Grazing-induced changes of soil mechanical and hydraulic properties

Abstract

Grazing affects the physical conditions of soils. The effects which can be attributed to grazing are limited to the upper 10–15 cm below surface. Most of the changes are unfavorable in terms of productivity and ecosystem services of grassland ecosystems. Grazing induced changes of soil mechanical and hydraulic properties are strongly interrelated with each other and lead to an increased sensitivity to wind and water erosion. The susceptibility of soils to structural deterioration increases with increasing water content. Repeated mechanical loading combined with shearing forces as applied by grazing animals adversely affect soil structure along with diminishing angles of internal friction and a decrease of the total and coarse pore volume. This in turn affects the pore water pressure and the occurrence and orientation of water menisci, which in dry states are concave, thus stabilizing. If due to the compression of coarse pores the hydraulic conductivity of the soil becomes too low to remove excess soil water from the pore system during loading, soil compaction leads to less negative and sometimes even positive pore water pressure during loading, resulting in convex, thus mechanically destabilizing menisci according to the effective stress equation (Nuth and Laloui, 2008). With incomplete drainage of excess soil water, soil strength can be severely decreased. Soil mechanical changes are interconnected with changes in the pore system; accordingly, soil functions such as hydraulic conductivity and air conductivity are influenced by grazing. Water infiltration and saturated hydraulic conductivity, which are highly depending on the pore diameter and pore continuity, are also highly sensitive to deformations induced by mechanical stresses exerted by grazing animals. Natural soil recovery due to wetting and drying cycles, soil fauna, root growth, and decay occurs but takes several years to decades. It is also not proofed that recovery will ever lead back to the state before being influenced by grazing.