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.

Background and objectivesNowadays, it is believed that carbon distribution and emission between different pools of soil, plant and atmosphere have an essential role in sustainability of agroecosystems, environmental challenges especially global warming and climate change. CO2 is the most important of greenhouse gases that increasing of its concentration in the atmosphere has been caused global warming and climate change (8). Carbon sequestration is increase soil organic carbon (SOC) that improves depth distribution of SOC and stabilize SOC by encapsulating it within soil and the simplest ways to mitigation levels of this atmospheric gas (18, 52).
Materials and MethodsIn order to evaluate the potential of carbon sequestration in above ground and below ground organs for canola as an important oil crop in Khorasan Razavi region, an experiment was conducted. Sampling was performed with random-systematic method by using 30 plots of 0.5 m2 and along three transects of 50 m. Conversion coefficients of above ground and below ground organs including silique seed, stem, leaf and root of canola were determined with combustion method separately. Sequestration carbon potential for above ground and below ground organs of canola and soil were measured. After the calculation of emission for greenhouse gases including CO2, N2O and CH4 based on emission indices, global warming potential (GPW) were calculated.
Results The results showed that mean yields of silique seed, stem, leaf and root were achieved with 33.46, 36.60, 17.45 and 22.91 kg.ha-1, respectively. The mean carbon sequestration of soil for canola fields was 3.46 t.ha-1. Conversion coefficients and carbon sequestration potential were significantly different between above ground and below ground organs of canola. The highest conversion coefficient was observed in silique seed with 51.65%. Total carbon sequestration potential for above ground and below ground organs was obtained 5.12 t.ha-1 that the maximum and the minimum were belonged for stem and leaf with 1.81 and 0.76 t.ha-1, respectively. The highest emission of greenhouse gas was achieved for nitrogen fertilizers with 1.35 ton CO2 and GWP was 0.688 ton CO2 equivalent per ha.
ConclusionIt is therefore concluded that reduction of chemical fertilizer and more use of above ground and below ground residues for canola seems to be a rational ecological approach for sustainable management of agroecosystem with a consequence of reduction in greenhouse gases and mitigation of climate change.

In this work, the damage produced in Carbon and Silicon by fast neutrons in the energy range of 0.15 to 10 MeV was calculated using TRIM code. In this range of energy, the dominant interaction is the elastic scattering. The information about recoil, nuclei energy, scattering angle and the depth in which the interaction was occurred is necessary to run TRIM code. For calculating the energy and scattering angle of recoil nuclei, the spectrum of scattered nuclei should be known. The spectrum of scattered Silicon and Carbon was calculated by using an analytical method to relate the statistical distributions to each other. Comparison of the calculated scattered spectrum with experimental one confirmed the validity of the calculation method. Calculations revealed that the damage produced in silicon is higher than the one produced in the Carbon. This is because the displacement energy threshold in Carbon is higher than the Silicon. So, using of Carbon in neutron exposure is recommended.

Recently the limitation of water resources and necessity of using unusual waters is among the most important issues, especially in dried and semidried area. Less attention to water quality standards can led to less soil quality and reducing theirs functions. This study was done to investigate the spatial distribution of soil fertility characteristics including: pH, phosphorus, nitrate, organic carbon and electrical conductivity. For obtaining maps of the spatial distribution, sampling was done from 57 points in two difference depths, first (0-30 cm) and second (30-60 cm) in 1200 hectares of study area, with using systematic random sampling. To creating the maps, the GIS software V 9.3, was used. The resulted map of the spatial distribution of pH in the two depths indicates that according to standards of Soil and Water Research Institute, at the studied area the limitation of alkalinity is high. Also the results of spatial distribution maps for phosphorus indicates that 360 hectares of study area at depth 1, and 330 hectares at depth 2, has concentrated more than 15 mg kg,-1 , which is more than acceptable range, according to standards of Soil and Water Research Institute. Furthermore the results of spatial distribution maps for nitrate at two depths indicated that concentration of nitrate in study area was not more than acceptable standard, but its concentration is increased from north up to south. In addition maps of spatial distribution for changes in organic carbon at two depths showed that the amount of this parameter is suitable according to standards of Soil and Water Research Institute. Finally the results of electrical conductivity showed that the amount of this parameter at two depths in whole studied area, except in small area, is not more than acceptable amount.

