White-tailed deer and an invasive shrub facilitate faster carbon cycling in a forest ecosystem

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Forest soils sustain productivity and ecosystem function through nutrient cycling and by acting as a carbon stock. By retaining carbon, these soils delay its release into the atmosphere as a greenhouse gas. Forest disturbances may alter the ability of soil to recycle nutrients and decrease its potential to act as a carbon stock. In the Midwestern US, many forests are experiencing two simultaneous disturbances likely to permanently alter the soil ecosystem: invasion by the exotic shrub Amur honeysuckle (Lonicera maackii) and dense populations of white-tailed deer (Odocoileus virginianus). To capture how these disturbances are impacting nutrient cycling, we measured potential soil enzyme activity and soil physicochemical properties in a fully factorial deer and honeysuckle removal experiment, where initial removal occurred three years prior to soil sampling. We measured β-glucosidase, phenol oxidase and peroxidase activities, which are associated with carbon cycling, and phosphatase activity which is associated with phosphorus cycling. We also measured soil pH, moisture, and texture to determine if honeysuckle and/or deer are changing the habitat of microorganisms, which may indirectly alter enzyme release. Using structural equation modeling, we then compared direct and indirect effects between combinations of honeysuckle and deer presence/absence, soil characteristics, and enzyme activities. Honeysuckle presence increased soil moisture and pH, but these relationships were weak. Additionally, deer presence directly increased phenol oxidase and peroxidase activities and altered the environment to be more conducive for phenol oxidase and peroxidase activities by decreasing pH. Neither deer nor honeysuckle presence altered β-glucosidase or phosphatase activity. Since we could detect changes in soil enzyme activity when deer were excluded independently of honeysuckle, it is likely that soil microorganisms can recover from overabundant deer within shorter time periods and thus the effect of deer may be reversible. In a Midwestern US forest invaded by both Amur honeysuckle and overabundant deer, removing or reducing deer presence could reduce C emissions, but more research is needed to further elucidate how honeysuckle alters nutrient cycling.