Mānuka-Dominated Riparian Plantings for the Mitigation of Diffuse Agricultural Nitrogen

Olivia María Adamson (University of Waikato), Maria J Gutierrez Gines (ESR), Tanya O’Neill (University of Waikato), Matthew Taylor (Waikato Regional Council), GlenTupuhi (Ngaa Muka Development Trust), Tawera Nikau (Matahuru Marae)

The disruption of nutrient cycles in agricultural settings, particularly pastoral farming, is responsible for up to 70% of the nitrogen (N) loads entering streams in Aotearoa New Zealand, resulting in the widespread degradation of freshwater ecosystems. Riparian plantings are one strategy to mitigate the losses of N from land to water, removing N by denitrification and plant uptake. Some types of vegetation are more effective than others at intercepting N, due to their ecology, impact on soil quality and production of root exudates, which can alter N cycling. Previous research suggested that mānuka (Leptospermum scoparium) could be a biological nitrification inhibitor (BNI), limiting nitrate (NO3- ) production in soil. For this reason, this species could be a good candidate to mitigate N losses and add co benefits such as farm diversification through apiculture or essential oil production.

A 1.6 ha experimental site was set up in 2017 in Nikau Farm to investigate the role of mānuka-dominated riparian plantings in reducing the inputs of N from farming activities in an adjacent waterway. The experimental site consisted of grassed and unplanted plots, as well as mānuka-dominated plots. The area was equipped with suction cups and dip wells for monitoring the movement of water. Soil physical properties were also measured. 

Results showed that both the mānuka and grassed plots effectively reduced Nconcentrations throughout the buffer up to  80%. Water management by the mānuka was the most important factor regulating the losses of NO3- , which were 21% smaller compared than the grassed plots. The higher concentration of NO3- at 50 cm soil depth, and lower concentration of ammonium in the topsoil, demonstrate that apart from water management, further mechanisms (i.e. mineralization, nitrification and/or denitrification) might be playing an important role in the movement of N in these systems. Soil bulk density and macroporosity improved by 17% and 38% respectively, which will likely have a positive effect in further mitigation of water pollution in the future, as well as recovering biodiversity and ecosystem functioning.


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09 Nov 2022