Stratification Ratio of Soil Organic Matter and Aggregate Stability under Fire in Semi-steppe Rangelands in Chaharmahal and Bakhtiari

Stratification ratio of soil organic matter and aggregate stability are considered as indicators of soil quality under different conditions such as burning rangeland vegetation. The objective of this study was to investigate the impact of fire on stratification ratio of soil organic matter and aggregate stability and distribution of organic matter in two macro and micro fractions in semi-steppe rangeland of Karsanak region in Chaharmahal and Bakhtiari province. Several sites which were affected by fire three، two، and one years before this study (i. e. 2008، 2009، and 2010، respectively) were chosen. Since the soil samples were taken from two depths (0-10 cm and 15-25 cm) of each site with 9 replications، the numbers of soil samples for laboratory analysis were 108. Independent t-test (P= 5%) was used to assess the difference between the measured properties at the burned and control sites. The results showed that the soil organic matter and mean weight diameter of aggregates (MWD) in the surface layer decreased significantly in 1 and 2 years after fire. Particulate organic matter (POM) significantly decreased in surface layer of the burned areas compared with the control areas. The effects of fire on stratification ratio of soil organic matter in 1- and 2-years after fire were significant compared with control areas، and stratification of soil organic matter decreased 26% and 22 percent in 1 and 2 years after fire، respectively. Stratification ratio of aggregate stability was lower in all burned areas compared with control areas، but the differences were not significant. Macro organic matter of both surface and subsurface layers in 1 year after fire was significantly reduced compared with the control، while، micro organic matter of subsurface layer showed significant increase. The study results showed that the effect of fire on physical and chemical soil properties in the surface layer were more than subsurface layer، because of higher-severity fire in surface layer. Furthermore، due to reduction of organic matter and aggregate stability after fire، rangeland soil quality decreased.