Soil Organic Carbon Fractions in Cultivated Calcareous Soils

Soils play an important role in the balance of carbon on the global scale. The study of the carbon cycle in Calcareous soils has received less attention, while Calcareous soils cover a third of the world’s soils and most croplands of Iran. The aim of this study was to investigate soil organic carbon fractions and the relationship of soil organic carbon (SOC) with soil properties in 30 wheat farms of Calcisols of Alborz, Iran. To study the effects of soil geochemical properties, the data were subjected to cluster analysis to select samples with similar texture and low salinity. We applied a densimetric method using sodium iodide (NaI) to isolate light fraction (LF < 1.6 g/cm 3 ), occluded fraction (OF < 1.8 g/cm 3 ), and mineral-associated organic matter (MAOM > 1.8 g/cm 3 ) of selected soils. Results showed that SOC decreased up to 61% throughout the soil profile. Mineral-associated organic matter had the highest proportion (more than 75%) of SOC in agricultural soils. The mean value of C/N reduced in mineral-associated organic matter compared to that of bulk soil, which indicates nitrogen concentration in this fraction. The study of soil properties suggested that calcium carbonate equivalent (CCE) could explain SOC content in calcareous soils, while extracted metals (iron, magnesium, and calcium) had a significant relationship with the organic carbon content of heavy fraction.