Organic and inorganic carbon storage in soils along an arid to dry sub-humid climosequence in northwest of Iran

Catena 153 (2017) 66–74

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Organic and inorganic carbon storage in soils along an arid to dry sub-humid climosequence in northwest of Iran Alireza Raheb, Ahmad Heidari ⁎, Shahla Mahmoodi Department of Soil Science, Faculty of Agricultural Engineering and Technology, College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran

a r t i c l e

i n f o

Article history: Received 5 June 2016 Received in revised form 6 November 2016 Accepted 30 January 2017 Available online xxxx Keywords: Climate Mean annual precipitation Semi-arid Soil carbon content

a b s t r a c t The importance of soil carbon in different biomes of the earth is well understood. Most of the studies on terrestrial carbon cycle have been focused on the surface horizon of soils, but deeper soils have rarely been considered. The effects of arid, semi-arid and dry sub-humid climates on soil organic carbon (SOC), soil inorganic carbon (SIC) and soil total carbon storage (STC) along a soil climosequence, on basaltic underlying rocks were investigated. SOC and SIC content and storage showed reverse trends with increasing soil depth. STCs increased by increasing mean annual precipitation (MAP) from 3.75 and 6.28 kg m−2 in the arid and semi-arid regions, respectively to 11.32 kg m−2 in the dry sub-humid. Despite lower SOC in the soils of arid region, the highest SICs/SOCs ratio was obtained in the arid climate, which indicates the importance of climate on SIC storage compared to the SOC content. The average times to store SIC in the dry sub-humid, semi-arid and arid regions were calculated as 15,400, 23,100 and 26,000 years, respectively; this indicates that SIC is stored more rapidly in wetter climates due to more weathering. SICs constituted the dominant proportion of STCs which decreased by increasing MAP from in the arid region in comparison with other wetter regions (65%, 74.4% and 84.8% in the semi-arid, dry sub-humid and arid regions, respectively). © 2017 Elsevier B.V. All rights reserved.

1. Introduction Carbon (C) storage has attracted a significant amount of attention from researchers (Shi et al., 2012; Zhang et al., 2015), policymakers and environmental scientists in recent decades due to the effect of carbon levels on various aspects of human life and the environment (e.g. reducing greenhouse gases emissions) (Shi et al., 2012; Lal, 2013b). Rising temperature and elevated atmospheric carbon dioxide (CO2) simultaneously affect the dynamics of soil total carbon (STC) (Wang et al., 2016). There are five main C pools on the earth: (1) the lithosphere, including fossil fuels and sedimentary rock deposits such as limestone, dolomite and chalk (66–100 million Pg); (2) oceans (38,000–40,000 Pg); (3) soil organic carbon (SOC) (1500–1600 Pg) (Lal, 2004, 2013a), and measured soil inorganic carbon (SIC) up to 1 meter (695–1738 Pg) (Eswaran et al., 2000; Hirmas et al., 2010); (4) the atmosphere (863 Pg) and (5) the biosphere (540–610 Pg) (Rice, 2004).

Abbreviations: MAP, mean annual precipitation; MAT, mean annual temperature; MAI, mean annual aridity index; MAPET, mean annual potential evapotranspiration; AWB, annual water balance; WSWB, wet season water balance; BD, bulk density; SOC, soil organic carbon; SIC, soil inorganic carbon; Nt, total nitrogen; C/N, carbon/nitrogen ratio; SOCs, SOC storage; SICs, SIC storage; STCs, STC storage; CF, coarse fragments. ⁎ Corresponding author at: Department of Soil Science, Faculty of Agricultural Engineering and Technology, College of Agriculture and Natural Resources, University of Tehran, P.O. Box: 31587-77871, Karaj, Iran. E-mail addresses: [email protected] (A. Raheb), [email protected] (A. Heidari), [email protected] (S. Mahmoodi).

http://dx.doi.org/10.1016/j.catena.2017.01.035 0341-8162/© 2017 Elsevier B.V. All rights reserved.

Many studies have been concentrated on the changes of STC in the topsoil (i.e., 0–20 cm depth) (Harrison et al., 2011) due to the ease of sampling and data collection, but there have been few studies on STC changes in the deeper soil horizon. However, the subsurface soil has a large carbon storage capacity (Jobbágy and Jackson, 2000), and there is a lot of evidence showing that the STC content of subsoil is sensitive to climate changes, land use (Knops and Kate, 2009; Carter and Gregorich, 2010) and management (Khan et al., 2007). The soil C pool consists of two distinct components: SOC and SIC (Zhang et al., 2015). The SOC component, a key indicator of soil quality, influences the essential biological, chemical and physical soil functions such as nutrient cycling, water retention and soil-structure maintenance (Vitti et al., 2016). Due to the sensitivity to environmental changes, SOC is one of the most important components involved in global climate change (Lal, 2004; Selim et al., 2016). Climate and parent materials can introduce a range of C levels in the ecosystems (McLauchlan, 2006). Jenny (1980) expressed that the best climosequence was observed over long transects with gradual slope gradients. Climate provides water and temperature (energy), two main components involved in soil formation. Changes in precipitation and temperature are responsible for the biomass entering to soil, which can also manipulate soil properties. Increasing rainfall decreases soil pH, while increasing the maximum depth of carbonate accumulation, as well as SOC, nitrogen and clay contents (Buol et al., 2011). Soil formation needs long time sequences and it is subjected to different climates. Furthermore, chemical, physical and mineralogical

