The Influence of Pasture Managements on Soil Aggregate-Size Distribution and Stability Using Wet and Dry- Sieving Methods in Sabzkouh and Boroujen Rangelands in Chaharmahal Va Bakhtiari Province

Free and uncontrolled pasture grazing by animals may decrease soil aggregate stability through reductions in plant cover and subsequent soil organic C، and trampling. This could expose the soil surface layer to degradation and erosion. The objective of this study was to determine the influence of pasture management (free grazing، controlled grazing and long-term non-grazing regimes) on aggregate-size distribution and aggregation parameters by wet and dry sieving methods in two native pastures، protected areas in Chaharmahal va Bakhtiari province. The studied pastures were 1) SabzKouh pastures protected from grazing for 20 years، and 2) Boroujen pastures protected from grazing for 25 years. Soil samples were collected from 0-15 cm depth during the grazing season in summer 2008. Samples (finer than 2 mm) were analyzed for aggregate-size distribution and aggregation parameters by wet and dry sieving methods. Results showed that pasture management had a significant influence on aggregate-size distribution and aggregation parameters in the two areas. The two methods indicated that macro-aggregates in non-grazing and controlled grazing regimes were higher than those in free grazing regime، whereas in free grazing management micro-aggregates showed an opposite trend، and were greater compared with the other grazing regimes. Similarly، soil aggregate stability indices (i. e. mean weight diameter، aggregate geometric and ratio mean diameter) were all improved by non-grazing regimes، suggesting that animal grazing and trampling break down large soil aggregates due largely to compaction and reduced plant coverage. However، the extent to which grazing affects soil aggregation depends in large part on grazing intensity and duration، and the area involved.