Nitrogen cycling in gorse-dominated ecosystems in New Zealand

The legume gorse (Ulex europaeus) is one of the most widespread nitrogen (N) fixing species and is also one of the most invasive weeds of New Zealand. Despite its widespread occurrence, little research has been undertaken on N cycling in gorse ecosystems. This lack of knowledge is important since gorse covers up to 900 000 ha in New Zealand and there is a potential for a large environmental impact from the cycling of large amounts of N fixed by gorse entering waterways and water bodies. We undertook this overview to summarise the current understanding of N cycling in gorse-dominated ecosystems and identify knowledge gaps and provide recommendations for future research in New Zealand. Specifically, we describe some key biological processes associated with N cycling under gorse-dominated ecosystems. Gorse is capable of fixing up to 200 kg ha^–1 N annually during the period of rapid dry-matter accumulation. External factors such as temperature, fertiliser application, moisture, pH and atmospheric CO₂ can affect biological N fixation in legumes, but such information is scarce for gorse. Gorse produces large quantities of litter, and N concentrations are generally higher in gorse litter than in the litters collected under other shrubs and trees. During and after litter decomposition, N is released into the surrounding soil, resulting in higher N concentration in the soils under gorse sites compared with under other species. This is due to the ability of gorse to fix N and produce large quantities of litter. The contribution of gorse-infested land to surface water eutrophication is also discussed because gorse has invaded large areas within the catchment of some significant lakes in New Zealand. From this review a good understanding of N cycling under gorse ecosystems has emerged. To better understand the wider environmental impact of gorse-dominated ecosystems in New Zealand, further research is needed including monitoring and modelling the impact of climate change on N fixation, leaching and C sequestration in gorse-dominated ecosystems.