Grazing management, slope aspect and canopy effects on the compression characteristic of soils of the Gonbad experimental watershed in Hamedan, Iran

Arid and semiarid environments are very sensitive to unsuitable land management practices like livestock overgrazing. Animal trampling and overgrazing can cause land degradation through soil compaction, and this can be controlled by slope aspect. Soil compaction changes soil structure, reduces soil water infiltration and root penetration and decreases vegetation cover and soil organic matter. Livestock overgrazing and the slope aspect can affect many soil properties, but their effects on the confined compression curve (CCC) of soils and other land degradation indices have not been investigated so far. We studied the effects of slope aspect, grazing intensity, and sampling position on the characteristics of the CCC in Gonbad experimental watershed, Hamedan, Iran. Undisturbed and disturbed soil samples were collected at the end of the grazing season in November. The soil CCC was measured, and the Gompertz model was fitted to the measured data. The smallest void ratios, compactability, and swelling indices occurred with a southern aspect due to the greater sand content and coarser texture. Free grazing increased the soil bulk density and decreased the void ratio to decrease all of the CCC characteristics and Gompertz model coefficients (except m). Soil on a northern aspect and with no grazing improved the compression and degradation indices, and would be the best management to prevent compression and consequent land degradation. Path analysis demonstrated that topography and grazing management changed the compression characteristics, which affected land degradation strongly, by affecting the cation exchange capacity, organic matter, and textural and structural properties. In consequence, compression of rangeland soils can be mitigated by selecting suitable grazing management systems by considering topography of the region.