Influence of Livestock Grazing on the Distribution of Organic Carbon, Total Nitrogen and Carbon Mineralization within Primary Particle-Size Fractions in Shayda Rangelands with Cropping History

Intensive livestock grazing may change soil properties and ecosystem functioning through reductions in soil quality and fertility. The objective of the current study was to establish the influence of three different rangeland grazing regimes (i.e., free grazing, controlled grazing and short-term ungrazing regimes for 8 years) on carbon (C), nitrogen (N) and C mineralization associated with different primary particle-size fractions in Shayda natural rangelands with cropping history in Chaharmahal VA Bakhtiari province. Results indicated that organic C and total N concentrations were higher in grazed than ungrazed rangelands, but the C/N ratio did not show any difference between grazing regimes. Controlled grazing regime had a higher amount of particulate organic matter (POM) while ungrazed regime had a higher amount of hot-water extractable carbohydrate (HWC). The amount of C and N increased with decreasing particle size from sand to clay fractions in all grazing regimes. Rangeland grazing regimes altered organic C and total N associated with primary particle-size fractions. Free grazing regime increased C contents in the silt-size fraction but had no influence on C contents in the other fractions. Additionally, although grazing regime had no influence on the sand-size fraction N, free grazing increased the amount of N in the silt and clay-size fractions. Bulk soil C mineralization was higher in the grazed regime compared to the ungrazed regime most likely because of higher organic C and POM under these conditions. Results revealed that C mineralization was higher in the sand and silt fractions with higher weight proportion in the bulk soil and a low degree of humification in all grazing regimes.