DÍAZ-HERNÁNDEZ et al. (2003) - Organic and inorganic carbon in soils of semiarid regions: a case...

8/7/2019 DAZ-HERNNDEZ et al. (2003) - Organic and inorganic carbon in soils of semiarid regions: a case study from the G

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Organic and inorganic carbon in soils of semiarid

regions: a case study from the GuadixBaza basin

(Southeast Spain)

Jose L. Daz-Hernandeza,

*, Enrique Barahona Fernandezb

,Jose Linares Gonzalezb

aCentro de Investigaciones Agrarias, Camino de Purchil s/n, Aptdo. 2027, 18080 Granada, Spainb Estacion Experimental del Zaidn, CSIC, c/Profesor Albareda nj 1, 18080 Granada, Spain

Received 10 October 2001; received in revised form 23 October 2002; accepted 8 November 2002

Abstract

Intramontane sedimentary basins are an important landscape component for the evaluation of soilcarbon stored in semiarid regions with Mediterranean climate. We determined the soil organic and

inorganic carbon (SOC and SIC) stored at various depths in 81 profiles sampled specifically in one such

basin (3317 km2) located in Southeast Spain. Sampling reached down to 2.04.0 m and the samples

were analysed by conventional methods. The mean SOC content for a 2.0-m depth column was 7.8 kg

m 2, and the mean SIC content (calcite plus dolomite) was 134.4 kg m 2, 23% of total SOC contents

and 51% of total SIC contents in the 2.0-m column were found in the second meter. The figures for SIC

are considerably higher than those reported in other evaluations for semiarid regions.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Calcareous soils; Caliches; Deep profiles; Late Pleistocene; Organic and inorganic carbon; Soil carbon

storage

1. Introduction

The quantification of terrestrial carbon pools is essential for the modelling of carbon

fluxes (Eswaran et al., 2000). Recent estimates of soil organic carbon (SOC) give values of

15001600 Pg for the first meter. Some of the amounts reported are: 1576 Pg (Eswaran et

al., 1993), 14621548 Pg (Batjes, 1996) and 1502 Pg (Jobaggy and Jackson, 2000). There

0016-7061/02/$ - see front matterD 2002 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0016-7061(02)00342-7

* Corresponding author. Agroalimentarya y Pesquera, Tecnologia y Formacion, Apartado de Correos 2027,

18071 Granada, Spain. Tel.: +34-958-267311; fax: +34-958-258510.

E-mail address: [email protected] (J.L. Daz-Hernandez).

www.elsevier.com/locate/geoderma

Geoderma 114 (2003) 6580


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is less agreement on the amount of SOC for depths below 1.0 m. Thus, Batjes (1996)

provides SOC values of 23762456 Pg to a depth of 2.0 m, while Jobaggy and Jackson

(2000) give a value of 1993 Pg to 2.0 m and 2344 Pg SOC to 3.0 m.

The estimate of soil inorganic carbon (SIC) at global level is as yet tentative. Thereported values range from 695748 Pg (Batjes, 1996) to 1738 Pg (Eswaran et al., 1995).

Other estimations are: 780930 Pg (Schlesinger, 1982) and 720 Pg (Sombroek et al.,

1993). A later report (Eswaran et al., 2000) gives a value of 940 Pg for the global SIC.

According to these authors, most of the SIC stocks are found under arid (77.8%), semiarid

(14.2%) and Mediterranean (5.4%) conditions.

Within the Mediterranean area, SIC is frequently present in soils as primary carbonates

(carbonatic rock fragments < 2 mm, including dolomite) and also as secondary calcium

carbonate in calcic and petrocalcic horizons. The genesis of these soils was discussed in

detail by Ruellan (1971) and Vogt (1984). In our area, this type of horizons is abundant in

piedmont plains occurring in intramontane sedimentary basins, but when petrocalcic

horizons are present, many of the pedons sampled for taxonomic purposes are not suitable

for establishing deep SIC (and SOC) profiles since only a few decimeters of the indurated

layer can be sampled. Thus, an increase in the number of cases studied would be useful in

this respect.

