Catchment-scale contribution of invasive nitrogen fixing shrubs to nitrate leaching: a scoping study

ABSTRACTWoody nitrogen (N) fixers are known to fix atmospheric N at very high rates. However, N leached from woody N fixers is rarely accounted for in catchment nutrient budgets. We conducted a scoping study to determine the potential impact of the invasive N fixer gorse (Ulex europaeus) on water quality in an agricultural catchment in New Zealand. With current gorse cover (596 ha) our model-based leaching estimates suggest that gorse accounts for 1.7%–2.4% of total N leaching in the catchment. This is equivalent to leaching from 1200–1800 ha of pasture (averaged across both dry stock and dairy land uses). In our worst-case scenario, with gorse covering over 5000 ha, we estimate that N leaching from gorse would account for 12%–25% of the catchment total. The expected increase in N leaching under this scenario is equivalent to leaching for 9000–14000 ha of pasture, suggesting that gorse could significantly constrain capacity for agricultural expansion or intensification if a cap on N loads is imposed in th...