Invasive Nitrogen-Fixing Plant Amplifies Terrestrial–Aquatic Nutrient Flow and Alters Ecosystem Function

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Nitrate pollution is a global issue threatening the health and function of many lowland freshwater ecosystems. Quantifying nitrate loads and instream attenuation associated with land use is a critical requirement for improving freshwater management. One often overlooked nitrate source in catchments is invasive N-fixing trees such as Ulex europaeus (European gorse). This study compared nitrate concentrations in conjunction with stable isotope analyses (nitrate δ15N and δ18O) to investigate the effects of catchment gorse cover on stream nitrate export relative to three other land uses. These were regenerating native forest, low-intensity (dry-stock) and high-intensity (dairying) agriculture. We tested two hypotheses: (1) gorse is a regionally significant nitrate source; (2) instream nitrate attenuation is land-use dependent. The study was conducted in 24 reaches across six small, mixed land-use coastal catchments located on Banks Peninsula, New Zealand. Our results demonstrated that gorse-dominated stream reaches had significantly higher nitrate concentrations than all other land uses. Within the gorse-dominated reaches, nitrate concentration was significantly correlated with upstream catchment gorse cover. Furthermore, nitrate oxygen and nitrogen stable isotope compositions demonstrated that elevated nitrate concentrations in gorse streams were associated with decomposition of dead gorse foliage. The isotope data revealed sub-catchment-scale land-use-specific patterns of nitrate attenuation within streams. All three anthropogenic land uses (gorse, dry stock and dairy) showed distinctly different N-cycling from native-forested reaches where nitrate was efficiently cycled with evidence for highly localised nitrification. Stable isotope data demonstrated that overall nitrate attenuation became less efficient with higher nitrate loads. Our research demonstrates the significant impact N-fixing plants have on nitrate concentrations and instream attenuation. Quantifying the effects of N-fixing plants on water quality is an important step in achieving globally significant goals of sustaining ecosystem health and (sub)catchment-scale nutrient management.