The aims of this study were:

to gather further data on deep carbonate profiles, and to obtain an average value of the SOC and SIC stored in a stretch of Mediterranean

landscape large enough to be considered adequate as a sample case for carbonevaluation in the Mediterranean area.

2. Materials

2.1. Geological setting

The GuadixBaza basin is located in the southeast of the Iberian Peninsula (Fig. 1).

The area of this study occupies about 3500 km2, and developed as an endorheic depression

from the Late Neogene to the Late Pleistocene (Vera, 1970; Pena, 1985; Garca Aguilarand Martn, 2000). During this period, the basin was filled with alluvial and lacustrine

materials whose composition varies zonally depending on the nature of source materials.

Those from the Betic ranges (Sierra Nevada, Sierras de Baza and Estancias) are rich in

quartzite, micaschist and dolomite, while those from the Subbetic ranges (Sierras de

Castril, La Sagra, Orce and others) are rich in limestone. Uplifting, tilting and faulting

during the Pleistocene facilitated the capture of the internal drainage network by a tributary

of the Guadalquivir river.

2.2. Geomorphology and soils

Changes in the regional base level, as well as climatic changes during the Pleistocene,

resulted in the modelling of the landscape into five main geomorphological units (Daz

J.L. Daz-Hernandez et al. / Geoderma 114 (2003) 658066


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Fig. 1. Geomorphological map of GuadixBaza basin. Limits approximately represent the watershed limit.

J.L. Daz-Hernandez et al. / Geoderma 114 (2003) 6580 67


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Hernandez, 1998). These units (see Fig. 1) are, in decreasing order by altitude, the

following.

S1: the top surface, consisting of broad relics of the ancient basin floor and fan-

piedmonts formed along the mountain fronts. This surface, gently sloping towards thecentre of the basin, is capped by caliche soils with thick petrocalcic horizons. The soils are

Petric Calcisols and Chromic Luvisols (FAOUNESCO, 1988).

S2: a second series of fan-piedmont surfaces encroaching on the previous one. The

petrocalcic horizons are less developed than in S1. The dominant soils are Petric and

Haplic Calcisols (FAOUNESCO, 1988).

S3: a third planation surface formed by the dissection of S2. Petrocalcic horizons

are less frequent and therefore Haplic Calcisols are more abundant than Petric

Calcisols.

S4: densely gullied land with Gypsic, Calcaric and Eutric Regosols, mostly loamy.

S5: recent fluvial valleys whose soils are Calcaric Fluvisols, mostly sandy.

Some units are difficult to classify in regard to their genesis and relative age, and have

been pooled together and labelled as others (S6). The soils in this unit are Petric and

Haplic Calcisols and Chromic and Calcic Luvisols.

The age of surfaces S1 and S2 is still a matter of discussion, but can be

roughly estimated to be between that of La Solana del Zamborino (a paleon-

tological and archaeological site, which is about 300 ky BP (Ruiz Bustos, 1997))

and that of the oldest Alicun travertine (in Fardes river valley), dated at 150 ky

BP by U/Th (Daz Hernandez et al., 2000). The landscape has remained largely

unchanged since the Solutrean Aurignacian (Casas Morales, 1949), about 20 kyBP.

2.3. Climate

The climate is semiarid, hot in summer, with frequent frosts in winter. The mean annual

temperature is 15 jC. Mean annual precipitation is about 300 mm in the central part, and

about 400 mm in the higher fans located along the mountain fronts (MAPA, 1989).

Rainfall occurs mainly during the winter months, while in summer, rain is scarce and falls

in a few, short but frequently violent storms.

2.4. Vegetation and use

Two mesomediterranean series were identified in this zone (Sanz Toro, 1995): Quercus

coccifera (Rhamno lycioidiQuerceto cocciferae sigmetum, Rivas Martnez, 1987) and

Quercus rotundifolia (Paeonio coriaceae Querceto rotundifoliae sigmetum, Rivas Mart-

nez, 1987).