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properties of parent rocks play a key role in soil formation. Soils formed on ferromagnesian rocks, such as basalt, are usually rich in nutrients such as iron, manganese, potassium, calcium, magnesium and phosphorus. Basalt weathers rapidly and yields fine textured (clayey) soils (Buol et al., 2011). SIC (both lithogenic (LIC) and pedogenic (PIC)) (Batjes, 1996; Zamanian et al., 2016) also plays a significant role in carbon sequestration (Chang et al., 2012; Monger et al., 2015). Inorganic carbon is not considered as a carbon stock in several studies (Wu et al., 2009; Tan et al., 2014), probably due to the longer time needed for changes in carbonates compared to the shorter time for SOC (Rawlins et al., 2011; Yang et al., 2012). Recent studies, however, have demonstrated that soil biota plays an important role in the generation of secondary carbonates (Lee et al., 2008), highlighting potentially rapid changes in SIC. Monger et al. (2015) suggested that the source of calcium (Ca) is the main factor in CO2 sequestration by soil inorganic carbon. Pedogenic calcium carbonate (CaCO3) resulting from limestone weathering, however, does not sequester atmospheric carbon because the source of the Ca is from preexisting CaCO3, and the CO2 consumed in the reaction to form carbonic acid is released upon the reprecipitation of CaCO3 (Drees et al., 2001). In contrast, if the Ca or Mg derives from the weathering of silicates bearing Ca-Mg, two moles of CO2 is consumed per mole of carbonate precipitated, and CO2 is sequestered during this process. Yang et al. (2012) stated that acidification leads to a significant decrease in carbonates in some Chinese soils, especially, those that contained higher amounts of inorganic carbon. This study proves that SIC is dynamic rather than the conventional concept considering it a stable C pool. Sedimentation of carbonates and/or release of CO2 to the atmosphere from SIC acidification influence the terrestrial carbon sequestration processes. SIC migration from the surface horizon to the deeper ones or down slopes in landscapes are other important processes leading to the decrease of SIC in soils. The emitted CO2 from SIC sources may influence the terrestrial C cycle in the ecosystems resulting in global warming. Without considering SIC dynamics, an understanding of the C cycle remains incomplete; thus SIC should be regarded as one of the C stocks (Yang et al., 2012). SIC pools and dynamics in the arid and semi-arid regions, which cover around one-third of the earth's surface, are very important due to their overall higher accumulation rate of SIC than the accumulation seen in other biomes (Drees et al., 2001; Lal, 2004, 2008). The greatest extent of the SIC stock is found under arid, semi-arid and Mediterranean conditions with 77.8%, 14.2% and 5.4%, respectively (Eswaran et al., 2000; Jobbágy and Jackson, 2000). There is an equilibrium between the soil components and the environment; therefore, SOC and SIC, which originated from the same sources during soil formation, reflect the existing thermodynamic equilibrium across the period of soil evolution. With the above-mentioned considerations, the current study compares organic and inorganic carbon content and storage in natural soil pedons along an arid to dry sub-humid climosequence in the soils of northwestern Iran, aiming to: 1) evaluating the relationship and variability of SOC and/or SIC content and some soil properties between the solum and the underlying rock; and (2) quantifying the soil organic and inorganic carbon storage along an arid to dry sub-humid climosequence.

2. Materials and methods 2.1. Study area and field sampling The study was conducted at three regions in the arid, semi-arid and dry sub-humid areas as a climosequence located in northwestern Iran (Fig. 1). Stable positions without anthropic effects were selected to reduce the effect of other variables in the relationships between SOC and SIC.

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The geographic midpoint coordinates of the studied regions are shown in Table 1. Soil moisture and temperature regimes and evapotranspiration were determined using long-term mean annual precipitation and temperature data by jNSM software (java Newhall Simulation Model) (USDA-NRCS, 2012b). Aridity index (AI) = MAP / MAEP (UNEP, 1997) was used to quantify precipitation availability over atmospheric water demand (Table 1). The studied areas located in the Alborz geological zone. Bedrock underlying the soil in all areas consisted of mid to late Eocene basalts (Sahandi and Soheili, 2005). After identification of the areas on geology maps (1:100,000), parent rocks were checked by field control. The arid region consisted of very thick, coarse, textured, massive, non-porous and homogenous megaporphyric trachybasalt without fracture (Chaichi and Mohabbi, 1995). The semi-arid region composed of very thick, textured, massive structure with some planar cracks and homogenous gray basalt, trachybasalt and andesitic basalt complex (Radfar, 2002). The dry sub-humid region contained very thick, fine, textured, massive (with some veins and pores in fractures) and homogenous basaltic lava (Nazari and Salamati, 1998). The studied regions had hilly geomorphic units and about 220–2200 m elevation above sea level. No conflicts with tephra episodes were observed for the basalts in the regions. There was no evidence of covering layers on basalts before denudation; thus, different soil thicknesses in the regions were the results of weathering of underlying rocks (Fig. 2). The upper boundaries of basalt were limited to soil thicknesses in each region (40–80 cm in arid, 80–120 cm in semi-arid and 150 to N 250 cm in dry sub-humid), but their lower depths went beyond 100 m up to 200 m in different regions (Sahandi and Soheili, 2005). Depending on climatic conditions, weathering intensity in different regions varied between low in the arid region to high in the semihumid region. Field observations, physical characteristics and the results of clay mineralogy of surface and subsurface horizons (data not shown) did not show any evidence of dust deposition. All the sampling sites were on stable upland positions with a southeast to northwest aspect and slopes averaging less than 25%. The sampled pedons did not exhibit evidence of human perturbation or accelerated wind and water sedimentation. Land use in the studied regions was rangeland with somewhat different vegetation cover in each region in accordance with their climatological properties. The vegetation cover in the arid region consisted of mainly annual herbs, along with sparse perennial shrubs with low fine-root density. However, in the semi-arid and dry sub-humid regions more intensive deep-rooted annual grasses or herbs, accompanied by perennial shrubs and small trees with moderate to high-root densities, were observed. Ten pedons inside hill geomorphic units from each region (about 500 ha extent each, 30 pedons in 1500 ha in total) were described and sampled according to the standard methods (USDA-NRCS, 2012a). The soils were classified based on Keys to Soil Taxonomy (Soil Survey Staff, 2014). 2.2. Physico-chemical analyses 27 out of 30 pedons comprising 85 samples from the genetic horizons/layers were selected for laboratory analyses. After complete analyses of the samples and classifying the soils according to Keys to Soil Taxonomy (Soil Survey Staff, 2014), nine bulk pedons with the most distinctive characteristics of the soil taxonomic units were selected to be presented (Table 2). All analyses were performed on air-dried and sieved (2 mm sieve) soil samples (Pansu and Gautheyrou, 2006). Coarse fragments' percentage was determined from the weight of fragments with diameter N2 mm/weight of whole sample ratio multiplied by 100 (SCS-USDA, 1967). Particle size distribution in fine earth fractions was determined using the hydrometer method (Gee and Bauder, 1986). Bulk density was determined by the core method (Blake and Hartge, 1986). SOC and SIC (as calcium carbonate equivalent (CCE)) content was determined using the Walkley-Black and calcimetry methods (with increasing reaction time), respectively (Carter and

