“Do a manu” “Do a bomb”. On a hot summers day these are the chants that ring out across Aotearoa as packs of kids and adults line up on bridges or climb atop rocks and get ready to jump into a lake or a river. A ‘bomb’ or a ‘manu’ is a very precise manoeuvre that involves jumping from somewhere high, curling into a ball and making the biggest splash you can when you hit the water. Some would argue that a manu involves more technical aerial acrobatics than a simple bomb. Either way, the bomb or the manu is a rite of passage for many New Zealand kids.

But increasingly this treasured national pastime is under threat. The Ministry for the Environment painted a grim picture of waterway health in its recent Our Freshwater report. Nearly half of New Zealand’s lakes are in poor or very poor health. Only two in every hundred lakes are in good or very good health. Many rivers have become so polluted that they are now unsafe for swimming at times. And it is not only humans who can no longer safely swim in some of the country’s rivers. Native freshwater fish are struggling to survive. More than three-quarters of them are threatened with extinction.

The native freshwater birds that depend on rivers, lakes and estuaries, are also in peril. More than two thirds of them are threatened with extinction or at risk of becoming threatened. Introduced predators, like trout and stoats, are one of the main culprits behind the decline in native freshwater fish and bird populations. The degradation of freshwater habitat by pollution is another driver that is pushing these precious species closer to extinction. Cleaning up waterways is important not only for protecting the long-held tradition of doing a ‘manu’, it’s also critical to the protection ngā manu (the birds) of Aotearoa.

Nitrogen pollution is one of the leading causes of the degradation of New Zealand’s freshwater ecosystems. When excess nitrogen on the land seeps down through the soil past the rootzone of plants it can get into the groundwater. From there it can move into the aquifers that many communities and cities get their drinking water from, or it can re-emerge in springs and get into lakes and rivers. Once in those lakes and rivers nitrogen can cause algal blooms, which can suck oxygen out of the water making it difficult for freshwater fish to survive. These algal blooms also make rivers a lot less appealing for jumping into on a hot summers day. Some algae are even toxic and can cause human health issues as well as kill sensitive animals like dogs.

There are lots of different forms of nitrogen, but one of the main forms that leaches in this way is nitrate. The vast bulk of nitrate pollution getting into New Zealand’s freshwater comes from agriculture. That’s mainly because New Zealand’s pastures are loaded up with synthetic nitrogen fertiliser and the urine of the livestock feeding on these pastures has huge amounts of nitrogen in it. When livestock, particularly dairy cows, urinate the plants can’t always use all the nitrogen for their growth and so the excess nitrate can leach into waterways.

A promising new solution to this nitrate leaching problem has been explored by researchers from Lincoln University, Canterbury University and Plant and Food Research. In 2017, the scientists simulated a dairy cow urinating (not something many of us do in our day jobs) and compared the nitrate leaching rates under three tree species that could be planted into dairy pastures; radiata pine (an exotic species), mānuka (native) and kānuka (native). They found that mānuka and kānuka leached far lower amounts of nitrogen (2 kg/ha) than pine (53 kg/ha).

Speaking about the project Dr Juergen Esperschuetz, the lead researcher from Lincoln university said, “These results show mānuka and kānuka could be even more effective at protecting water systems than anyone expected.”

Intentionally planting trees into pasture where animals continue to graze is a farming system called silvopasture. Silvo is derived from the latin word for forest and pasture, well we all know what that is. Silvopasture is not just about shelterbelts, windbreaks, and riparian buffers, systems which relegate trees to the margins of a farm. Instead, silvopasture systems often plant trees into the paddock itself. It has been said that silvopasture, and other agroforestry systems like it, represent a shift away from monocultural production and towards an agricultural system that more closely mimics natural forest ecosystems. Mānuka and kānuka are native trees to Aotearoa so incorporating them into dairy pastures also provides a way to bring more native biodiversity back into farming landscapes.

The researchers also found that soils under the mānuka and kānuka emit far less nitrous oxide, with the mānuka soils emitting the least of the three. Nitrous oxide is an extremely potent greenhouse gas, it is long lived and in Aotearoa, the vast bulk of nitrous oxide emissions come from livestock farming. So planting mānuka and kānuka into dairy pastures could also help in the fight against climate change. On top of that, both trees produce high-value products in the form of oils and honey and that could be used to supplement farm income.

Since the publication of this study, other researchers have gone on to use parts of its methodology and draw on its findings in their research. In the Wairarapa, a study done in the field found much lower nitrate levels under manuka than under pasture, corroborating the findings from this glasshouse study done by Dr Esperchuetz and his team. In Spain, researchers also drew on the study when they investigated nitrate leaching risk under walnut silvopasture.

This study has added to the toolkit of options available to help reduce the environmental impact of pastoral farming in Aotearoa. Incorporating mānuka and kānuka trees into pastures will not only bring biodiversity into farming landscapes. Thanks to this research, we now know it will likely help clean up our lakes and rivers too, protecting both ngā manu and the manu now and into the future.

This article was prepared by Master of Science postgraduate student Genevieve Toop as part of the ECOL608 Research Methods in Ecology course.

You can read the full article here: Esperschuetz, J., Balaine, N., Clough, T., Bulman, S., Dickinson, N. M., Horswell, J., & Robinson, B. H. (2017). The potential of L. scoparium, K. robusta and P. radiata to mitigate N-losses in silvopastural systems. Environmental pollution, 225, 12-19.