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The role of mānuka (Leptospermum scoparium) riparian plantings in the mitigation of diffuse agricultural nitrogen

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 New Zealand (NZ) — 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 root exudates. Mānuka (Leptospermum scoparium) is a plant native to NZ, and the species could be a good candidate to mitigate N losses due its ability to tolerate high N loads and co-benefits such as farm diversification through apiculture or essential oil production. Previous work also suggests that L. scoparium could be a biological nitrification inhibitor (BNI), limiting nitrate (NO₃⁻) production in soil. The aim of this study was to investigate the potential of mānuka to intercept and remove N in an experimental riparian buffer in Lake Waikare. Currently, the lake is eutrophic due to high nutrient and sediment inputs. Significant restoration efforts have been made in recent years, led by many hapuu around the lake. All are very passionate and determined to recover the mauri of this ecosystem and reconnect the people to the lake, as well as provide economic opportunities. The riparian band had been established for four years and was set up as a series of experimental plots with different vegetation types, including plots solely in mānuka, and grassed controls. The plots are on a Perch- Gley Ultic Soil underlain by a slowly permeable clay layer— the Hamilton Ash. Further, the study sought to explore the relationship between soil physical properties and N cycling, and to identify influences expected by the local hydrology— as there was evidence of perching and lateral flow in the site. A series of suction-cup lysimeters were installed and pore water was sampled seven times between May and July 2021. Samples were analysed for total N (TN), total Kjeldahl N (TKN), NO₃⁻ and ammonium (NH₄⁺). The results show that the riparian buffer is effectively removing N from subsurface flows, with TN declining from an average of 9.32 mg/l at 1 m into the buffer to 2.03 mg/l at 7 m. Although N concentrations were higher under mānuka, the total amount of N extracted from those plots was 21% less due to high rainfall interception by the canopy (63.9%), limiting the transport of the solute. Indicators of soil physical quality, bulk density (ρb) and macroporosity (MP), were investigated as potential causes for differences in the movement of N. There was a 17% improvement in ρb, and 38% improvement in MP relative to 2017 measurements, before the riparian band was established. No appreciable differences in soil physical properties were found between vegetation types at this early stage in the inception of the riparian buffer, although changes are likely to occur as the mānuka trees mature and the impact of roots is more developed. Shallow wells were deployed to monitor the dynamics of the water table, as there was evidence of winter perching. Dip wells demonstrated the existence of a short- lived perched water table, resulting in lateral flow, and likely impacting the degree of soil gleying in different parts of the riparian band. Water and contaminants are likely to move laterally over the Hamilton Ash towards the drain, supported by the accumulation of NO₃⁻ at the boundary with the clay layer. Future work should be devoted to better quantifying NO₃⁻ leaching from the riparian plot by incorporating a drainage model, as well as further investigating the relative importance of N removal pathways at this site (i.e. denitrification and plant uptake)— including the potential for the lower part of the riparian band, closest to the drain, to become a hotspot for nitrous oxide (N₂O) emissions. It would also be of value to revisit this study in 4-10 years to explore how the buffer impacts soil quality and N dynamics in the subsurface as it matures.
Type of thesis
The University of Waikato
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