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Fig. 1. Mean annual aridity index and geographic position of the studied regions.

Gregorich, 2008). STC content was calculated as the sum of SOC and SIC. Total nitrogen (Nt) was determined using the Kjeldahl method (Bremner, 1996) and the C:N ratio was calculated by dividing SOC by Nt. 2.3. Calculations and statistical analysis Eq.uation (1) was used for the calculation of the amount of whole SOC in a pedon with k horizon (Batjes, 1996; Wang et al., 2010): k

k

SOC ¼ ∑i¼1 SOCi ¼ ∑i¼1 ρi  Pi  Di  ð1−SiÞ  103

ð1Þ

where, k is the number of horizons, SOCi is the SOC content (kg m−2), ρi denotes the bulk density (Mg m−3), Pi presents the proportion of organic carbon (g C g−1) in horizon i, Di is the thickness of horizon (m), and Si is the volume fraction of fragments N 2 mm. Similarly, SIC was calculated using Eq. (2): k

k

SIC ¼ ∑i¼1 SICi ¼ ∑i¼1 0:12  ρi  Pi  Di  ð1−SiÞ  103

ð2Þ

where, k is the number of horizons, SICi is the soil inorganic carbon content (kg m−2), ρi is the bulk density (Mg m−3), Pi is the proportion of

inorganic carbon (g C g−1) in horizon i, Di is the thickness of horizon (m), and Si is the volume fraction of fragments N 2 mm. The coefficient of 0.12 is the molar fraction of C in CaCO3 to convert the measured carbonates into SIC (Li et al., 2007). The volume fraction of fragments (Si) was calculated according to the Soil Survey Staff (2011) method for each horizon. Furthermore, to understand the importance of various forms of carbon in deep soil, SIC, SOC and STC storage in 0–25, 25–60 and 60–120 cm soil depths was determined based on weighted averages. A factorial design was carried out in order to compare the storage of SOC, SIC and STC in different depths (0–25, 25–60, 60–120 cm), in the soils under different climates (arid, semi-arid and dry sub-humid), with nine replications located in homogenous delineations. Data were analysed by mixed linear model and all of the comparisons were tested at 0.05% significance level using SAS software version 9.4. In the simpler of them the effect of different climates was considered a fixed effect and the effect of nine sites from each area was taken as a random effect. In order to find the relationships between the soil properties and soil carbon forms in each region, a Pearson correlation was calculated using Excel and SPSS 17.0 softwares.

Table 1 Geographic and climatological properties of the studied regions. Regions

MAP (mm)

MAT (°C)

MAET (mm)

AWB (mm)

WSWB (mm)

MAI

Elevation (m a.s.l)

Longitude (E)

Latitude (N)

Arid Semi-arid Dry sub-humid

137.7 320.2 390.4

15.6 13.9 11.7

903.2 810.1 674.7

−765.5 −489.9 −284.3

+45.4 +132.1 +167.8

0.15 0.4 0.56

1297–1330 2080–2200 220–546

50° 26′ to 27′ 50° 02′ to 03′ 49° 32′ to 36′

35° 41′ to 42′ 36° 26′ to 27′ 36° 50′ to 52′

MAP, mean annual precipitation; MAT, mean annual temperature; MAI, mean annual aridity index; and MAPET, mean annual potential evapotranspiration, AWB annual water balance, WSWB, wet season water balance.

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Fig. 2. A. Pedon No. 3 in arid region, basalt rock, saprolite and about 45 cm soil thickness. B. Pedon No. 10 in semiarid region, basalt rock, saprolite and B horizon with about 100 cm soil thickness. C. Pedon No. 26 in subhumid region basalt rock soften parent rock and about 160 cm thickness.