There are only vestiges of the potential communities, due to anthropogenic factors.

Serial shrubs are mainly formed by Stipa tenacissima with Rosmarinus, Thymus, Phlomis,

etc. (25%). Dry farming crops are extensive in this landscape (50%). Other units are

forests of Pinus halepensis and Quercus ilex (7%), irrigated lands in the bottom ofprincipal valleys (11%), and ramblas (wadis) devoid of vegetation (based on MAPA,

1986).



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3. Methods

To obtain deep profiles, we made use of natural scarps, ravines or deep man made

trenches or pits. Nevertheless, we tried to distribute the sampling points as evenly aspossible to simulate a probabilistic sampling. For this purpose, the area was subdivided

into square cells of 10 10 km in side and one or two sites were selected per cell, in as

impartial a manner as practical. We did not attempt of stratifying the sampling by soil map

units as the soil maps available did not have a suitable scale. Nevertheless, the geo-

morphological units were a good guide as they are very clearly expressed and closely

related to soil distribution.

In total, 81 sites were sampled (Fig. 1). The number of pits taken in each geo-

morphological unit were: 32 (S1), 18 (S2), 3 (S3), 11 (S4), 9 (S5), 8 (S6). The exposures

were cleaned to revel fresh material, and samples were taken at 20-cm depth intervals

down to a depth ranging from 1.20 to 4.0 m. The thickness of the sampled section was

over 2.0 m in most cases. The soil types found were (FAOUNESCO, 1988) 8 Fluvisols,

17 Regosols, 52 Calcisols and 4 Luvisols. We believe that, given the type of sampling

made, these figures reflect the frequency of soil types within the area.

The samples were air dried, rolled and sieved to separate the fine earth (< 2 mm). The

gravels were weighted and stored separately. Indurated petrocalcic horizons were stored in

bulk and the fine matrix was obtained in the laboratory by drilling between gravel

fragments.

Organic carbon was determined in the fine earth by wet oxidation (WalkleyBlack

method). The CaCO3 equivalent was determined by a manometric method. Calcite anddolomite contents were obtained from the CaCO3 equivalent and the peak height ratios

found in the XRD powder diagrams.

Mass fractions of SOC and SIC determined in the fine earth of individual samples were

converted into total amounts contained in a soil column of unit area by taking into account

layer thickness, bulk densities of cemented caliches and fine earth, and correction to allow

for the presence of coarse fragments (Cm). Bulk densities were determined by coating

clods and caliche fragments with saran resin (SCSUSDA, 1967).

The fine earth fraction (Cm coefficient, SCSUSDA, 1967) was calculated by the

weight of gravel separated by sieving, in non-cemented samples, and by the volume

of gravels embedded in cemented horizons, which was determined by image analysisof polished cross-sections. Soil texture was determined by the Robinson pipette

method.

4. Results and discussion

4.1. Distribution in depth

4.1.1. Organic carbon

The average organic carbon distribution with depth (Fig. 2) is similar in allgeomorphic units, decreasing rapidly with increasing depth tending to near-zero

values below 0.80 1.20 m.



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Fig. 2. Mean value curves for organic carbon distribution in depth per geomorphic unit.



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A feature common to units S1 and S2 is the presence of individual samples with

relatively high carbon contents down to 0.600.80-m depth or even deeper. This can be

explained by the frequent occurrence of indurated caliches within this depth range. The

impedance of caliches to rooting frequently results in a root enrichment on this contactcausing organic matter enrichment after root decay.

The organic carbon content at the soil surface in unit S4 (badlands) is clearly lower

(0.9%) than in units S1, S2 and S3, and very low values (0.20.25%) are reached at

shallow depth (0.40 m).

The distribution curve of unit S5 (alluvial land) shows the irregularities and relatively

high carbon contents (around 0.4%) within 1.60 m of the soil surface which are

characteristic of alluvial soils.

The curve habit of unit S6 is uncertain and displays irregularities, which can be

attributed to very high individual values. This feature is common in caliche soils, and

indicates the mixed character of the unit.

