Mixed and Single Species Grazing Impacts On Soil Structural Properties.

Grasslands support a unique combination of plant and animal species. Vegetation helps to prevent erosion by providing cover to protect the soil surface from raindrop impact, and by increasing root mass to hold surface aggregates together; animal grazing behavior directly affects vegetation and soil surface. Understanding the environmental effects of mixed species grazing as compared to a single species grazing system will help to refine our agricultural practices, increasing sustainability and protecting the environment. The objective of this study was to characterize soil properties as affected by single and mixed species grazing systems. To achieve the study’s objective a completely randomized factorial experiment composed of one factor (grazing system) with two treatments, and three replicates was established at WVU Reedsville Animal Research Farm. Soil structural properties, such as bulk density, soil texture, aggregate stability (wet and dry), infiltration rate, penetration resistance, and runoff volumes were measured. Runoff, infiltration, and sediment production were obtained from erosion/infiltration plots installed in May 2011.

The mixed species grazing system exhibits a cumulative runoff volume (1010 m3/ha) similar to the single species grazing system (1062 m3/ha) between the months of May 2011 and July 2012. However, the single species grazing system produced more sediment (39.02 g/m2) than the mixed species grazing system (19.13 g/m2) for the same period of time. Surface saturated hydraulic conductivity was highly variable, however the mixed species grazing system exhibited a higher (0.97 cm/hr) saturated hydraulic conductivity than the single species grazing system (0.70 cm/hr).

Surface bulk density between treatments ranged from 0.838 g/cm3 to 1.13 g/cm3, was not affected by treatment, and showed slight changes within the two years. Soil organic carbon (SOC) content was affected by treatment. The mixed species treatment had a higher surface (0-5 cm) SOC content (48.5 g/kg) than the single species grazing treatment (44.5 g/kg). In 2011 and 2012, surface penetration (0-5 cm) resistance was higher in the mixed species (than the single species grazing treatment. In 2011, dry and wet aggregate stability exhibited differences between the treatments. The single species treatment had larger aggregates when dry aggregate stability was measured. The wet aggregate stability for the mixed species treatment in 2011 and 2012 exhibited larger aggregates. Runoff and the aggregate geometric mean diameter are negatively correlated, implying that presence of large aggregates decrease the average amount of runoff.

These data indicate the single grazing system may produce more runoff, and creates an environment in which the negatively impacted soil is more easily transported than in the mixed species grazing system. The data indicate that, over time, land management practices may affect the sustainability of the grassland based on soil and grassland hydrologic properties.