3. Results 3.1. Climatological and physico-chemical properties Climatological and geographical data for the studied regions are presented in Table 1. Soil moisture and temperature regimes in the arid, semi-arid and dry sub-humid regions were Typic Aridic-Thermic, Dry Xeric-Thermic and Typic Xeric-Mesic, respectively (USDA-NRCS, 2012b). Mean aridity indices (MAI) in the studied areas showed remarkable differences (Table 1) indicating that they can be considered as powerful tools for distinguishing between the arid, semi-arid and dry sub-humid climates in the studied climosequence regions from the southeast to the northwest. Annual water balance (AWB) in all regions was highly negative, demonstrating the limited leaching from the soils. However, wet season water balance (WSWB) that is responsible for translocation processes during soil formation was positive (Table 1). Table 2 shows soil classification, some of the physico-chemical properties and SOC, SIC and STC storage for the selected pedons from different regions of the climosequence. In comparison with wetter regions, the arid region had less developed soils, including Entisols and Aridisols with shallower solums, higher bulk densities, coarser soil textures, lower SOC, SIC and STC contents and storages (Table 2). Moving from the arid to dry sub-humid regions, more developed soils including moderately deep Inceptisols and Mollisols in the semi-arid and very deep Inceptisols in the dry sub-humid regions were developed. Besides thickening of the solum from 45 cm in the arid-region Entisols to more than 160 cm in the dry sub-humid region, the other soil properties indicated greater soil development across the climosequence from the arid to dry sub-humid regions. The range of bulk densities in the dry sub-humid, semi-arid and arid regions were 0.93–1.19, 0.92–1.21 and 1.12–1.56 g cm− 3, respectively (Table 2). 3.2. Organic and inorganic carbon contents SOC and SIC contents were measured to evaluate the effects of different climates on soil formation rate and carbon sequestration. As shown in Table 2, all soil horizon of the arid region contained lower SOC content (decreasing by depth) than the semi-arid and dry sub-humid regions; however, SIC content in the upper soil horizon of

the arid region was higher than in the semi-arid region due to more leaching caused by higher humidity, stronger soil structure, and the existence of mollic epipedons (Table 2). The highest SIC content (48%) was observed in the Cr horizon of pedon 11 in the semi-arid region. A similar trend was also observed in other pedons of the semi-arid region. The deepest pedons were observed in the dry sub-humid region with high SOC content even in the deepest horizons. Although all pedons in this region contained thick calcic horizons, SIC content did not sharply increase by depth. The average SIC content increased according to the MAP of each region: dry sub-humid (15.84 %) N semi-arid (11.96 %) N arid (8.99 %). The pedogenic concentration of carbonates including different types and amounts of carbonates depended on climate and stage of soil development. In Entisols located in the arid region (pedon 4; Table 2) carbonates were observed only as disseminated forms; however, a few visible nodules of carbonate, though not significant for calcic horizon diagnosis, were found in the cambids (pedon 1; Table 2). The most developed pedons in the arid region was a calcids (pedon 6; Table 2), which contained N 5% visible carbonate nodules. Maximum development of carbonates in the arid region was in stages I–II in Gile's model. The most distinctive carbonate features recorded in the semi-arid region were typic calcic horizons (pedons 11, 13 and 17; Table 2) that stages II–III in Gile's model and carbonate accumulations in C or Cr horizons. The studied pedons in the dry sub-humid region showed thick calcic horizons (pedons 19, 22 and 23; Table 2), which in some pedons were developed to petrocalcic (e.g. pedon 26; Figure 2C). The pedons of the dry sub-humid region were stages II–IV in Gile's model (Gile et al., 1966; Brock and Buck, 2009). 3.3. Soil carbon stocks Table 2 shows SOC, SIC and STC storages calculated from SOC, SIC, and STC contents based on the horizons’ thicknesses. The greatest amount of SOCs (SOC storage) was observed in the surface horizons in all regions. The opposite trend was found for SICs (SIC storage), in which the maximum amounts were found in subsurface horizons. A similar trend for SIC was also observed for STCs. Overall, the average storage of different forms of carbon, from the soil surface up to the bedrock in the dry sub-humid region (SOCs = 2.92 kg m−2, SICs = 8.4 kg m−2 and STCs = 11.32 kg m−2) was higher than in the semi-

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Table 2 Soil classification and properties in selected studied pedons at different regions. Depth cm

CFN2 mm %

Sand

Silt

Clay

SOC

SIC

SICs

STCs

0.85 0.29 0.46 0.19 – 1.79

1.22 1.01 2.49 5.17 – 9.89

2.07 1.30 2.96 5.36 – 11.69

7.83 9.67 5 2 –

0.81 0.72 0.31 0.07 – 1.91

1.18 2.60 4.31 7.09 – 15.18

1.95 3.38 4.61 7.16 – 17.1

0.05 0.02 0.02 0.02 –

8.4 10 7.5 5 –

0.96 0.54 0.43 0.37 – 2.30

2.92 6.04 4.96 6.97 – 20.89

3.88 6.58 5.39 7.34 – 23.19

2.1 9.2 48 – SUM

0.1 0.07 0.04 –

9.3 8.71 8.25 –

2.64 1.71 3.68 – 8.03

0.72 3.1 64.26 – 68.08

3.36 4.81 67.95 – 76.12

1.49 0.82 0.51 –

1.7 7.6 18.4 – SUM

0.14 0.09 0.06 –

10.6 9.11 8.5 –

2.20 1.76 2.44 – 6.4

0.30 1.94 10.55 – 12.79

2.51 3.69 12.98 – 19.18

42 58 52 49 –

1.93 0.84 0.39 0.37 –

Trace 3.1 18 24 – SUM

0.16 0.09 0.05 0.04 –

12.1 9.3 7.8 9.25 –

2.76 2.80 1.31 1.13 – 8

Trace 1.24 7.26 8.78 – 17.28

2.76 4.04 8.57 9.91 – 25.28

34 36 34 –

44 34 34 –

0.98 0.35 0.18 –

16.1 19 21 – SUM

0.1 0.05 0.02 –

9.8 7 9 –

3.74 2.30 0.67 – 6.71

7.37 15.0 9.34 – 31.71

11.11 17.31 10.01 – 38.43

15 14 15 –

34 39 37 –

51 47 48 –

0.95 0.47 0.31 –

18.4 19.3 21 – SUM

0.1 0.06 0.04 –

9.5 7.83 7.75 –

1.88 1.84 1.45 – 5.17

4.38 9.07 11.75 – 25.2

6.26 10.91 13.19 – 30.36

32 23 21 –

32 32 29 –

36 45 50 –

1.15 1 0.46 –

12.5 12.5 15.4 – SUM

0.1 0.09 0.06 –

11.5 11.1 7.67 –

2.34 6.29 2.99 – 11.62

3.05 9.43 12.03 – 24.51

5.39 15.72 15.02 – 36.13

BD g cm−3

Soil texture

%

Arid region Pedon No. 1 - Typic Haplocambids A 0–20 29 Bw 20–35 36 Bk 35–60 42 C 60–130 51 R N130 –

