Biogeographic differences in the effects of Centaurea stoebe on the soil nitrogen cycle: Novel weapons and soil microbes

The success of some invasive plants may be due in part to native organisms lacking adaptation to species-specific biochemical traits of invaders-the Novel Weapons Hypothesis. We tested this hypothesis in the context of soil microbial communities by comparing the effects of Centaurea stoebe and the root exudate (±)-catechin, on ammonification and nitrification in both the non-native and native ranges of this species. In a non-native range (Montana), soil nitrate (NO₃^-) concentrations were lower in invaded than uninvaded grasslands. This did not appear to be due only to higher uptake rates as both C. stoebe plants and catechin significantly reduced resin extractable NO₃^-, the maximum rate of nitrification, and gross nitrification in Montana soils. Thus, reduced NO₃^- in invaded communities may be due in part to the inhibition of nitrifying bacteria by secondary metabolites produced by C. stoebe. The effects of C. stoebe on N-related processes were different in Romanian grasslands, where C. stoebe is native. In Romanian soil, C. stoebe had no effect on resin extractable NH₄⁺ or NO₃^- (compared to other plant species), the maximum rate of nitrification, nor gross nitrification. A relatively high concentration of catechin reduced the maximum rate of nitrification in situ, but substantially less than in Montana. In vivo, gross ammonification was lowest when treated with catechin. Our results suggest biogeographic differences in the way a plant species alters nitrogen cycling through the direct effects of root exudates and adds to a growing body of literature demonstrating the important belowground effects of invasive plants.