Soil chemistry and microbial community functional responses to invasive shrub removal in mixed hardwood forests

Invasive woody plant species are a threat to biodiversity and ecosystem function in forests of the eastern U.S., due in part to their effects on soil properties and nutrient cycling. Controlling invasive shrubs can benefit the ecosystem at multiple scales, but these species tend to resprout when cut, and post-cutting flushes of root exudates have been linked to accelerated decomposition of soil organic matter (SOM), a rhizosphere priming effect. We removed the invasive shrub Lonicera maackii (Rupr.) Herder (Amur honeysuckle) from forested sites in central Indiana, USA, using cut-stump and forestry mulching head treatments. For two growing seasons after the initial removal treatments, we compared the soil chemistry and microbial community function of bulk and honeysuckle rhizosphere soils in shrub removal areas to those in invaded reference areas. Microbial activity measured using multiple substrate induced respiration (MSIR) in bulk soils was generally lower in removal areas than reference areas the first year, coinciding with relatively elevated responses in the rhizosphere soils of resprouting shrubs. Elevated SOM and organic C in rhizosphere soils of resprouting shrubs suggested a flush of rhizodeposits from cutting and regrowth. However, bulk soil chemistry responses to shrub removal did not show any evidence of a rhizosphere priming effect, but instead reflected the reduced effects of honeysuckle on throughfall precipitation chemistry-higher ammonium and lower magnesium. Nonetheless, changes in bulk soil chemistry between years were driven by the chemical characteristics of rhizosphere soil associated with resprouting honeysuckle. This study is the first to document the potential for invasive shrub control to affect soil properties through rhizodeposition by the target species.