The interaction of New Zealand native plants with nitrogen in Canterbury’s agricultural landscapes

Abstract

Less than 0.5 % native vegetation cover remains in the productive Canterbury Plains region of New Zealand. Incorporating native plants into agricultural landscapes could provide numerous benefits including shelter, supplementary stock fodder, production of essential oils or honey, wildlife-corridors, and protection of waterways. New Zealand’s native species are adapted to environments where nitrogen (N) occurs at low concentrations. Such environments are in stark contrast to New Zealand’s agricultural landscapes, where high inputs of fertilisers and animal effluents have elevated soil N. There is a lack of knowledge on how native species will interact with N in agricultural environments. Potentially, native species may alter nitrate leaching to receiving waters and emissions of nitrous oxide (N₂O), a potent greenhouse gas. This research aims to investigate the interaction with soil N of selected native species and their rhizospheres to gain an understanding of species-specific differences and potential effects on N fluxes. The native species investigated were typical of those used in restoration projects. Perennial ryegrass (Lolium perenne), an introduced species that dominates New Zealand pasturelands, was used as a control. I studied rhizosphere soil and foliar N status at two planted restoration sites. Plant growth and uptake in response to agriculturally elevated N levels were investigated in greenhouse pot trials. A field experiment explored the effect of Kunzea robusta (kānuka) on N₂O fluxes from soil. Finally, farm-scale N uptake and reduction in N losses were modelled for various native planting scenarios. At the restoration sites New Zealand native species Austroderia richardii (toetoe), Phormium tenax (flax), Cordyline australis (cabbage tree), Coprosma robusta (karamu), K. robusta, Olearia paniculata (akeake) and Pittosporum tenuifolium (black matipo) had similar foliar N concentrations to L. perenne. While native species with winter leaf loss, Plagianthus regius (ribbonwood) and Sophora microphylla (kōwhai), had higher foliar N than these other species. There was significant inter species variation in rhizosphere soil mineral N concentrations among native species, with A. richardii and P. regius having higher nitrate status than L. perenne. Pot trials revealed that while native species tolerate high N loading (up to 1600 kg ha⁻¹), there was negligible growth response. Increased soil N concentrations resulted in increased foliar N in native plants, of which the high-biomass-producing monocotyledons assimilated the most. Nevertheless, foliar N concentrations were higher for L. perenne receiving N and farm-scale calculations showed L. perenne to extract more soil N than the native species. K. robusta reduced N₂O emissions following effluent application by 80 % relative to control soil, which emitted significant amounts. Modelling revealed that incorporating native species into agricultural landscapes reduced the N loading per hectare due to the reduced area of fertilised and grazed soil. The native monocotyledons, in particular P. tenax and Carex virgata (pukio), have greater potential to reduce nitrate leaching than the woody species and are the most suitable for receiving effluent, whereas K. robusta in farm paddocks may mitigate N₂O emissions following urine deposition by sheltering stock. Further work could involve lysimetry to quantify simultaneously the effects of native species on nitrate leaching and N₂O emissions. These findings provide a first step towards targeted native planting strategies in sustainable agricultural management.