Soil organic matter is an indicator of soil, agriculture and environmental quality, playing important roles in soil strength and sound environmental functions. Size and chemical fractionation of soil organic matter are long-standing approaches in clarifying their key role in soil processes. Twenty seven surface samples (0-20 cm) and 6 pedons from different landform units were sampled and described according to USDA, 2002 and by means of illustrating the surface as well as vertical distribution of different size and varied chemical fractions of organic carbon. Physico-chemical, mineralogical, physical and chemical fractionations were carried out following standard methods. The results revealed that organic carbon content increases by decrease in the fraction size, reaching its maximum at <250 μ. Also it was observed that 2:1 expandable clays show relatively higher capability in organic carbon complication in comparison with the other mineralogical suits. The results for chemical fractionation demonstrated that humic to fulvic acid ratio increases by increase in organic matter input, itself being followed by increase in soil biological activity.

To investigate spatial variability of soil characteristics in the most valuble forest stands in the northern Iran using geostatistical approach، a twenty hectare area in Experimental Forest Station of Tarbiat Modares University was considered. Soil samples were taken from pits، mounds، canopy gaps، under single trees and closed canopy positions at 0 - 15، 15 - 30 and 30 - 45 cm depths using core soil sampler (81cm2 cross section). pH and organic matter were measured in the laboratory، and then carbons to nitrogen ratio and carbon sequestration were calculated. Spatial variability for soil characteristics revealed anisotropic variogram due to the same variability of surface variogram in different depths. pH in all the depths and carbon sequestration in 30 - 45cm depth showed exponential model whereas the other soil characteristics resulted in a linear model in different layers. The results of spatial structure showed pH with medium structure in all the depths، organic matter and carbon to nitrogen ratio with pure nugget effect (non structure)، carbon sequestration in 0 - 30cm depth with weak structure and 30 - 45cm depth with medium spatial structure.

The aim of this study was to determine the possible difference in the amount of organic carbon and total nitrogen in the soil under the crown of tree species in a semi-arid forest at the level of a single tree. Also, the pattern of possible differences between these two main soil materials was observed in different size classes of trees and in top- and subsoil. For this purpose, 60 trees from four species of juniper (Juniperus excelsa), narrow-leafed ash (Fraxinus angustifolia), Brant's oak (Quercus brantii) and hawthorn (Crataegus azarolus) were randomly selected in a mixed stand of central Zagrosian forest that had been enclosd for over 30 years. The amounts of organic carbon and total nitrogen of topsoil (0-15 cm) and subsoil (15-50 cm) were measured and compared. The results showed that, first, both the amount of soil organic carbon and the total nitrogen from the samples beneath the tree crowns were higher than their corresponding values in soils outside the crowns. Second, only the amounts of soil organic carbon were affected by the species, and these values ranged from a minimum of 1.37% in topsoil and 0.52% in subsoil for hawthorn, to maximum values of 1.64 and 0.73 percent for Juniper in top- and subsoil, respectively. Third, the amount of total nitrogen in the soil with an average of 0.21 and 0.08 percent of dry weight in top- and subsoils, respectively were the same beneath the crowns of the four species. Fourth, the size of the trees only affected the amount of organic carbon and total nitrogen in the topsoil. This study showed that there is little difference in soil organic carbon and total nitrogen under the canopy of different tree species in a standard Zagros forest and therefore the whole stand may act as a mega-organism and has more or less homogeneous soil.

Nuclear reaction analysis (NRA) is a powerful method for the characterization of light elements in heavy matrix. The capability of this method to determine carbon in iron-based substrates was studied. The systematic error of this method was estimated to be 6%. It was found that the sensitivity of the method for determining carbon in the metal substrates is 0.05% by weight. Three different standard samples of which have specific carbon content, were used to measure carbon in the bulk substrate. The difference between measured and actual carbon of standard samples is within the method error spread and less than 6% reported. To determine the depth profile, carbonized steel-304L samples by the DC plasma method in two various substrate temperatures were used. The effect of the substrate temperature on the formation of the carbon layer and its depth profile was studied. The formation of Fe3C layer in the depth of the carbon samples under the 385 °C has been determined using the NRA method and confirmed with other SEM and XRD methods.