Soil genesis and iron nodules in a karst environment of the Apodi Plateau 1

The Apodi Plateau, located in the northeastern part of the state of Ceará in Brazil, has limestone as its parent material, with significant variation in the soils formed in this region, as well as the presence of iron nodules, having been identified. The iron oxides have been used as indicators for evaluating the weathering processes of some soils, with the nodules being sources of information about pedogenic processes. Seeking to evaluate the influence of the karstic parent material and of the iron nodules on soil genesis in the Apodi Plateau, the morphological, physical and chemical properties of five profiles were evaluated, as well as of the iron nodules present in some horizons. The profiles were classified as RED-YELLOW ARGISOL Abruptic eutrophic plinthosol (P1), Tb HAPLIC CAMBISOL Eutrophic latosol (P2), RED-YELLOW ARGISOL Abruptic eutrophic plinthosol (P3), Tb HAPLIC CAMBISOL Eutrophic latosol (P4) and RED ARGISOL Eutrophic nitosol (P5). Changes in chemical, physical and morphological characteristics were observed at depth in some profiles, suggesting variation in the characteristics of the parent material. Nodules of class C1 (0.053 to 0.25 mm) and C2 (0.25 to 2.00 mm) were concentrated on the surface, alternating with a sub-surface concentration of class C3 (2.00 to 4.76 mm), C4 (4.76 to 7.90 mm) and C5 (7.90 to 19.1 mm). Profile P5, considered as more evolved, showed a predominance of classes C1, C2 and C3 in the surface horizons, indicating degradation of the larger-diameter nodules with the advancing weathering process.


INTRODUCTION
Iron oxides can be used as pedogenic indicators as they make possible a better understanding of the evolution of the processes of weathering, being sensitive to variations in the environment, mainly to changes in the redox conditions of the soil, and so allow us to infer changes in the past forms of landscapes (COELHO; VIDAL-TORRADO, 2003;INDA-JUNIOR;KÄMPF, 2003).
The segregation and accumulation of iron can take place in many ways, being generally related to mottling, nodules, concretions and petroplinthite, among other classifications (CONSTANTINI; PRIORI, 2007).The formation of iron accumulated through nodules is influenced by seasonal variations in the water table (TAN et al., 2006), where materials having diffuse internal structures are formed by the continuous supply of iron.These materials can provide information about very common pedogenic processes in the soils of tropical and subtropical regions (CONSTANTINI et al., 2006;HUANG et al., 2008).
Colour is an important aspect of soils derived from limestone, and is used as one of the defining elements of the classification 'Terra rossa', employed for soils which have their origins in calcareous materials, with a clayey texture and reddish colours of varying hues ranging from 5YR to 10 R, and which are found in the Mediterranean region (AYDINALP;FITZPATRICK, 2009), being related to soils influenced by such iron oxides as hematite or goethite.Soils with similar characteristics, developed in other areas and also originating from calcareous materials have been documented, however under semi-arid climates (KHORMALI et al., 2003).
In Brazil, studies can be found related to soils which developed on calcareous materials with an occurrence of iron oxides; these generally being clayey soils of a reddish colour (5YR) and of oxide mineralogy (hematite and goethite), kaolinitic and micaceous.Lynch (2009) found Chernosols, Nitosols and Latosols in the Planaltina region of the state of Goiás.Oliveira et al. (2000) analysed Cambisols, Red Argisols and Red Latosols developed on limestone in the north of the state of Minas Gerais, and highlighted the presence of nodules and concretions of iron-manganese in the Cambisol profile.Pereira et al. (2013) carried out evaluations on the Serra da Bodoquena, in Mato Grosso do Sul, finding Organosols, Chernosols and Gleysols.Running further evaluations on the same region of Mato Grosso do Sul, Silva et al. (2013) found Red Argisols and Haplic Cambisols and stressed polygenesis as having strongly influencing this variation.
The Apodi plateau, located in the northeast of the state of Ceará in Brazil, is a fruit-producing region of importance in the state, and in soil surveys carried out in the 70s, large areas of Cambisols (BRASIL, 1973) were identified.Some more recent studies carried out in the region however, have shown variations in soil class and the presence of iron nodules (ALENCAR, 2002;MOTA et al., 2007), albeit with few detailed evaluations as to what factors may be contributing to these changes.Thus, with the aim of assessing the influence of the source material and of the iron nodules on the genesis of soils of the Apodi Plateau, morphological, chemical and physical analyses were carried out in five profiles formed from limestone.

