The mycorrhizal nutrient economy of nitrogen (N) in forest soils strongly influences the long-term productivity and sustainability of these ecosystems. By disrupting this nutrient economy, garlic mustard (Alliaria petiolata) may be promoting its short-term interests at the expense of long-term N retention in forests its invasive range. We hypothesize that garlic mustard’s inhibition of soil fungal activity, specifically fungal-associated N-cycling, promotes a more inorganic nutrient economy, increasing susceptibility to future N loss through groundwater leaching. To test this hypothesis, we measured gross N-cycling soils from garlic mustard present, absent, and removed treatments in nine forests throughout central Illinois before, during, and after garlic mustard’s reproductive period. To understand the fungal-associated contribution to these processes we replicated these measures with and without a fungicide to inhibit fungal activity. We also measured subsurface N loss after garlic mustard senescence to assess the plant’s long-term impact on forest N retention.
Results/Conclusions
Prior to garlic mustard flowering, fungal-associated processes were 60-80% lower in garlic mustard treatments, which corresponded to significant inhibition (P < 0.05) of overall N-cycling activity relative to where garlic mustard was absent. Fungal suppression decreased as the plants began flowering, mitigating garlic mustard’s effects on N-cycling processes. After senescence, the release of antagonistic pressure from the garlic mustard and the accumulation of mineralizable-N in garlic mustard plots led to an increase in N-cycling activity (P < 0.05) relative to earlier time points. The post-senescence stimulation of mineralization and nitrification in soils where garlic mustard was present promoted a more inorganic nutrient economy, which has been previously shown to lead to increased N-losses to subsurface drainage as the mineral NO3- pool is poorly held in these soils. Here we found garlic mustard to significantly affect the important biogeochemical processes that sustain forest productivity, showing a potentially problematic pattern for its long-term success in its non-native habitat due to its disruption of the fungal nutrient economy. Furthermore, this study highlights increasing evidence for the importance of fungal activity in promoting forest vitality and how this relationship can be exploited by invasive competitors.