Organic and Inorganic Carbon Storage in the Soils of Sardooeyeh Semi-arid Region in South of Kerman

Soil carbon (C) sequestration is recognized as a potentially significant option to off-set the elevation of global atmospheric carbon dioxide (CO2) concentrations. Soils are the main sink/source of carbon and also, an important component of the global C cycle. Total soil carbon (C) comprises of the soil organic C (SOC) and the soil inorganic C (SIC) components. The soil inorganic C (SIC) stock mainly consists of carbonates and bicarbonates. Processes governing the dynamics of the soil carbon stock differ among ecoregions and strongly interact with soil properties. Understanding the distribution of organic and inorganic carbon stocks in soil profiles is essential for assessing carbon storage at the regional and global scale. Although global estimates provide a general view of carbon stock levels, accurate local estimates and factors affecting soil carbon dynamics are very important. As a result, there is an essential requirement for accurately estimating the distribution of carbon reserves and their differences with regard to soil properties. The study area is located in the Sardooeyeh region, South of Kerman, under semiarid conditions. A total of 5 soil profiles were excavated. Percentage of coarse fragments (> 2 mm) using a 2 mm sieve, total organic C by the K2Cr2O7-H2SO4 oxidation method of Walkley-Black, soil inorganic carbon using the Gravimetric carbonate meter method were determined. Bulk density was measured by drying core samples in an oven overnight and dividing the weight of dry soil by the volume of the core occupied by the soil after correction for coarse fragments. Organic carbon in the surface horizons of all profiles is maximum due to vegetation and decreases with increasing soil depth. As the altitude increased, the amount of organic carbon increased in the surface horizons. Lower temperature and higher humidity at higher altitudes lead to the lower organic matter decomposition and consequently higher organic carbon content of the soil. Although the upper soil layers had the maximum soil organic C content, the maximum soil inorganic C content was observed in the sub-surface layers. The soil organic carbon storage was between 5.52 to 9.48 kg m-2 and the storage of soil inorganic carbon in profiles was between 14.41 and 91.34 kg m-2. The total soil carbon storage in the profiles varied between 19.92 to 100.83 kg m-2 and the average was 42.66 kg m-2. The average of soil organic carbon storage in 0-25, 25-60, 60-120 cm depths were 2.6, 1.97 and 1.26 kg m-2, respectively. The amount of soil inorganic carbon storage in 0 -25, 25-60 and 60-120 cm depths were equal to 2.7, 10.40 and 8.26 kg m-2, respectively. Therefore, it seems that more than 50% of the total soil inorganic carbon storage is stored at a depth of 25-60 cm from the soil surface. The portion of inorganic carbon storage of total soil carbon was 77.5%, and about 89% of it was stored in sub-surface horizons (below 25 cm). The portion of organic carbon storage of total soil carbon was 22.4%. It seems that an increase in the partial pressure of CO2 in soils leads to some dissolution of the pedogenic carbonate in the top soil. Dissolved pedogenic carbonate transfers to the deep soil and then re-crystallizes under relatively dry conditions and low CO2. The results showed that soil organic carbon storage was mostly higher in surface horizons, and soil inorganic carbon storage was higher in sub-surface horizons. On average, the ratio of soil inorganic carbon storage to soil organic carbon storage was 4.27. The high percentage of soil inorganic carbon storage in total soil carbon, shows that inorganic carbon plays a very important role in semi-arid regions. Almost 89% of the soil inorganic carbon content and about 80% of the total soil carbon were accumulated in the sub-surface horizon of soil (below 25 cm), indicating the importance of sub-surface soil for storing carbon in semi-arid regions.