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 abstract article info Article history: Received 5 June 2016 Received in revised form 6 November 2016 Accepted 30 January 2017 Available online xxxx 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. Keywords: Climate Mean annual precipitation Semi-arid Soil carbon content 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 car- bon levels on various aspects of human life and the environment (e.g. reducing greenhouse gases emissions) (Shi et al., 2012; Lal, 2013b). Ris- ing temperature and elevated atmospheric carbon dioxide (CO 2 ) simul- taneously affect the dynamics of soil total carbon (STC) (Wang et al., 2016). There are five main C pools on the earth: (1) the lithosphere, in- cluding fossil fuels and sedimentary rock deposits such as limestone, do- lomite 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). 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 mainte- nance (Vitti et al., 2016). Due to the sensitivity to environmental chang- es, 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 accumula- tion, as well as SOC, nitrogen and clay contents (Buol et al., 2011). Soil formation needs long time sequences and it is subjected to dif- ferent climates. Furthermore, chemical, physical and mineralogical Catena 153 (2017) 66–74 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: araheb@ut.ac.ir (A. Raheb), ahaidari@ut.ac.ir (A. Heidari), smahmodi@ut.ac.ir (S. Mahmoodi). http://dx.doi.org/10.1016/j.catena.2017.01.035 0341-8162/© 2017 Elsevier B.V. All rights reserved. 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