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Keywords:

  • agricultural pollution;
  • broom (Cytisus scoparius);
  • gorse (Ulex europaeus);
  • invasion ecology;
  • invasive plants;
  • N cycling;
  • N excess;
  • N isotopes;
  • riparian N cycling

Summary

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.