Garlic mustard (Alliaria petiolata (Bieb.) Cavara & Grande) -- an invasive understory plant in North American forests -- damages soils by reducing fungal mutualists to native species. By depleting soil fungi also important in nitrogen (N) cycling, garlic mustard may be further disrupting the integrity of forests. We hypothesize garlic mustard’s inhibition of fungal N-cyclers is promoting a more inorganic N economy, “opening” up the N-cycle in forest soils and creating a greater possibility for N loss from these systems. In order to characterize the effects of active garlic mustard invasions on soil communities and processes, we established plots in the early spring with three treatments (garlic mustard present, absent, or removed via hand-pulling) in a heavily-invaded maple-dominated forest and a lightly-invaded oak-hickory forest in Illinois. We collected soil from these plots in the early- and late-summer, corresponding to the flowering and post-senescence life stages of garlic mustard. We characterized the bacterial and fungal community composition using Illumina sequencing of 16 rRNA and ITS3/4 genes, respectively, and quantified fungal to bacterial biomass ratios with phospholipid fatty acid analysis. We also measured gross and potential N-cycling rates and the fungal contributions to these processes using a 15N stable isotope tracer study and enzyme activity assays.
Results/Conclusions
During garlic mustard flowering, we found lower potential nitrification rates in the garlic mustard present and removed treatments compared to garlic mustard absent plots (P < 0.05) in the heavily invaded forest. The fungal contribution to potential nitrification was lower in garlic mustard present and removed treatments than in the garlic mustard absent plots for both forests (P < 0.05). However, after garlic mustard plants had senesced, we found the fungal contribution to nitrification was greater in garlic mustard present plots compared to both other treatments (P < 0.05). We found significantly lower fungal:bacterial biomass in garlic mustard present plots compared to the other two treatments in the lights invaded forest (P < 0.05), but there were no treatment differences in the heavily invaded forest. We found no differences in the soil concentrations of inorganic N (NO3- and NH4+) among treatments at any time. These results show that living garlic mustard plants inhibit nitrification, but senescent garlic mustard plants can increase nitrification rates. As there were no differences in concentrations of soil N related to nitrification, it is possible that excess N is being lost to the environment due to garlic mustards invasion.