Temporal dynamics of hydraulic and mechanical properties of an Andosol under grazing

Soil physical properties continuously change as a result of wetting and drying cycles (WD), soil tillage and grazing events. In order to define sustainable grazing management, it is necessary to determine how the soil responds during and after applied external stresses. This is relevant in southern Chile where pastures are the main livestock feed throughout the course of the year and Andosols are exposed to great mechanical stresses by a wide range of soil water contents. Therefore, the purpose of this paper was to illustrate the effect of grazing, considering two grazing intensities in the winter: 50 and 200 cows ha−1, on the temporal changes of structure-dependent properties of an Andosol in southern Chile. In order to characterize the impact of grazing, 15 soil samplings (5–10 cm depths) were conducted over a 2 year period. The penetration resistance (PR) and soil water content were determined in the field. Air capacity (AC), air (kl) and saturated hydraulic (ks) conductivity, precompression stress (Pc), and aggregate strength (AS) were measured. Changes in the soil mechanical stability and pore functions were observed as a consequence of grazing events and WD cycles, indicating that the structure-dependent properties are dynamic. Animal trampling induced an increase in PR; however, the increase in soil mechanical strength may not affect root growth, considering the changes in time of the PR and the specific properties of Andosol (e.g. bulk density <0.9 Mg m−3). Temporal variations in Pc did not exceed critical values for root growth even under high grazing intensities, indicating that the soil recuperates after the impact of trampling during grazing. The air capacity/conductivity values measured in our study reached critical values for gas exchange as a consequence of animal trampling; however, the Andosol showed its ability to recuperate its integral pore functionality. Finally, in spite of the low bulk density of the Andosol, the increasing grazing intensity during the winter did not affect their structural properties. This, however, does not mean that the soil can support large stresses indefinitely. Therefore, long-term investigations on the effects of grazing management on Andosol physical properties in southern Chile are strongly needed.