1. This study examined the seemingly paradoxical proliferation of invasive, N2-fixing broom (Cytisus scoparius) and gorse (Ulex europaeus) in N-saturated riparian areas of intensive agricultural land in Canterbury, New Zealand.
2. A field study of natural abundance δ15N suggested that broom and gorse along the Selwyn River fix approximately three times more N than they take up from soils, and are thus a potentially large source of N in the landscape. Broom N fixation rates based on mass balance calculations from a glasshouse study were similar.
3. In the controlled glasshouse study, broom grown at both c. 1× and 6.5× field NO3− supply fixed N at the same rate per unit biomass (0.061 mg N day−1 g−1 dry wt) over a 9-month period. Broom plants grown under the high-N supply, however, grew c. 1.6 times larger, and thus fixed more N per plant. Above-to-below-ground biomass ratios and %N in above- and below-ground pools were the same under the two levels of N supply.
4. Each broom plant in the greenhouse study contributed at least 0.02 g N year−1 to soils, but leaching from the soils was surprisingly low (<2% of total plant and soil stocks) suggesting that plants less than 1 year old are not contributing substantially to high NO3− concentrations in Selwyn ground and surface water.
5. Synthesis. This study shows that both broom and gorse growing in the Selwyn riparian area are an additional source of bioactive N in this N-saturated ecosystem. Additionally, broom grows more quickly as N availability increases and therefore fixes more N per plant. This suggests a positive feedback whereby agricultural nutrient pollution leads to increased per-plant N2 fixation in broom, and probably in gorse, given the taxonomic and physiological similarity of the species. The Selwyn is representative of a large number of New Zealand rivers with riparian zones that are dominated by invasive N2 fixers. The likelihood that these invasive plants increase the amount of bioactive N in rivers and downstream ecosystems presents new considerations and challenges for management.