Bracken fern (Pteridium aquilinum L. kuhn) promotes an open nitrogen cycle in heathland soils

In spite of the broad array of studies conducted on the ecology of bracken fern (Pteridium aquilinum (L.) kuhn), there is currently only a limited understanding of how P. aquilinum alters the soil environment in which it succeeds. P. aquilinum is one of the world's most aggressive invasive species and is known to effectively invade conservation priority habitats such as Calluna vulgaris (L.) heathland. The aim of this study was to evaluate differences in soil properties between intact stands of C. vulgaris and neighboring P. aquilinum to assess how P. aquilinum alters soil N transformations in a manner that might promote its success. Replicate plots in five independently paired stands of P. aquilinum and C. vulgaris were established on land in which P. aquilinum is actively invading. Soils under the two plant types were evaluated for total N, mineralisable N, net nitrification, nitrifier activity, denitrification enzyme activity, polyphenol N complexing capacity, and resin sorption of inorganic N. Soils under P. aquilinum were consistently higher in NO3 (-) and NH4 (+) concentrations compared to C. vulgaris. Extractable organic and inorganic N concentrations for soil under P. aquilinum were respectively 65 %, 77 % and 358 % greater in amino N NH4 (+)-N and NO3 (-)-N compared to that under C. vulgaris. In-situ net nitrification (NO3 (-) sorption to ionic resins) was found to be nearly 300 times greater under P. aquilinum than under C. vulgaris. P. aquilinum alters the soil environment as to create an inorganic N-rich environment that is favorable to its growth and development.