1.54 1.48 1.56 1.51 –

S.L S.L S.L S.L –

63 68 66 60 –

20 16 19 24 –

17 16 15 16 –

0.34 0.17 0.17 0.04 –

4.1 5 7.6 9.1 – SUM

Pedon No. 4 - Typic Torriorthents A 0–16 30 AC 16–45 40 C1 45–80 45 C2 80–120 47 R N120 –

1.29 1.22 1.2 1.14 –

S.C.L S.C.L C.L C.L –

46 46 42 34 –

20 23 26 28 –

34 31 32 38 –

0.47 0.29 0.1 0.02 –

Pedon No. 6 - Typic Haplocalcids A 0–22 32 Bk 22–50 39 BCk 50–83 45 Ck 83–130 51 R N130 –

1.28 1.26 1.2 1.12 –

C.L C.L C.L S.C.L –

45 38 43 46 –

27 25 22 22 –

28 37 35 32 –

Semi-arid region Pedon No. 11 - Calcic Haploxerolls A 0–26 19 Bk 26–57 20 Cr 57–150 10 R N150 –

1.21 0.99 1.03 –

C C C.L –

26 25 43 –

29 29 28 –

Pedon No. 13 - Typic Calcixerepts A 0–14 3 Bk 14–35 10 Cr 35–80 2 R N80 –

1.07 1.06 1.07 –

S.C.L S.C.L L –

54 48 44 –

Pedon No. 17 - Calcic Argixerolls A 0–15 19 Bt 15–45 15 Btk 45–80 22 Bk 80–110 18 C/R N110 –

1.04 1.2 1.07 1.11 –

C C C C –

Dry sub-humid region Pedon No. 19 - Typic Calcixerepts A 0–40 7 40–110 8 Bk1 Bk2 110–150 9 C/R N150 –

0.98 0.97 0.96 –

Pedon No. 22 - Typic Calcixerepts A 0–20 7 Bk1 20–60 3 Bk2 60–110 5 C/R N110 – Pedon No. 23 - Calcic Haploxerepts A 0–21 8 Bk1 21–90 17 Bk2 90–170 3 C/R N170 –

Horizon

Nt

SOCs C/N ratio

kg m−2

0.04 0.02 0.01 0.01 –

8.5 8.5 17 4 –

5.6 8 11.7 17.9 – SUM

0.06 0.03 0.02 0.01 –

0.42 0.2 0.15 0.1 –

10.6 18.6 14.3 15.9 – SUM

45 46 29 –

0.93 0.61 0.33 –

26 28 30 –

20 24 26 –

26 17 18 19 –

32 25 30 32 –

C C.L C.L –

22 30 32 –

1.02 0.99 0.95 –

C C C –

1 0.98 0.94 –

C.L C C –

%

Abbreviations: BD, bulk density; SOC, soil organic carbon; SIC, soil inorganic carbon; Nt, total nitrogen; C/N, carbon/nitrogen ratio; SOCs, SOC storage; SICs, SIC storage; STCs, STC storage; CF, coarse fragments.

arid (SOCs = 2.23 kg m− 2, SICs = 4.05 kg m− 2 and STCs = 6.28 kg m− 2) and arid regions (SOCs = 0.55 kg m− 2, SICs = 3.20 kg m− 2 and STCs = 3.75 kg m− 2). Table 3 shows the results of comparison of SOCs, SICs and STCs in different fixed depths (0–25, 25–60, 60–120 cm) based on mixed linear models in the studied regions.

3.4. Correlation analysis Analysis of Pearson correlation coefficients for the sampled horizons (N = 85) showed that there were evident differences in the relationships of SOC, SIC, SOCs, SICs, STCs, Nt and C/N ratio among the studied regions (Table 4). Although SOC was negatively correlated with SIC in

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Table 3 Maximum, minimum, means and standard error (SE) of calculated SOCs, SICs and STCs at different depths (0–25, 25–60, 60–120 cm) in the studied regions. Depth (cm)

SOC (kg m−2)

SIC (kg m−2)

STC (kg m−2)

Min.

Max.

Mean

SE

Min.

Max.

Mean

SE

Min.

Max.

