sandy clay loam
4Czgn3         m         97       2     1       Sand

P5. Typic Sulfaauent (hypersulfidic),
mangrove

Cgjn1          m         50      16      34     Sandy clay loam
Cgjn2          m         52      16      32     Sandy clay loam
Cgjn3          m         56      16      28     Sandy clay loam

P6, Typic Sulfaquent (hypersulfidic),
mangrove

Cgjnk1         m         35      35      30     Clay loam
Cgjnk2         m         47      33      20     Loam
Cgnk3          m         52      30      18     Loamy sand

Horizon    Boundary

P1, Typic Quartzipsamment,
coastal tableland

c          cs
Cn1        cs
Cn2        cs
Cn3        cs
Cn4        cs

P2, Typic Quartzipsamment,
coastal tableland

Ap         cs
ACn        cs
Cn1        cs
C2         cs
C3         cs
C4         cs
C5         cs

P3, Typic Fluvaquent (hyposulfidic),
apicum salt flat

Azgnk      cs
2Czgnk1    cs
3Czgn2     cs
4Czgjn3    cs
5Czgjn4    cs

P4, Typic Sulfaquent (hypersulfidic),
apicum salt flat

Azgnk      cs
2Czgnk1    cs
3Czgjnk2   cs
4Czgn3     cs

P5. Typic Sulfaauent (hypersulfidic),
mangrove

Cgjn1      --
Cgjn2      --
Cgjn3      --

P6, Typic Sulfaquent (hypersulfidic),
mangrove

Cgjnk1     --
Cgjnk2     --
Cgnk3      --

Table 2. Chemical properties of the studied representative
soil pedons

TOC, Total organic carbon; SB, sum of base cations; CEC,
cation exchange capacity; Clay act., clay activity;
V%, base saturation (V%= 100 x sum of bases-cation
exchange capacity); ESP, exchangeable sodium percentage,
CCE, CaC[O.sub.3] equivalent, EC, electrical conductivity
of saturation extract; Oxy-Fe, oxyhydroxide-Fe; Pyr-Fe,
pyrite-Fe; DOP, degree of iron pyritisation;
n.d., not determined

Horizon     Depth       TOC (g        Ca     Mg    Na
             (m)      [kg.sup.-1])

PI, Typic Quartzipsamment, coastal tableland

C          0-12           13.5       0.6    0.4    0.3
Cnl        12-50          1.3        0.6    0.0    0.3
Cn2        50-107         0.6        0.6    0.4    0.3
Cn3        107-156        5.2        1.0    0.0    0.3
Cn4        156-200+       4.3        0.6    0.4    0.3

P2, Typic Quartzipsamment, coastal tableland

Ap         0-12           8.4        1.6    2.2    0.4
ACn        12-24          1.9        1.0    1.1    0.4
Cnl        24-46          2.7        1.0    2.4    0.3
C2         46-97          1.3        2.6    0.8    0.3
C3         97-139         3.1        0.8    2.8    0.3
C4         139-182        4.8        1.0    2.4    0.3
C5         182-200+       5.7        1.0    2.2    0.3

P3, Typic Fluvaquent (hyposulfidic), apicum salt flat

Azgnk      0-6            0.57       1.8    3.0    1.6
2Czgnkl    6-15           0.41       1.8    3.4    3.9
3Czgn2     15-23          0.41       2.0    2.8    4.1
4Czgjn3    23-30          0.19       1.2    2.8    2.6
5Czgjn4    30-90+         0.39       2.4    0.8    1.2

P4, Typic Sulfaquent (hypersulfidic), apicum salt flat

Azgnk      0-10           7.7        1.8    6.8    5.6
2Czgnkl    10-16          12.9       1.2    5.2    5.7
3Czgjnk2   16-65          25.8       1.6    4.4    5.8
4Czgn3     65-92+         0.6        0.8    2.0    4.2

