The reuse of treated municipal wastewater (TMW) for irrigation rather than disposal into waterways, has net positive effects on water quality. TMW could be irrigated onto native vegetation to create zones of enhanced ecological value, and possibly generate valuable native products. However, there is a lack of knowledge on how TMW irrigation will affect the performance of native plant species. The fluxes of nutrients and contaminants in such systems need to be assessed to determine the risks of soil degradation as well as ground- and surface water contamination.

Long-term field trials in Duvauchelle (Banks Peninsula) and Levin were used to study the soil-plant interactions of New Zealand native vegetation irrigated with TMW. The growth and survival of native species at the sites were monitored and the chemistry of the plants, as well as the underlying soil, was analysed. Results from both field sites showed that irrigation of TMW accelerated the growth of native vegetation. At Duvauchelle, TMW increased the overall average plant height by 10% after 3.5 years of irrigation. However, weed growth and competition was also accelerated by TMW irrigation. Weed control resulted in a ninefold increase in the survival of TMW irrigated plants at Levin. This shows that weed management is a critical success factor for the establishment of native ecosystems with TMW.

The application of TMW increased phosphorus, nitrogen, and sodium concentrations in the soil. Nitrogen fluxes were affected by plant species, and not all of the nitrogen applied with TMW could be accounted for. At Duvauchelle, soil nitrate concentrations were lower in the rootzone of Coprosma robusta than other species. TMW irrigation did not significantly increase soil nitrate concentrations at application rates of 1000 mm yr-1 (equivalent to approx. 200 kg N ha-1 yr-1). In contrast, preliminary results from Levin showed that 28-38% of the applied nitrogen leached from the soil in the form of nitrate at TMW irrigation rates >4000 mm yr-1, with no significant difference between Kunzea robusta and pasture.

A pot experiment was set up to quantify nitrifying and denitrifying microorganisms under native species compared to pasture. Ammonia oxidising bacteria were less abundant under monocotyledonous than dicotyledonous species. This was likely due to higher nitrogen acquisition by the monocotyledonous plants, which limited the nitrogen availability to the bacteria. The abundance of nosZ, the gene encoding nitrous oxide reductase that reduces nitrous oxide to dinitrogen during denitrification, differed between plant species and was significantly lower under Coprosma robusta than most other New Zealand native species and Lolium perenne. This reflects the importance of species selection to mitigate nitrogen losses from TMW irrigated soil.

If TMW irrigated plants were utilised to produce value from the land through native products, increased soil nutrient concentrations with TMW irrigation may affect the quality of products such as mānuka (Leptospermum scoparium) honey and essential oils. Data from a field study was analysed to determine the effects, if any, of soil properties on the quality of mānuka honey. Results showed that honey trace element and soil nitrate concentrations were negatively correlated with mānuka honey methylglyoxal concentration, which largely determines the honey’s market value. It is possible that accelerated growth of exotic weeds in high fertility soil led to a dilution of mānuka honey. This is particularly relevant at sites with high nutrient application rates through TMW.

This research revealed that TMW could be beneficially reused to establish native ecosystems in New Zealand and elsewhere. Such multi-purpose restoration presents an opportunity to increase the percentage of TMW that is applied onto land. Globally, establishing native vegetation with TMW irrigation has the potential to improve water quality, increase biodiversity, and accelerate carbon sequestration.