Sheep grazing is known to influence soil organic carbon (SOC) storage and dynamics in rangelands. However, very little is known of grazing impacts on measurable SOC pools associated with primary particles, particulate organic matter (POM) and carbohydrates in the heavily grazed rangelands of Zagros Mountains, Iran. Bulk SOC, total nitrogen (N), POM and hot-water extractable carbohydrates (HWC) as well as different SOC and N fractions in particle-size separates were studied in natural mountainous rangelands of arid (Boroujen with 255 mm annual rainfall) and semi-arid (Sabzkouh with 860 mm annual rainfall) sites, Central Zagros. Two sheep grazing regimes including grazed and ungrazed (for 20-25 years) rangelands with four replicates were identified as the grazing treatments at each site. Soil samples (0-15 cm) were taken and analyzed for bulk SOC, total N, POM, HWC, and SOC and N associated with physical fractions. Bulk SOC contents were similar for both ungrazed and grazed regimes, while total N contents significantly decreased under grazed conditions. Bulk soil POM and HWC contents decreased considerably and clearly by sheep grazing, indicating that these fractions of total soil organic matter (SOM) pool may be suitable indicators for detecting the grazing effects on bulk SOC changes and storage in these arid and semi-arid ecosystems. Semi-arid rangeland sites contained more bulk SOC, total N and POM contents than arid rangeland sites. These differences were primarily due to the large differences in vegetation composition, annual rainfall and soil conditions between the two rangelands. After 20-25 years of grazing over 10-33 % of SOC and total N losses occurred in the sand-and clay-size fractions with 10 % increases in the silt fraction. This means sheep grazing increases the contribution of the silt fraction to bulk soil N. We found evidence that sheep grazing decreases soil POM and HWC pools, and the sand fraction C, suggesting a lower recent annual input of decomposable organic C in heavily grazed rangelands. Sheep grazing had no influence on the potential C mineralization of the bulk soil at the semi-arid site (Sabzkouh), but reduced C mineralization at the arid-site (Boroujen), indicating that sheep grazing may affect SOC dynamics by changes in substrate quality at the former, but by substrate quantity at the later. In brief, long-term sheep grazing can potentially lead to losses of both labile and no-labile SOM in these arid and semi-arid rangelands.

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The study of the kinetics of non-exchangeable potassium (NEK) release is very important for a better understanding of K availability for plants in different soils. Moreover, aggregates with different sizes have different effects on the release of nutrients. Therefore, the aim of this study was to examine the release of NEK in 5 calcareous soils of chaharmahalva-bakhtiari province, and small and large aggregates (<250 μm and >250 μm) using CaCl2 0.01 M at 25±1ºc for 2-2017 h. The results showed that cumulative released NEK in soils, and small and large aggregates was 173.5-372.7, 215.1-426.1 and 178.9-381.5 mg kg-1 , respectively. The results revealed that coefficients of the cumulative released NEK in small aggregates was lower than those of the soils and large aggregates. Based on the coefficient of determination (R 2) and standard error (SE), the released NEK was well described by the first order, the power function, parabolic diffusion, and simplified Elovich equations. The rate coefficients of the release of K were different in different soils. The cumulative released amount of K and its rate of release in a solution of calcium chloride in small aggregates was more than those of large aggregates.