Bioclimatic condition and its effect on the genesis of inorganic carbon in soils developed on basalt

Study of soil carbon, including organic and inorganic carbon (carbonates), is essential for the proper management of soil carbon on a global scale. It is too important the balance between different parts of carbon sources in the environment and soil quality. As regards much of Iran is covered by arid and semi-arid regions, source of inorganic carbon of soil can have very important and effective to better understanding of the topics of the soil sciences. Carbonates are abundant mineral in the Earth’s crust and about include four percent of it. Carbonates importance especially in arid and semi-arid regions is essential to understand the dynamics of relations in the morphological, physical, chemical and biological soil characteristics.
In this study macro and micro morphological characteristics and carbonates genesis has done on basaltic bedrock without carbonate under different bioclimatic conditions. Morphological characteristics of carbonates described in eight profiles located in three bioclimatic region arid, semi-arid and semi-humid of northwest of Iran and micromorphological properties were studied in 21 thin sections.
Chemical results indicated that prevailing increase the amount of calcium carbonate in all three regions with increasing depth and the depth of the accumulation of calcium carbonate becomes more with the increase of precipitation. The only source of carbon in these soils is CO2 from biological activity, which provides the possibility of genesis carbonates, but due to precipitation restrictions their movement is limited to water depth of penetration and does not happen the carbonate complete withdrawal on the soil profile. The micromorphologic properties of pedogenic carbonates in the studied thin sections revealed the calcite coating> micritic nodule> pores infilling with micritic and sparitic carbonate> hypo-coating> capping and pendant> coating and infilling with needle fiber calcite trend. Size and frequency of micritic nodules and calcite coatings increase from drier moisture regime (Weak Aridic) to the wettest moisture regime (Typic Xeric). Microscopic studies showed that the needle shaped calcite concentrations were exist in semi humid region thin sections only. These pedofeatures also were observed as veins and mycelia in field studies of soils with higher biological activity. These concentrations probably are the result of decomposition of in voids plant residues and of settlement of the Ca-containing components of cell walls, and also of the calcification of dead plant roots.
Due to the lack of carbonate in primary bedrock, total carbonate of these soils was pedogenic and acidification of soil solution by the dissolution of CO2 leads to more quickly basalts weathering and cations availability for combination and eventually carbonates deposition with various forms in soil. Powdery carbonates in the arid region were observed as thin discontinuous coatings that represents primary stage of carbonate formation. While in the semi-arid region, existence of few fine carbonate nodules refers to the second stage of the carbonates pedogenesis according to Gile’s model. Simultaneous presence of common to many calcite pseudomyceliums and nodules and carbonatic hard layers (petrocalcic horizons) in semi-humid region represent extreme stages of carbonates formation and accumulation.