Mean

SE

0.49 0.19 0.05

1.19 0.68 0.42

0.74d 0.42d 0.24d

0.07 0.05 0.04

0.81 0.29 1.82

2.44 4.49 6.66

1.61b 2.59b 4.00b

0.17 0.45 0.56

1.54 0.55 2.12

3.34 5.14 7.08

2.35c 3.02c 4.25bc

0.20 0.49 0.57

Semi-arid (N = 24) 0–25 1.07 25–60 0.97 60–120 1.05

3.05 2.57 3.46

2.04c 2.13c 2.38bc

0.22 0.18 0.38

Trace Trace Trace

0.99 7.96 60.34

0.36b 1.47b 13.13a

0.14 0.88 9.61

1.07 0.97 1.49

3.05 10.15 63.79

2.39c 3.59bc 15.51a

0.24 0.89 9.81

Dry sub-humid (N = 27) 0–25 1.39 25–60 1.59 60–120 1.39

5.01 5.61 6.08

2.70abc 3.36a 3.27ab

0.40 0.47 0.50

Trace Trace Trace

11 13.68 20.64

3.68b 7.66ab 11.42a

1.14 1.58 2.23

2.21 3.74 4.48

16.01 18.86 22.61

6.39bc 11.03ab 14.69a

1.48 1.67 1.99

Arid (N = 26) 0–25 25–60 60–120

all regions, but the correlation coefficients varied from no significant difference in the arid region (r = − 0.35) to significant (r = − 0.48; P b 0.05) in the semi-arid region and highly significant (r = − 0.61; P b 0.01) in the dry sub-humid region. The strong and positive correlations between SOC content and Nt showed the expected trends in those regions. Similar to the trend between SOC and SIC, the correlations between SIC and Nt as well as SOC and the C/N ratio were not significantly different in the arid region; however, they were significantly different in both the semi-arid (P b 0.05) and dry sub-humid (P b 0.01) regions. In the arid region, SOC content was strongly correlated with SOC storage (SOCs (r = 0.90; P b 0.01)), which can be attributed to lower effects of soil horizons and soil solum on reducing SOCs variations caused by the products of SOC and horizons thickness. 4. Discussion 4.1. Vertical distribution of soil carbon in the pedons Basalt weathering plays a major role in the terrestrial C cycle (Dessert et al., 2003); therefore, it weathers rapidly, exposing the

influence of H+ dissociated from H2CO3, which results from atmospheric CO2 dissolution in soil water and releases earth alkaline cations to react with bicarbonates and produce carbonates. Based on the igneous parent rock and position of the studied soils in the landscape (lack of wind and water sediments), the only source of carbon in these soils was probably CO2 from biological activity, including the respiration of plant roots and atmospheric processes, which provided the possibility of the genesis of various forms of carbonates in the soils. The existence of carbonate-free parent rocks and rangeland vegetation cover in the studied regions suggests that the soil-formation initiation of the stored organic and inorganic carbons was pedogenic. The lowest amounts of SOC and SIC were observed in the arid region due to less chemical weathering, lower biomass growth, and residue production. The wetter semi-arid and dry sub-humid climates compared to the arid climate stimulated soil formation, in terms of physico-chemical and biological weathering (Wang et al., 2013). The arid climate is a limiting factor in biomass production, and therefore, the lowest SOC content was obtained in the arid region. Evans et al. (2011) reported that SOC increased with increasing precipitation and decreased with increasing temperature in Inner Mongolia. This can be attributed to higher speed of SOC production than decomposition in

Table 4 Results of Pearson's correlation between the different soil carbon forms and the other soil properties for all sampled horizons (N = 85) in three regions. SOC Arid region (N = 32) SOC 1 SIC −0.35 Nt 0.97⁎⁎ C/N SOCs SICs STCs

0.17 0.90⁎⁎ −0.50⁎⁎ −0.28

Semi-arid region (N = 25) SOC 1 SIC −0.48⁎ Nt 0.95⁎⁎ C/N 0.40⁎ SOCs SICs STCs

−0.01 −0.36 0.32

Dry sub-humid region (N = 28) SOC 1 SIC −0.61⁎⁎ Nt 0.97⁎⁎ C/N 0.85⁎⁎ SOCs SICs STCs ⁎ P-value b 0.05. ⁎⁎ P-value b 0.01.

