Soil health captures the ability of soil to sustain animal and plant productivity. Microorganisms found in the soil play a key role in maintaining soil health. In the Eastern US, white-tailed deer (Odocoileus virginianus) have skyrocketed in population size and opened a niche for invasive species through preferential grazing. One such invasive is the shrub Amur honeysuckle (Lonicera maackii), which is allelopathic and reduces the success of neighboring plants. To determine the influence that deer and honeysuckle have on microbial function, and therefore soil health, we quantified soil exoenzyme activities in paired deer exclosure and access plots each with areas with honeysuckle removed and intact. We measured three important soil enzymes as a proxy for microbial function: β-glucosidase (decays labile C), polyphenol oxidase (decays recalcitrant C), and peroxidase (decays recalcitrant C). We also measured soil pH and gravimetric soil moisture. We predicted that honeysuckle would decrease polyphenol oxidase and peroxidase activities and increase β-glucosidase activity due to its input of labile litter to the system. In contrast, deer would have the opposite impact on activity because they preferentially graze labile plant material leaving behind recalcitrant litter.
Results/Conclusions
While honeysuckle did not have a direct influence on exoenzyme activity (p>0.05), deer increased polyphenol oxidase (p<0.001), peroxidase (p<0.001), and β-glucosidase (p<0.05) activities. Both deer and honeysuckle altered the soil environment which in turn impacted enzyme production. Deer decreased soil pH (p<0.001), and within deer treatments, honeysuckle increased soil pH (p<0.001). Polyphenol oxidase (p<0.001), peroxidase (p<0.001), and β-glucosidase (p<0.05) activities were higher where soil pH was lower, independent of deer and honeysuckle treatments. This implies that deer are indirectly increasing exoenzyme production while honeysuckle may be decreasing exoenzyme production through changes in soil pH. Deer also decreased soil moisture where honeysuckle was removed (p<0.001), but neither deer nor honeysuckle influenced soil moisture alone. Phenol oxidase (p<0.001) and peroxidase (p<0.001) activities increased with decreasing soil moisture overall, whereas β-glucosidase did not have a clear relationship with soil moisture (p>0.05). Under conditions of honeysuckle removal, deer may be increasing soil enzyme production by decreasing soil moisture. Altogether, in a forest invaded by both honeysuckle and large populations of white tailed deer, honeysuckle did not clearly change microbial function while deer stimulated exoenzyme production. Reduction in deer densities in invaded forests may contribute to forest sustainability by improving soil health.