4.1.2. Inorganic carbon

Accumulation of carbonates as calcic or petrocalcic horizons is frequent in old

planation surfaces (units S1, S2 and S3), but these horizons are absent in units S4(badlands) and S5 (alluvial land). The depth to the top limit of petrocalcic horizons is

shown in Table 1.

The petrocalcic horizon mostly occurs within 0.40 m of the soil surface, but it can

sometimes be as deep as 1.20 m. There is not a clear relationship to surface age. On the

other hand, the thickness of petrocalcic horizons (Table 2) seems to be related to surfaceage. In the oldest unit (S1), the most frequent thickness is about 1.00 m, whereas in S2, it is

0.60 m and in S3, it is 0.40 m.

Fig. 3 shows the average of inorganic carbon distribution with depth. Units S1, S2 and

S3 show similar patterns of carbon distribution, characterized by:

(a) a relative decarbonatation of near-surface horizons,

(b) a well-defined bulge in carbon content at certain depths which probably results from

the frequent presence of carbonate accumulation horizons at these depths,

(c) a progressive decrease in carbon content from this maximum as depth increases.

Table 1

Depth of the upper limit of caliches

Geomorphological unit S1 S2 S3 S6 Total

Depth (cm) Class interval Number of cases

10 29 20 8 4 1 13

30 49 40 12 2 2 1 17

50 69 60 7 1 1 9

70 89 80 4 1 5

90 109 100 2 1 3

110 129 120 1 1

Mean depth 44.5 50.0 33.3 80.0



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Fig. 3. Mean value curves for inorganic carbon (without dolomite) distribution in depth per geomorphic unit.



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2.0-m depth come from non-indurated deep subsoil materials in Fluvisols, but also from

more or less indurated materials underlying petrocalcic horizons, which were sampled to

obtain a deep carbonate profile.

Regardless of the section thickness considered, the organic carbon stored per unit areadecreases as follows:

Fluvisols > Calcisols > Regosols > Luvisols

SOC seems to be higher in calcaric, petric and calcic soil classes, which could be

accounted for by the stabilizing effect of Ca2 + ions in humic complexes.

The SOC contents agree with those reported by Batjes for equivalent soil units of the

FAOUNESCO (1974) system.


Table 4 summarizes separately the amounts of inorganic carbon contained in calcite and

dolomite. Calcite and dolomite carbon contents are of broadly similar orders of magnitude,

although dolomite carbon is, in general, less abundant. Calcitedolomite carbon ratios can

vary within a very wide range as they depend on parent material lithology and degree of

leaching and/or calcification of soils. Both calcite C and dolomite C tend to increase

monotonically with section thickness.

Mean calcite-carbon (01.0 m) ranges from 0.4 kg m 2 (Chromic Luvisols) to 70.7

kg m 2 (Petric Calcisols). The differences between soil units in Regosols and

Fluvisols are to be accounted for by regional differences in soil lithology. However,the high SIC contents in Petric Calcisols must be a consequence of the added effect of

pedogenesis.

Table 3

Mean organic carbon contents for several depth intervals by FAO UNESCO (1988) soil units

Soil organic carbon

Soil units 0 0.5 m 0 1.0 m 0 2.0 m 0 3.0 m 0 4.0 m

(kg m 2) S.D. N (kg m 2) S.D. N (kg m 2) S.D. N (kg m 2) S.D. N (kg m 2) N

Calcaric

fluvisols

5.2 1.9 8 7.6 3.5 8 10.0 6.9 3 19.6 1

Regosols 3.5 2.0 17 5.2 2.8 17 8.6 4.6 4

Eutric 2.5 0.6 2 4.1 1.1 2 6.2 1

Gypsic 2.4 1.5 5 3.6 1.9 5

Calcaric 4.2 2.1 10 6.3 3.1 10 9.4 5.4 3

Calcisols 5.2 2.5 52 7.0 3.9 52 9.4 5.0 35 10.4 5.0 7 7.7 1

Petric 5.5 2.5 46 7.5 3.9 46 9.7 5.0 33 10.4 5.0 7 7.7 1

Haplic 2.7 1.2 6 3.3 1.0 6 4.2 1.4 2

Luvisols 2.3 1.2 4 2.9 1.2 4 3.9 2.0 2Chromic 1.5 0.0 2 2.1 0.1 2 2.5 1

Calcic 3.2 1.1 2 3.7 1.3 2 5.3 1

S.D. = standard deviation; N= number of profiles.