0.28 −0.62⁎⁎ 0.63⁎⁎

SIC

Nt

C/N ratio

SOCs

SICs

STCs

1 −0.34 −0.18 −0.25 0.93⁎⁎ 0.92⁎⁎

1 −0.07 0.84⁎⁎ −0.47⁎⁎ −0.25

1 0.21 −0.26 −0.26

1 −0.33 −0.12

1 0.98⁎⁎

1

−0.14

1 0.14 −0.01 −0.38 0.41⁎

1 0.07 −0.21 0.02

1 0.33 0.09

1 −0.14

1

1 −0.68⁎⁎ −0.44⁎ −0.28 0.84⁎⁎ −0.37

1 0.74⁎⁎ 0.26 −0.69⁎⁎ 0.58⁎⁎

1 0.41⁎ −0.49⁎⁎ 0.48⁎⁎

1 −0.05 0.02

1 −0.35

1

1 −0.48⁎ −0.30 0.11 0.91⁎⁎

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more rainfall, while higher pace of SOC decomposition than production in higher temperatures. The results of current study are inconsistent with other studies reporting the effect of precipitation and temperature on the SIC content (Wang et al., 2010; Evans et al., 2011), but consistent with the effect of precipitation reported in Wang et al. (2013). This could be attributed to the deeper penetration of rainfall that distributed SIC through the pedon with a thicker solum, which developed on more deeply weathered parent rock. Soils in the arid and semi-arid regions accumulate pedogenic calcium carbonate in non-carbonate parent materials from the combination of Ca2 + ions, water from rainfall and CO2 from plant root respiration (Stevenson et al., 2005; Brock and Buck, 2009). The results showed that the SIC content increased with increasing the precipitation. This can lead to the better solubility of CO2 in soil solution and its retention in the soil with later carbonate formation. Similarly, Stevenson et al. (2005) stated that depth of carbonate precipitation decreases with decrease in MAP; furthermore, the effect of temperature on pedogenic SIC formation, accumulation and localization are complicated. But the highest SIC content was observed in some pedons of the semi-arid region because of higher H2CO3 formation in the upper SOC-rich mollic epipedon and its migration to reach lower horizons and even to the surface of underlying rock located in relatively low depths, which caused rapid weathering and carbonates forming reactions. An increase in MAT directly influences the supersaturation of the soil solution with CaCO3 (Barker and Cox, 2011) and increases microbial respiration and CO2 concentration in soil air (Lal and Kimble, 2000). Moreover, temperature has two different impacts, (I) with increased temperature, the weathering rate also increases, but only in the presence of water, which is, by definition, scarce in arid regions. (II) With decreased temperature in the presence of water (wetter regions), the dissolved CO2 in water increases and ultimately reacts with cations released through weathering and precipitates as soil inorganic carbon. Cerling (1984) suggested that the mixing of atmospheric CO2 in most arid soils occurs in the top 50 cm and may reach as far as 100 cm below the soil surface. The depth where the atmospheric mixing occurs is a function of both soil porosity and soil CO2 flux (Stevenson et al., 2005). By increasing the clay content and SOC in the dry sub-humid regions, total porosity and soil CO2 flux increased, which greatly facilitated the diffusion of atmospheric CO2 into the soil up to 250 cm (Fang et al., 1998; Stevenson et al., 2005). SIC accounted for 84.8%, 65% and 74.4% of STC in the arid, semi-arid and dry sub-humid regions, respectively (Table 2). Comparing the results from the poor vegetation cover in the arid region to the better cover in the semi-arid and dry sub-humid regions showed that more SOC entered the soil as humidity increased and, in turn, more SIC formed following its decomposition; that resulted in higher differences between SOC and SIC contents in the wetter climates. The respiration of plant roots in the dry sub-humid area was rather high than other regions indicating its importance in inorganic carbon formation in this area. Active roots caused the increased carbonate dissolution by 5 to 10 times (Gocke et al., 2011). Carbonate solubility increases near roots due to higher CO2 concentrations in the rhizosphere versus atmosphere (up to 100 times) and lower local pH (Gocke et al., 2011). SOC was negatively correlated with SIC in all regions, but the correlation coefficients varied. This trend showed that although SIC content almost completely originated from SOC in the studied regions, but it was rather stable compared to SOC resulting in the accumulation of SIC during soil formation. Therefore, by aging the soils the SIC content increased continuously because of leaching. However, the added SOC could not resist against biological decomposers, and it decayed rapidly and partly converted to SIC by subsequent chemical reactions. Arkley (1963) calculated the minimum time required for the accumulation of soil carbonates in the present climate. According to Henry's law (KH = [A(aq)] / PA) where KH = 34.2 at 25 °C, [A] is the concentration of gas A in the soil solution (mol m−3), and PA is the partial pressure of A in soil air (atm); at the partial pressure atmospheric CO2 of

0.0003 atm, the concentration of H2CO3 equals 0.01026 mol m−3; this finally reacts with Ca2 + and produces 0.01026 mol m− 3 or about 1 g m−3 of CaCO3. Taking into account the climatological data (Table 1), the maximum amount of water that penetrates into soil per year (in some months of year that precipitation exceeds evapotranspiration) is about 45.4 mm that equals to 0.0454 m3 per m2 per year. In other words, 0.0454 g CaCO3 per m2 per year, or 454 g ha− 1 y−1 or 4.54 × 10−4 Mg ha−1 y−1, enters the soil. Considering this as an approximation, the required time for the accumulation of total carbonates throughout the pedons of the arid region would be between 177,000 and 350,000 years. In the semi-arid pedons, accumulation time showed wide variability from 95,000 to 480,000 years and in the dry sub-humid pedons the required accumulation time would be about 138,000 to 180,000 years. Considering the CO2 partial pressure in well drained soils is 10 times more than the atmosphere (Bohn et al., 2001), by dividing the above calculated times by 10, the minimum and maximum times in semiarid region would be about 9500–48,000 years, respectively. This could possibly reflect the weathering processes leading to soil development and sufficient solum thickening that were more rapid in the semiarid and dry sub-humid regions than the arid region. Thinner solum, more stoniness and coarser texture of the arid region soils are also in accordance with the obtained results. 4.2. Soil carbon storages Bulk density (BD) in natural soils, which is an essential factor for calculating carbon storages and soil strength and/or mechanical resistance to plant growth, can impact the distribution of soil carbon content (Gregorich et al., 1997; Drewry et al., 2008). Regarding each pedon, the highest organic matter content and porosity were observed in the surface soil horizons (A horizons), but bulk density showed a reverse trend (highest BD obtained in the surface of some pedons). Clay content and a moderate amount of calcium carbonate are the factors that increase aggregation and consequently porosity (Buol et al., 2011). In the studied soils, clay and calcium carbonate increased with depth. These parameters improved aggregation and increased soil porosity (resulting in low bulk density). In addition, XRF results (data not shown) displayed high content of Fe2O3 in the upper horizons especially in A horizons that could increase the bulk densities. Lynn et al. (1974) reported that the bulk density of a soil with high organic matter, increases with increasing the mineral content. Table 2 shows total SOC, SIC and STC storages in each pedon. The lowest amounts of SOCs and SICs were obtained in the arid region due to poor vegetation cover and lower inputs of organic carbon to soil and inorganic carbon formation. Moreover, thicker solums and horizons in the semi-arid and dry sub-humid regions compared with the arid region led to more irregular SOCs values, resulting in no significant difference correlations between SOCs and SOC (Table 4). The above discussion about the effect of thickness factor could probably explain the correlations between SOC and SICs and STCs. The SICs/SOCs ratios in the arid-region pedons (pedon 1 (5.54), pedon 4 (7.72) and pedon 6 (9.07)) showed an average of 7.44 in comparison to about 4.21 and 3.9 for the pedons of semi-arid and dry sub-humid regions, respectively. The SICs/SOCs ratio in the semi-arid and dry sub-humid regions was lower than that of arid region possibly due to the wetter climate that prevented the precipitation of carbonates and higher occasional leaching likely occurred during storm rainfalls compared to the arid region. 4.3. Importance of carbon storage in deep soil (N60 cm) Although subsurface horizons are known to be important in soil carbon storage, most of the studies have been focused on soil carbon storage in upper soil horizons (Mi et al., 2008; Zhang et al., 2015). Some studies mentioned that soil carbon stocks would be greatly