P5, Typic Sulfaquent (hypersulfidic), mangrove

Cgin1      0-30           50.2       5.6    9.4    6.4
Cgjn2      30-60          40.0       5.8    9.8    8.5
Cgjn3      60-90          33.8       9.4    7.2    8.5

P6, Typic Sulfaquent (hypersulfidic), mangrove

Cgjnk1     0-30           29.8       8.2    1.4    6.4
Cgjnk2     30-60          23.3       9.0    11.0   7.9
Cgnk3      60-90          22.8       11.0   7.0    6.4

                 K
           ([cmol.sub.c]                          Clay
Horizon     [kg.sup.-1]    H + Al    SB    CEC    act.     V

PI, Typic Quartzipsamment, coastal tableland

C               0.1         2.9     1.3    4.2     --     30.8
Cnl             0.1         1.9     0.9    2.8     --     33.8
Cn2             0.0         2.1     1.4    3.5     --     39.1
Cn3             0.0         1.6     1.3    2.9     --     45.7
Cn4             0.0         2.3     1.3    3.6     --     36.5

P2, Typic Quartzipsamment, coastal tableland

Ap              0.1         1.8     4.3    6.0     --     70.9
ACn             0.1         1.5     2.5    4.0     --     63.7
Cnl             0.1         1.1     3.8    4.9     --     78.5
C2              0.1         1.2     3.7    4.9     --     75.7
C3              0.1         1.4     4.0    5.3     --     74.5
C4              0.1         1.6     3.8    5.4     --     70.1
C5              0.1         1.4     3.5    4.9     --     72.4

P3, Typic Fluvaquent (hyposulfidic), apicum salt flat

Azgnk           0.5         1.2     6.9    8.0      -     85.7
2Czgnkl         1.0         1.1     10.1   11.2   74.7    90.6
3Czgn2          0.9         1.5     9.8    11.3   59.5    86.7
4Czgjn3         0.7         1.5     7.3    8.7    62.1    82.8
5Czgjn4         0.4         1.3     4.8    6.1    101.7   78.9

P4, Typic Sulfaquent (hypersulfidic), apicum salt flat

Azgnk           1.5         0.3     15.7   16.0    --     98.1
2Czgnkl         1.2         4.3     13.3   17.6   80.0    75.6
3Czgjnk2        1.1         1.8     12.9   14.7   70.0    88.1
4Czgn3          0.1         4.9     7.1    12.0    --     59.4

P5, Typic Sulfaquent (hypersulfidic), mangrove

Cgin1           2.0         3.4     23.4   26.8   78.2    87.3
Cgjn2           0.2         3.9     24.2   26.4   82.0    91.7
Cgjn3           0.1         4.7     25.3   28.9   103.1   87.5

P6, Typic Sulfaquent (hypersulfidic), mangrove

Cgjnk1          2.0         1.6     18.0   19.6   65.3    91.9
Cgjnk2          1.8         1.4     29.7   31.1   155.5   95.5
Cgnk3           1.4         2.0     25.8   27.8   154.4   92.8

                             EC        Oxy-Fe   Pyr-Fe
           ESP               (dS          ([micro]mol    DOP
Horizon    (%)-   CCE    [m.sup.-1])      [g.sup.-1]     (%)

PI, Typic Quartzipsamment, coastal tableland

C          5.9    2.6        0.3        n.d.     n.d.    n.d.
Cnl        9.7    2.2        0.2        n.d.     n.d.    n.d.
Cn2        9.3    2.1        0.1        n.d.     n.d.    n.d.
Cn3        9.8    2.3        0.1        n.d.     n.d.    n.d.
Cn4        8.0    2.0        0.1        n.d.     n.d.    n.d.