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4.3. Carbon contents by geomorphic units

Table 5 summarizes the average organic and inorganic carbon contents found for eachgeomorphic unit, as well as other values characterizing the unitsUarea and number of

profiles studied.

Although the profiles were sampled to the greatest depth possible, the results are given

(Table 6) for a standard depth of 2.0 m in order to make comparisons easier.

4.3.1. Organic carbon

The highest SOC content is that of unit S5 followed by that of unit S1. As regards unit

S5, this was to be expected since the formation and posterior burial of A horizons are a

common process in alluvial soils (Alonso-Zarza et al., 1998). This could also be the reason

for the relatively high SOC content found in the lower 1.02.0-m section of unit S 1,mostly formed by indurated CaCO3 caliches, which suggests upward growth of the soils

by alluviation, prior to CaCO3 impregnation, as a frequent process in this unit. We should

Table 4

Mean inorganic carbon contents (kg m 2) for several depth intervals by FAOUNESCO (1988) soil units

Calcite SIC

Soil units 0 0.5 m 0 1.0 m 0 2.0 m 0 3.0 m 0 4.0 m

(kg m 2) S.D. N (kg m 2) S.D. N (kg m 2) S.D. N (kg m 2) S.D. N (kg m 2) N

Calcaric

fluvisols

19.4 15.3 8 37.7 29.0 8 67.0 51.0 3 140.9 1

Regosols 18.1 21.3 17 35.8 44.0 17 58.5 80.1 4

Eutric 0.6 0.6 2 1.0 0.8 2 0.6 1

Gypsic 8.6 12.8 5 17.5 29.3 5

Calcaric 26.4 23.3 10 51.9 48.2 10 77.8 85.9 3

Calcisols 32.2 17.1 52 66.8 38.0 52 123.4 76.6 35 234.8 165.7 7 125.8 1

Petric 34.2 17.0 46 70.7 38.3 46 125.3 77.3 33 234.8 165.7 7 125.8 1

Haplic 16.9 8.8 6 36.6 17.0 6 91.7 77.8 2

Luvisols 2.0 3.3 4 4.9 6.9 4 16.0 19.6 2Chromic 0.2 0.2 2 0.4 0.4 2 2.1 1

Calcic 3.7 4.4 2 9.5 7.9 2 29.8 1

Dolomite SIC

Calcaric

fluvisols

11.6 9.8 8 20.4 11.9 8 47.1 38.5 3 109.3 1

Regosols 11.0 11.6 17 25.7 31.4 17 56.2 95.6 4

Eutric 0.6 0.5 2 1.0 0.7 2 1.4 1

Gypsic 13.6 3.3 5 34.3 13.7 5

Calcaric 11.8 14.3 10 26.3 38.6 10 74.5 108.2 3

Calcisols 6.8 13.9 52 20.0 36.8 52 40.1 48.2 35 64.2 62.2 7 170.4 1Petric 7.1 14.6 46 21.4 38.8 46 41.9 49.0 33 64.2 62.2 7 170.4 1

Haplic 4.3 6.7 6 8.6 12.6 6 9.9 13.0 2

Luvisols 0.2 0.2 4 0.3 0.2 4 0.6 0.5 2

Chromic 0.1 0.1 2 0.1 0.2 2 0.3 1

Calcic 0.4 0.2 2 0.5 0.1 2 1.0 1

S.D. = standard deviation. N= number of profiles.



