The effects of Sable Island horses on microbial biomass and soil chemistry

Grazing animals often have significant impacts on vegetation and soil in their environment. Introduced horses are the dominant herbivore on Sable Island off the coast of Nova Scotia. The large population of horses currently present on the island may be affecting biodiversity and ecosystem functioning, but the effects belowground on soil biological and chemical properties have yet to be studied. We conducted a five-year study that excluded horses from grazing on large areas (0.25 – 1.7 ha) to investigate effects of horses on microbial biomass and soil properties. Each fenced exclosure site (n = 8) was paired with a nearby control site, which was chosen based on similarities in vegetation density and species composition. Horse exclosure were hypothesized to increase microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), pH, cation exchange capacity (CEC), organic matter (OM), and phosphorus pentoxide (P2O5) due to the reduced grazing effects. Soil MBC and MBN were measured from a subset of plots using the chloroform fumigation extraction method, while the soil properties were measured for all plots. Microbial biomass did not significantly differ between exclosures and controls. Similarly, there were no significant differences between exclosures and control plots for pH, CEC, OM, or P2O5. However, analyses that included vegetation class suggested that some soil and microbial responses differed among habitats. Overall, these results suggest that horses did not significantly affect microbial biomass or the soil properties examined. However, the vegetation community modulated some of these relationships when included as a covariate.