P2, Typic Quartzipsamment, coastal tableland

Ap         6.4    2.1        0.3        n.d.     n.d.    n.d.
ACn        8.8    2.1        0.1        n.d.     n.d.    n.d.
Cnl        6.4    3.0        0.1        n.d.     n.d.    n.d.
C2         5.5    2.3        0.2        n.d.     n.d.    n.d.
C3         5.4    3.4        0.1        n.d.     n.d.    n.d.
C4         5.5    2.9        0.1        n.d.     n.d.    n.d.
C5         5.7    3.6        0.1        n.d.     n.d.    n.d.

P3, Typic Fluvaquent (hyposulfidic), apicum salt flat

Azgnk      19.8   5.1       42.8        79.0     0.4     0.5
2Czgnkl    35.3   5.4       31.7        82.6     0.4     0.5
3Czgn2     35.8   4.7       30.9        86.1     1.4     1.6
4Czgjn3    29.2   3.7       28.9       110.4     16.5    13.0
5Czgjn4    20.0   2.5       25.8        38.0     59.4    61.0

P4, Typic Sulfaquent (hypersulfidic), apicum salt flat

Azgnk      35.2   10.7      31.4       102.5     0.6     0.6
2Czgnkl    32.5   4.8       32.3       143.8     0.7     0.5
3Czgjnk2   39.6   4.6       44.0        63.3    151.1    70.5
4Czgn3     35.3   2.2       32.9        n.d.     82.7    n.d.

P5, Typic Sulfaquent (hypersulfidic), mangrove

Cgin1      24.0   3.4       28.0        11.8     44.3    79.0
Cgjn2      32.0   3.3       36.3        5.7      42.2    88.1
Cgjn3      29.6   2.4       37.5        n.d.     n.d.    n.d.

P6, Typic Sulfaquent (hypersulfidic), mangrove

Cgjnk1     32.7   5.6       37.2        28.8     13.0    31.1
Cgjnk2     25.4   6.4       50.3        6.0      40.0    87.0
Cgnk3      23.0   10.5      46.9        n.d.     n.d.    n.d.

                       Sulfidic material
                        Field     Post-ox.
Horizon       Eh         pH          pH

PI, Typic Quartzipsamment, coastal tableland

C            n.d.       n.d.        n.d.
Cnl          n.d.       n.d.        n.d.
Cn2          n.d.       n.d.        n.d.
Cn3          n.d.       n.d.        n.d.
Cn4          n.d.       n.d.        n.d.

P2, Typic Quartzipsamment, coastal tableland

Ap           n.d.       n.d.        n.d.
ACn          n.d.       n.d.        n.d.
Cnl          n.d.       n.d.        n.d.
C2           n.d.       n.d.        n.d.
C3           n.d.       n.d.        n.d.
C4           n.d.       n.d.        n.d.
C5           n.d.       n.d.        n.d.

P3, Typic Fluvaquent (hyposulfidic), apicum salt flat

Azgnk        452         8.3        7.5
2Czgnkl      460         7.7        6.7
3Czgn2       460         8.8        6.8
4Czgjn3      403         8.9        6.4
5Czgjn4      396         8.5        6.9

P4, Typic Sulfaquent (hypersulfidic), apicum salt flat

Azgnk        446         9.1        7.0
2Czgnkl      417         8.6        6.6
3Czgjnk2     394         5.3        1.5
4Czgn3       n.d.        5.0        2.5

P5, Typic Sulfaquent (hypersulfidic), mangrove

Cgin1        435         6.9        2.7
Cgjn2        -81         6.8        1.7
Cgjn3        n.d.       n.d.        1.7

P6, Typic Sulfaquent (hypersulfidic), mangrove

Cgjnk1       116         7.2        2.4
Cgjnk2        64         7.1        2.0
Cgnk3        n.d.       n.d.        5.5
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Author:Albuquerque, A.G.B.M.; Ferreira, T.O.; Nobrega, G.N.; Romero, R.E.; Junior, V.S. Souza; Meireles, A.
Publication:Soil Research
Article Type:Report
Geographic Code:3BRAZ
Date:Mar 1, 2014
Words:11263
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