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point out the relatively high SOC values found in the 1.02.0-m section of unit S4,

dominated by regosols. This could be accounted for by the high bulk densities of subsoil

materials, which must be used as a multiplier for computing volumetric C contents. The

decrease in organic carbon from unit S1 to S4 suggests that their organic carbon content

could be related to surface age. However, this is not plausible taking into account the fast

oxidation rate of organic carbon in well aerated soils and the fact that we are dealing with

surfaces whose ages span about 200 ky BP, although the rate of weathering of ancient

organic matter is not clear (Hedges, 1993; Torn et al., 1997; Giardina and Ryan, 2000;Kirschbaum, 2000).


The amounts of equivalent CaCO3 (as determined by gas manometry), calcite and

dolomite (XRD) were the data used for the computation of SIC (Table 5). Dolomite SIC is

clearly inherited from the parent materials and this can also be the case, at least partly, for

calcite SIC. Calcite SIC is, as a rule, approximately twice as abundant as dolomite. Calcite-

carbon is higher in the 01.0-m section while dolomite SIC is enriched in the 1.02.0-m

section. The total SIC (calcite- plus dolomite SIC) down to 2.0-m depth is much higher

than the corresponding SOC, as expected, given the calcareous nature of soil parentmaterial.

Table 6

Carbon contents (kg m 2)mean values for the GuadixBaza basin

Depth intervals (m) Organic carbon Inorganic carbon Total carbon

Calcite Dolomite

0 1.0 6.0 48.4 17.0 71.4

1.0 2.0 1.8 39.2 29.8 70.8

Total 7.8 87.6 46.8 142.2

Table 5

Organic and inorganic carbon contents (kg m 2) in different geomorphic units of the GuadixBaza basin

Geomorphic units

Depth(m)

S1(piedmont)

S2(piedmont)

S3(piedmont)

S4(badlands)

S5(alluvial)

S6(others)

Area (km2) 921 405 29 791 273 898

Number of profiles 32 18 3 11 9 8

Organic carbon 0 1.0 7.6 5.9 4.4 4.1 8.1 5.5

0 2.0 9.5 7.1 5.1 6.3 12.1 6.5

Inorganic carbon

Calcite 0 1.0 78.5 39.8 51.7 28.3 41.9 41.1

0 2.0 133.0 79.0 84.2 53.7 79.8 77.1

Dolomite 0 1.0 27.3 18.7 7.4 18.9 18.9 3.8

0 2.0 64.1 39.9 46.5 38.8 44.7 39.8



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The highest calcite SIC content is that of surface S1 which is the oldest, but it is difficult

to relate SIC enhancement to surface age given the opposite effect of carbonate trans-

locationwhich would tend to impoverish CaCO3 in the upper horizonsand soil erosion

which would make the horizons of carbonate accumulation shallower.

4.4. Mean SOC and SIC contents for the whole area

The mean SOC and SIC contents per square meter for the whole area (3317 km 2) were

computed by averaging the values for surfaces S1 to S6, weighted by their respective areas.

This weighting criterion was selected because geomorphic units are clearly expressed and

can be easily delimited upon examination of topographic maps and remote sensing images.

These results are given in Table 6, and characterize a semiarid landscape as a whole with

its variety of land units, soil types and vegetation cover.

The average SOC content within a depth of 2.0 m is close to 8 kg m 2 and most of it is,

as expected, contained in the upper 1 m of the soil; it is worth mentioning that the amount

of SOC in the 1.02.0-m section is by no means negligible (1.8 kg m 2), as it represents

23% of the total SOC. Similar results are reported by Jobaggy and Jackson (2000) for a

variety of biomes.

Table 7

Carbon contents in some similar soil types (kg m 2) and number of profiles

Soil types Organic carbon Inorganic carbon Reference

0 1.0 m 0 2.0 m 0 1.0 m 0 2.0 m

(kg m 2) N (kg m 2) N (kg m 2) N (kg m 2) N

Xerosols 6.0 Bohn (1976)

6.2 10 Sombroek et al. (1993)

4.8 73 8.7 8 Batjes (1996)

Xerosols (Calcisols) 7.0 52 9.4 35 66.8a 52 123.4a 35 This paper

Calcic Xerosols 6.0 8 12.8 1 Batjes (1996)

Calcisols

(Petric, Haplic)

7.0 52 9.4 35 66.8a 52 123.4a 35 This paper

Calcaric Fluvisols 6.3 87 12.8 8 Batjes (1996)

7.6 8 10.0 3 37.7a

8 67.0a

3 This paperRegosols 5.0 Bohn (1976)

5.6 6 Sombroek et al.