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Fig. 3. Percentage of soil organic and inorganic carbon at different depths (0–25, 25–60, 60–120 cm) in the studied regions.

underestimated if one does not include the amounts stored in subsoil (Li et al., 2007; Díaz-Hernández, 2010). Table 3 displays SOC storage in different soil depths indicating the storage of relatively high amounts of SOC in N60 cm depths, especially in the semi-arid and dry sub-humid regions. These amounts comprised about 17%, 37% and 35% of SOC in the arid, semi-arid and dry sub-humid regions, respectively (Fig. 3). More than 50% of SOC storage was found in the 0–25 cm depth in the arid region, while in other regions it did not exceed 30%. Soil organic carbon storage between fixed depths of three regions showed that in the arid region the mean of SOC storage decreases from the soil surface to depth, while in other regions it increases. It can be concluded that the higher annual litter input and fine root biomass may partly contribute to the greater SOC storage in the dry sub-humid region. In contrast, the results of this work showed that the SIC storage in the deeper soil is also very important and the highest proportion of SIC (more than 50%) was observed in the 60–120 cm depths in all regions. Also, more than 50% of total carbon (SOC + SIC) storage in the studied soils occurred in the 60–120 cm depths. Our results are thus consistent with the previous studies. Wang et al. (2010) estimated soil carbon storage in 1–3 m to full soil pedon to be 63% in grassland (with 350 mm MAP), 52% in shrub-grassland (with 295 mm MAP) and 50% in desert (with 102 mm MAP). Jobbágy and Jackson (2000) reported that SOC storage in 1–3 m to full soil pedon was about 39% in temperate grassland, 39% in scerophyllous shrub land, and 46% in desert. 5. Conclusion The results were not only interesting due to the present concerns about climate change, but also the studied region offered a good soil arid to dry sub-humid climosequence. Strong differences in soil evolution were distinctly observed across the climosequence from the arid region to the semi-arid and dry sub-humid regions. The solum as a criterion in soil evolution showed minimum, medium and maximum thicknesses in the arid, semi-arid and dry sub-humid regions, respectively. All of the evidences including decreased bulk density along with increased clay, SOC, SIC, STC contents, increased depth of carbonates accumulation and longer time for accumulation of total carbonates, show the accelerated weathering processes leading to soil development in the wetter conditions. The existing trends between the bulk density with clay percentage, SOC and SIC demonstrated that all of the

mentioned parameters had positive effects on soil structure and porosity. Soil inorganic carbon constituted the dominant proportion of STC (65%, 74.4% and 84.8% in the semi-arid, dry sub-humid and arid regions, respectively). SOC and SIC contents and storages showed reverse trends with increasing soil depth. Despite lower values for SOC in the arid-region soils, the highest SICs/SOCs ratio was obtained in the arid climate, which indicates that climate is relatively more important than SOC content and plant growth in determining SIC storage. The mean content of STC storage (STCs) in the dry sub-humid region (11.32 kg m−2) was higher than that of the semi-arid (6.28 kg m−2) and arid (3.75 kg m−2) regions. Furthermore, more than 50% of STC storage in the studied regions occurred in the deep soils (N 60 cm). Acknowledgments The financial support provided by the University of Tehran (Grant No. 7104017/6/21) and center of excellence for soil quality improvement for balanced plant nutrition, is gratefully acknowledged. References Arkley, R.J., 1963. Calculation of carbonate and water movement in soil from climatic data. Soil Sci. 96 (4), 239–248. Barker, S.L., Cox, S.F., 2011. Oscillatory zoning and trace element incorporation in hydrothermal minerals: insights from calcite growth experiments. Geofluids 11 (1), 48–56. Batjes, N.H., 1996. Total carbon and nitrogen in the soils of the world. Eur. J. Soil Sci. 47, 151–164. Blake, G.R., Hartge, K.H., 1986. Bulk density. In: Klute, A. (Ed.), Methods of Soil Analysis, Part 1, Physical and Mineralogical Methods. American Society of Agronomy and Soil Science, Madison, WI, pp. 363–375. Bohn, H.L., Mc Neal, B.L., O'Connor, G.A., 2001. Soil chemistry. third ed. John Wiley and Sons Inc, New York, USA. Bremner, J.M., 1996. Total nitrogen. In: Sparks, D.L. (Ed.), Methods of Soil Analysis: Chemical Methods. Soil Science Society America, Madison, WI., pp. 1085–1086. Brock, A.L., Buck, B.J., 2009. Polygenetic development of the Mormon Mesa, NV petrocalcic horizons: geomorphic and paleoenvironmental interpretations. Catena 77 (1), 65–75. Buol, S.W., Southard, R.J., Graham, R.C., McDaniel, P.A., 2011. Soil Genesis and Classification. sixth ed. John Wiley and Sons Inc. Carter, M.R., Gregorich, E.G., 2008. Soil Sampling and Methods of Analysis. second ed. Canadian Society of Soil Science. Carter, M.R., Gregorich, E.G., 2010. Carbon and nitrogen storage by deep-rooted tall fescue (Lolium arundinaceum) in the surface and subsurface soil of a fine sandy loam in eastern Canada. Agric. Ecosyst. Environ. 136, 125–132. Cerling, T.E., 1984. The stable isotopic composition of modern soil carbonate and its relationship to climate. Earth Planet. Sci. Lett. 71 (2), 229–240.

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