Area under study
The Apodi Plateau is inserted in the Potiguar Basin and is part of the Rift System of the Brazilian northeast.The emerged part of the Potiguar Basin displays the Acu and Jandaíra rock formations, the first consisting of red and green, clayey, micaceous and kaolinitic fine-grained sandstones and of siltstones of mostly fluvial origin, while the Jandaíra formation is characterised by the presence of calcite and dolomite, fine to medium grain limestones, which may be inter-bedded with cross-stratified sandy horizons (BRASIL, 1981;MOTA et al., 2008).
The area under study is of approximately 10 ha, planted with banana, and located in the town of Limoeiro do Norte, Ceará.The climate in the region according to the Köppen classification is BSw'h' (hot and semiarid), with an average annual temperature and precipitation of 28.5 °C and 772 mm respectively.The dominant topography of the region is flat, with a declivity ranging from 0.005 to 0.015 m -1 and an approximate elevation of 150 m.

Sampling and Analysis
A transect of approximately 300 metres was established, where five trenches were opened (Figure 1), described and collected as per Santos et al. (2005).The area is of flat terrain with a declivity of around 0.01 m -1 , the typical vegetation is hyper-xerophilic caatinga, and the small distances between profiles do not suggest variations in the climate.Samples from each horizon were broken up and passed through a 2 mm sieve in order to obtain fine, air-dried soil (FADS), in accordance with Empresa Brasileira de Pesquisa Agropecuária (1997).The soil profiles were classified according to Empresa Brasileira de Pesquisa Agropecuária (2013).
Granulometric composition was determined by the pipette method.The pH was determined in water and in a 1 mol L -1 KCl solution in the ratio of 1:2.5.The organic carbon was obtained using the method proposed by Yeomans and Bremner (1988).The Ca 2+ and Mg 2+ cations and exchangeable acidity (Al 3+ ) were Soil genesis and iron nodules in a karst environment of the Apodi Plateau Figure 1 -Representation of profiles P1, P2, P3, P4 and P5, located in the area under study, the Apodi Plateau, Ceara, Brazil extracted with a 1 mol L -1 KCl solution and determined by titration.The exchangeable Na + and K + were analysed by photometric method in a soil extract obtained with a Mehlich-1 solution.The potential acidity was determined by the volumetric method, with extraction by 0.5 mol L -1 calcium acetate at a pH of 7.0.
The levels of Fe in the fine-soil fraction were determined by atomic-absorption spectrophotometry after extraction with a solution of sodium dithionitecitrate-bicarbonate (DCB), in accordance with Mehra and Jackson (1960), and with Tamm's solution in the absence of light, according to Urley and Drees (2008).From the Fe content, extracted by ammonium oxalate (Fe o ) and dithionite-citrate-bicarbonate (Fe d ), the Fe o /Fe d and Fe d /clay ratios were calculated, and used as a qualitative index of the degree of crystallinity of the oxides (INDA-JUNIOR, KÄMPF, 2005).

Iron Nodules
The nodules of iron present in some horizons were separated from the soil matrix with the use of water-jet sieves and were then grouped into five diameter classes (C1 -0.053 to 0.25, C2 -0.25 to 2.00, C3 -2.00 to 4.76, C4 -4.76 to 7.90 and C5 -7.9 to 19.10 mm).
After separation of the nodules the relative mass, density of the nodules was determined by the volumetric flask method (EMPRESA BRASILEIRA DE PESQUISA AGROPECUÁRIA, 1997) together with the levels of iron extracted by the ammonium oxalate (Fe o ) and dithionite-citrate-bicarbonate (Fe d ) (MEHRA; JACKSON, 1960;URLEY;DREES, 2008).

