Soil physical properties in a natural highland grassland in southern Brazil subjected to a range of grazing heights

Abstract

Natural grasslands in southern Brazil are ecosystems that are naturally adapted to grazing. Despite the productivity and ecological relevance of these ecosystems, the effects of grazing intensity on their soil quality are still underexplored. This work aimed to determine whether increasing the grazing intensity would jeopardize the soil structure in a natural highland grassland in southern Brazil. Pastures were evaluated at four pregrazing canopy heights, 0.12, 0.20, 0.28 and 0.36 m, with Andropogon lateralis Nees as the reference species, and in one ungrazed control area. The pastures were intermittently stocked, and the postgrazing heights corresponded to 60% of the pregrazing targets (i.e., the postgrazing heights were 0.072, 0.12, 0.168 and 0.216 m). The experiment started in 2015 and was conducted for three years, with evaluations of soil physical properties performed in the final year. Forage growth and grazing were concentrated from October to May; during the winter, no grazing occurred for a period of approximately 4 months. The soil structure was evaluated at two points: after the end of the 2017/2018 grazing season, in June, and after the winter period, in October. Regardless of the canopy height, the presence of grazing animals increased the degree of compaction at the soil surface (0.0–0.05 m) to an intermediate level immediately after grazing, did not disaggregate the soil or change its carbon and nitrogen stocks, and increased soil macroporosity and water infiltration compared to those in the exclusion area (control). Increasing the grazing intensity (i.e., increasing the grazing frequency, corresponding to lower pregrazing canopy heights) increased the soil macroporosity and water infiltration rate, decreased the soil moisture and increased the soil penetration resistance. Except at the 0.12 m canopy height, the exclusion of grazing for 4 months allowed the soil water infiltration in the grazed plots to return to a level similar to that in the exclusion area. We conclude that there are a wide range of grazing intensities (corresponding to canopy heights of 0.12–0.36 m) that do not jeopardize the soil structural quality in highland grasslands. Furthermore, it was not possible to determine whether canopy heights below 0.12 m would disrupt the synergism at the soil-plant-animal interface in these natural ecosystems.