Effects of trampling by cattle on the hydraulic and mechanical properties of soil

Destroyed soil structure can increase surface runoff water, with adverse environmental impacts. The effects of trampling by cattle on physical parameters of a heavy clay (a Typic Cryaquept) were studied at the followings four Site types of a pasture after grazing for three seasons: (1) grass with no visible trampling; (2) pasture with some trampling; (3) vicinity of a drinking site with some signs of penetrated hooves, and; (4) a drinking site with totally homogenized surface soil and destroyed vegetation. Additionally, Site types 1 and 4 were studied on sandy loam (an Aquic Cryothent) derived from glacial till after one grazing season. Steady state infiltration rates were reached after 15-35 min of infiltration, depending on the degree of trampling and soil texture. On the sandy loam the infiltration rate at the drinking site was only 20% of that under natural pasture with no visible trampling. When the trampling occurred for longer periods in a soil with higher clay content, the infiltration rate of drinking site was only 10-15% of that in non-trampled pastures. These findings could be explained by a reduced porosity for both soils. In heavy clay at the depth of 10-15 cm, macropores (circle divide > 30 mu m) decreased from 5 to 1.5% (v/v) by the most severe trampling and mechanical strength was lost near the soil surface (cohesion approximate to 1 kPa). Deeper at 10-13 cm, cohesion (at water potential of -6 kPa) was near 30 kPa at the drinking site but around 20 kPa in non-trampled sites. At the 20-23 cm depth of Site 4, the angle of internal friction value (35) for smaller stresses (< 150 kPa) was reduced to smaller values (9) at greater strengths (150-300 kPa) due to a stress dependent aggregate strength and the conversion of a structured soil system to a mostly texture dependent one. The data showed that even a low intensity of grazing will reduce infiltration and hence increase susceptibility to erosion at the drinking sites. (c) 2004 Elsevier B.V. All rights reserved.