Use of nitrogen-fixing plants to improve planted forest soil fertility and productivity in New Zealand: A review

Background: Planted forests with low fertility soils are likely to require increased inputs of nitrogen (N) to satisfy increasing productivity demands. The use of N fertilisers will become more challenging due to their increasing cost and the risk of unwanted environmental impacts. Nitrogen-fixing plants may provide an alternative option to chemical fertilisers that is not only cheaper but has a lower greenhouse gas footprint. Methods: Information on N-fixing plants was collated from available literature, focusing on species previously associated with planted forests as well as other exotic N-fixing trees and New Zealand native plants not used in commercial planted forests. Results: Benefits to planted forest growth in New Zealand by N-fixing plants have been proven in only a few cases. Lupinus arboreus Sims was used in the 1970s and 1980s to increase N inputs in a planted Pinus radiata D.Don coastal forest, and improved P. radiata productivity was demonstrated. Productivity gains ceased when a blight disease infected the lupin population. Many N-fixing plants have been shown to tolerate low fertility soils and establish in planted forests, but there is limited information on their N contribution to the soil, and whether this N contribution increases P. radiata productivity. Such N-fixing plants include Lotus species, clover (Trifolium species) and weeds such as gorse (Ulex europaeus L.) and broom (Cytisus scoparius (L.) Link), which are commonly found in association with P. radiata forests. New Zealand native N-fixing shrubs may be suitable but there is limited information available describing N-fixation or forestry management for native plant species. Conclusions: Nitrogen-fixing plants are a valuable resource, which when managed properly could be used for enhancing productivity. Issues with pests and disease, and limited knowledge regarding the management of N-fixing species, restricts adoption in planted forests. Well-designed research is needed to assess impacts on whole forest system N cycling as well as selection of appropriate plant species for use in planted forests without unintended consequences such as competition for soil moisture or light. This research needs to be integrated with forest management and delivered as practical options to ensure success.