5.0 42 7.0 9 (1993)

Batjes (1996)

5.2 17 8.6 4 35.8a 17 58.5a 4 This paper

Calcaric Regosols 4.5 11 8.4 2 Batjes (1996)

6.2 10 9.4 3 51.9a 10 77.8a 3 This paper

Arizona (mean) 4.9 91 26.0 91 Schlesinger (1982)

basalts 4.8 17 25.5 17

mixed alluvium 4.2 53 24.2 53

limestones 6.6 21 31.0 21

GuadixBazaarea (mean)

6.0 81 7.8 81 48.4a 81 87.6a 81 This paper

a The inorganic carbon from dolomite is not considered.



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The average SIC content is 134 kg m 2, i.e. 17 times that of SOC. Most of it is

contained in calcite (88 kg m 2). Dolomite SIC is about half that contained in calcite

SIC and would be inherited from parent rocks. Nevertheless, the amount of dolomite SIC

found is circumstantial as it strongly depends on the geological setting. On the basis ofsoil morphology, it is clear that in the older surfaces (S1, S2 and S3), there is significant

translocation of carbonates and, therefore, a high proportion of the SIC must be

considered to be pedogenic. We should point out that calcite SIC is more abundant in

the 01.0-m section within which the top part of most illuvial carbonate horizons is

included.

The amounts of SOC found in the GuadixBaza basin agree quite well with those

found for similar situations or soil types by other authors (Table 7). Post et al. (1982)

give a value for SOC storage of 7.9 kg m 2 for warm temperate dry forest. Also, Adams

et al. (1990) and Prentice and Fung (1990) give a value of 8 kg m 2 for Mediterranean

shrub and woodland. These figures are given for the upper 1 m. The average SOC

storage in the GuadixBaza basin for this depth section is 6 kg m 2, a value somewhat

lower than those cited. The difference can be explained by the fact that most of the area

studied is covered by xeric shrub and cultivated land. The loss of soil SOC in previously

untilled soils following cultivation has been estimated to be about 30% by Davidson and

Ackerman (1993). This reduction applied to 8 kg m 2 would yield a value close to ours

(6 kg m 2).

On the other hand, our data for SOC in the upper 1 m are somewhat higher than those

reported by Schlesinger (1982) for Arizona soils. This difference could be explained by the

more arid conditions of Arizona.As regards SIC, the values found in the GuadixBaza basin are considerably higher

than those reported by Schlesinger (1982) for Arizona soils, even if only calcite SIC is

taken into account. The SIC to SOC ratio is remarkably higher than that reported by

Eswaran et al. (2000) for Mediterranean moisture conditions. Whether this is due to a local

anomaly in parent material composition or is a feature of the Mediterranean area can only

be ascertained by further sampling.

5. Conclusions

The average amounts of SOC (8 kg m 2) and SIC (134 kg m 2) found in the soils of

the GuadixBaza basin are an example of those that could be expected in a Mediterranean

landscape, modified by man, and could be useful for further evaluations of global soil

carbon contents.

Most of the SOC are found in the upper meter, but that contained in the second meter is

not negligible (1.8 kg m 2). The scarce data obtained from deeper sections indicate a rapid

decrease of SOC below 2.0 m.

The closest approximation to the SIC of pedogenic origin is that contained in calcite of

fine earth (or fine matrix of indurated horizons) and this mounts to 88 kg m 2. Calcite

carbon is abundant in both the first and the second meter sections. The lower limit ofcarbonate illuviation horizons could reach as deep as 2.03.0 m. Thus, this depth should

be reached in surveys concerning SIC.



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