Effects of deer-exclusion fences on soil microbial communities through understory environmental changes in a cool temperate deciduous forest in Southern Japan

The increasing deer population has significantly altered the forest ecosystems. Deer browsing affects not only plant species diversity and composition but also other organisms indirectly, along with soil properties. However, the effects of deer grazing on the soil microbial community and the underlying mechanisms have not been well understood. To assess these effects, we compared the understory environment and soil microbial community inside and outside fences at Mt. Shiraga, where severe soil environmental degradation has occurred due to sika deer grazing. We selected 20 Fagus trees inhabiting areas within and outside the fences with similar topological features. Eleven environmental variables and both soil prokaryotic and fungal communities were compared between samples inside and outside the fences. The area inside the fence had significantly higher dwarf bamboo density and soil carbon (C) and nitrogen (N) content, whereas soils outside the fences had higher pH and bulk density. The diversity index of the fungal community, in terms of the number of amplicon sequence variants, inside the fence was higher than that outside, whereas that of the prokaryotic community did not differ between fences. Both prokaryotic and fungal communities differed between inside and outside fences. The prokaryotic community changed with the soil C/N ratio, and the relative abundance of oligotrophic bacteria increased with decreasing soil C/N ratio. The fungal community also changed with soil pH and dwarf bamboo density, with the relative abundances of symbiotrophic fungi and ectomycorrhizae increasing with increasing soil pH. Comparative analyses between inside and outside fence samples might provide information on soil microbial community changes with the changes in soil properties after deer grazing.