Effects of slope aspect, grazing, and sampling position on the soil penetration resistance curve

The slope aspect, grazing intensity, and vegetation canopy are factors that affect the physical and mechanical properties of soil. However, their effects on the soil penetration resistance (PR) curve are unclear. Thus, we investigated the effects of northern and southern slope aspects, the grazing intensity (free and controlled grazing, and livestock exclusion), and the sampling position (beneath and between canopy) on the PR curve and bulk density in the Gonbad paired watershed in Hamedan, Iran. At the end of the grazing season, we collected and analyzed seven undisturbed samples (total = 252) and one disturbed soil sample (total = 36) from each experimental unit. The soil PR was measured at different suctions. We fitted the To and Kay (2005) and Stock and Downes (2008) models to the PR data. The effects of treatments on the bulk density, PR curve and its model coefficients, and PR were analyzed at different suctions. The slope aspect had the greatest impact on the soil bulk density and PR at suctions of 30 and 1500 kPa. The Stock and Downes (2008) model coefficients, which represent the degree of change in the PR relative to the moisture content, were affected mostly by the sampling position. Southern aspects had higher overall PR curves than northern aspects due to their lower soil organic matter contents and more coarse fragments. The overall PR curve increased with the grazing intensity. There were interactions between the effects of the three factors on the PR curve and all affected the PR.