Soil physical and chemical properties in response to long-term cattle grazing on sloped rough fescue grassland in the foothills of the Rocky Mountains, Alberta

Soil physical and chemical properties are critical indicators for soil health assessment. It is important to understand how these qualities respond to different grazing intensities at different landscape slope positions. This study evaluated how soil physical and chemical properties respond to cattle grazing intensity and slope position on a rough fescue grassland. Cattle have been grazing at four stocking rates, 0, 1.2, 2.4 and 4.8 animal-unit-months (AUM) ha(-1) since 1949 to simulate Control (CK), Light (L), Heavy (H), and Very Heavy (VH) grazing intensities. Surface soil penetration resistance was measured using a portable penetrometer and soil samples were taken from the top and bottom slope positions for each grazing paddock (but only from the top position for CK) in September 2016. Soil texture, aggregate stability, pH, available phosphorus (AP), water-soluble ions concentration and soil cation exchange capacity (CEC) were determined. Bulk density, porosity and hydraulic properties were measured using a HYPROP system. The S-index, the slope at the inflection point on the soil moisture retention curve, was calculated. Soil clay and silt contents were lower in VH than L treatments; total porosity, air capacity, and saturated water content followed the same pattern. However, the opposite was true for sand content, penetration resistance, and bulk density, with differences between the two grazing treatments greater at the top than the bottom slope position. Grazing changed the shape of soil moisture retention curve in both slope positions with the curve steeper in L than H and VH treatments. The S-index and saturated hydraulic conductivity were reduced by grazing but their responses to slope position and its interaction with grazing intensity were not significant. Soil pH, AP, and water-soluble Na+, K+, Mg2+, Ca2+ and SO42- concentration increased with the animal stocking rate while Cl- and CEC were not affected. Similarly, greater increases in soil pH with increased animal stocking rates were observed in the top than bottom position, while AP, Na+, K+, and SO42- increased with animal stocking rate only in the bottom position. Our results indicate that soil physical properties may be more sensitive to grazing at the top than bottom of a slope. Thus, slope should be considered when developing rangeland soil health assessment indices and grazing management strategies.