Carbon stock and mineral factors controlling soil organic carbon in a climatic gradient, Golestan province

Organic matter content of the many soils is less than 5% by weight; nevertheless this small amount of organic matter (OM) has a significant impact on soil properties. A study to evaluate the role of climate in distribution of organic carbon (OC) and mineral factors (such as soil composition, soil stability and clay minerals) controlling OC was performed. Seven surface soils were collected (0-20 cm depth) with three replications in a climate gradient (200 to 800 mm of rainfall). The results showed that aggregates < 0.053 mm in most of the soils had the lowest organic matter content (Figure 2), and carbon stock increased with increasing in the aggregates size in most soils. There was no significant relationship between OC content and clay mineral content, but there was significant relationship for illite and chlorite. It can therefore be deduced that OC stock is probably controlled mainly by climate factor than clay minerals. Oxalate iron was positively related to OC content, but this relationship does not apply for dithionite iron. The relationship between soil OC content and specific surface area (SSA, g m(-2)) of the soil was positive and significantly correlated. Labile soil OC content had a positive relationship with the climate index (P/ET degrees). As the index increased the potential loss of OC content increased, as well. Labile OC content was estimated between 0.49 to 16.64 g kg(-1) and a potential loss of OC was between 1.28 to 46.12 Mg of carbon per hectare on the soil surface horizon.