Genesis and Morphology
The soils have depths exceeding 150 cm and display well-developed A and B horizons, thus indicating an evolving pedogenesis; with the exception of profile P3 there was no contact with the parent material of these soils.In general, the soils are of red and reddish colourations, indicating the presence of iron oxides, mainly hematite.
The soils of the Apodi Plateau were generally classified as Cambisols (BRASIL, 1973).However, on the transect defined in the study area, two profiles were classified as Cambisols, two soils identified as red-yellow Argisols and one as a red Argisol.Given the negligible changes in the other factors of formation, this would suggest that changes in source material are influencing this variability, also been reported by other authors (ACHURRA et al., 2009;ALENCAR, 2002;MOTA et al., 2007).
The occurrence of variations in limestone rocks reported by Bautista et al. (2011), where different layers may vary from centimetres to a few metres, can be considered as a cause of the variation seen in profile depth, such as in profile P2 (4.3+ m) and profile P3 (1.9 m) (Table 1), separated by a distance of 70 m.
The prevailing hue of the profiles was 2.5 YR (red) (Table 1).These reddish colours in soils developed on a karst environment are strongly related to a rapid precipitation of iron oxides due to the high pH, which among other factors, favours the precipitation of ferrihydrate, a precursor of hematite, instead of goethite (SILVA NETO et al., 2008;TARDY;NAHON, 1985).
In profiles P2 and P3, vertic horizons were described just below the horizons that showed a greater accumulation of concretions (Table 1).Yaalon ( 2009)

Chemistry and physics
All of the profiles displayed high values for V% (Table 2), a fact also noted by Foster, Chittleborough and Barovich (2004), who found high CEC in soils identified as "Terra rossa', especially for Ca 2+ , to which is attributed the ability for particle aggregation.
The texture of the profiles is predominantly clayey and very clayey, reaching values of the order of 700 g kg -1 in profile P5 (Table 1).Moreover, the texture gradient present in profiles P1, P3 and P5, and the significant amount of waxiness in the last profile are evidence of clay illuviation, a feature also identified by Delgado et al. (2003) as one of the determining factors in the genesis of soils on karst environments in Spain.
The values for the CaCO 3 found were low, with the hypo-carbonate feature (EMPRESA BRASILEIRA DE PESQUISA AGROPECUÁRIA, 2013) only being identified in some of the deeper horizons of profiles P2 and P3 (Table 2).The reason for the low values may be due to better drainage in the profiles favouring solubilisation and leaching.
The Cr layers of profiles P2 and P3, materials which are still little weathered, look to be different material to that which gave rise to the overlying horizons, since they appeared strongly stratified, which may be related to wind deposition and a large selection of grains, as evidenced by the silt content (Table 1).Feng (2011), when evaluating insoluble residues of dolomitic limestones in China, found a predominance of fine materials, mainly sand and silt.
The Cr2 and Cr3 horizons of profile P2, and the Cv1 horizon of profile P3 showed an abrupt change in the values of the Fe o /Fe d ratio (Table 3).As they represent the basal horizons of the profiles, this increase may be related to greater humidity or suggest strata variation in the source material.
In the Cv1 horizon of profile P2, and the Cv1 horizon of profile P3, there is a strong reduction in the amounts of iron extracted through sulphuric acid attack, and in the underlying horizons of the same profiles the reduction is even greater, together with a decrease in the values of silica, reinforcing the hypothesis of variation in the properties of the source material (Table 4).
The values for TiO 2 in horizons Cr2 and Cr3 of profile P2, and Cr2 of profile P3 show a clear contrast (Table 4), as their values are of the order of 2 g kg-1 in these horizons, while in the above horizons they are of the order of 6 g kg-1, confirming the observations of Jiménez-Millán and Nieto (2008).Further, variation was observed in the values of TiO 2 in the above profiles, from the surface to where horizons showing a presence of concretions were identified, suggesting that the source material from the surface to the concretions differs from the source material of the last layers of these profiles.
Among the studied profiles, the Ki index (Table 4), used to relate the weathering intensity, shows that P5 is the profile displaying the highest degree of pedogenetic evolution, this can be evidenced by the lower CEC of the profile.The Kr index greater than 0.75 indicates the presence of kaolinitic soils (MOTA et al., 2007).
Average values for density were around 2.6 Mg m -3 and similar to those found by Singh and Gilkes (1996).It can also be seen that nodule density is greater on the surface, becoming smaller with depth (Figure 3).The increase in density takes place with a decrease in diameter.This inverse relationship between the size and density of the concretions is related to incorporation of the soil matrix, mainly quartz and kaolinite, during the formation and development of the nodules increasing the pore space, thereby resulting in a lower density.
In profiles P1, P2 and P3, high proportions of rounded-habit nodules were seen in the gravel fraction (classes C3, C4 and C5), which displayed a low mechanical resistance.However, nodules with smaller diameters, belonging to the sand fraction, were mechanically resistant, resembling lead shot.
Given the conditions for development of the limestone that makes up the Jandaíra Formation, such as shallow-water marine environments, the formation of rounded limestone structures is common and often associated with ferrous compounds (CASSAB, 2003).Also characteristic of this environment is a rise in water temperature, favouring the precipitation of carbonates, mainly Ca and Fe, while the movement of the waves may have rounded the bodies that were formed.
Soil genesis and iron nodules in a karst environment of the Apodi Plateau Analysing the levels of Fe d in the nodules of profiles P2 and P4, classified as Cambisols (Figure 4), values increase with diameter for classes C1, C2 and C3, suggesting greater variation in the degree of crystallinity.Whereas for profiles P3 and P5, classified as Argisols, there is greater homogeneity, especially in the last profile which has characteristics related to more evolved soil.
In profile P5 the horizons displayed a homogeneous distribution for nodule mass and density, a fact which demonstrates progress of the weathering processes, a slight increase can also be seen in the quantity of nodules of classes C1, C2 and C3 on the surface, which may be related to an environmental condition which favours their preservation, since in this horizon conditions of hydromorphism are virtually absent when compared to the subsurface layers.
Also in profile P5 no nodules of classes C4 and C5 were recorded (Figure 2), which confirms that, both in the horizons and in the more advanced profiles, there is a reduction in larger-diameter iron nodules, suggesting their degradation with the advancement of the weathering process.
From the observations made here, a significant relationship can be seen between the processes of oxidation/reduction in the dissolution of limestone material, and consequently a change in the dynamics of the iron accumulating in the form of nodules in horizons of currently oxidic conditions.In this study, the number of nodules identified and their characteristics are not seen as limiting the development of agricultural activities; a fact which is relevant faced with the economic importance of this region, however genetic peculiarities in the nodules should be pointed out which are giving rise to more extensive approaches.

Table 1 -
Anjos et al. (2007)hysical characteristics of profiles P1, P2, P3, P4 and P5 as common in the lower parts of the landscape, due to the lateral and surface flow of solutions rich in Ca 2+ and Mg 2+ , which favours the formation of 2:1 clays.The position of vertic horizons may be related to obstruction of the vertical flow of the soil solution resulting in the formation of such types of clay, further complemented by the alternating conditions of intense rainfall and prolonged dry periods which is characteristic of this environment, as reported byAnjos et al. (2007).

Table 3 -
Values for iron in sodium dithionite-citrate-bicarbonate (Fe d ), iron in ammonium oxalate (Fe o ), ratio of iron in the oxalate to iron in the dithionite (Fe o /Fe d ) and ratio of iron in the dithionite to clay (Fe d /Clay) in profiles P1, P2, P3, P4 and P5

Table 4 -
Percentage of SiO 2 , Fe 2 O 3 , Al 2 O 3 , MnO 2 and TiO 2 and the molecular indices Kr and Ki determined in the extract from the sulphuric acid attack for profiles P1, P2, P3, P4 and P5