Scientists are tracing the flow of hydrogen, oxygen and nitrogen to find out how nutrients flow through water and farms. By Anne Hardie.

Scientists are providing another tool for farmers to trace the sources of nitrogen in their farming systems with the use of natural isotope tracers.

Scientists like Dr Troy Baisden have been working on the science of pinpointing periods or locations of excess nitrogen (N). By doing that, farming and catchment groups then know where and how to mitigate nitrate losses.

The tracers work because water and nitrate molecules naturally contain different versions – isotopes – of the elements hydrogen, oxygen and nitrogen. With good instrumentation, these isotopes serve as tracers of water and nitrate, used to observe its progress through a system, or to determine its sources. For farmers, they can be used to find out how nutrients as well as water move through a farm.

Dr Baisden, a professor at Waikato University, and now connecting across Auckland University, Te Pūnaha Matatini, Motu and Land and Water Science,  has led teams developing this science for more than a decade. He has been using N isotopes to confirm details such as whether the nitrogen comes from urea or urine, or whether there is another source such as market garden fertilisers or maybe an abandoned septic tank in the catchment. Until now, water testing has shown where high nitrate concentrations exist, but isotope tracers can now reveal why those concentrations are high and where they come from.

Some natural isotope tracers can show whether excess nitrogen levels in a waterway after a high rainfall event comes from runoff capturing urea and urine from the land, or from underground sources. Others identify whether underground water feeding into the waterway has been there for one month or for 20 years.

In some catchments, N levels can rise three, four or five times in a storm event, yet he says little has been known about whether the N is from runoff or from underground. Or how water is bypassing soil and running into the stream.

“When and how do we get particular nitrates out of particular catchments? With this we can see where the nitrate excesses are and can look at how we can reduce them by working backwards.”

Research used the natural isotope tracers to work out how they could be used as tools to reduce uncertainties in the sources and magnitude of N losses, to clarify rates of change in N budgets and to identify opportunities to reduce the N excess causing losses to water.

The tracers reveal targets in space and time for potential mitigation, specifically N content in clover-ryegrass pastures which seasonally exceed N demand in grazing animals. He says that suggests farmers could use alternate species of feeds to reduce animal urinary N excretion and that would limit soil-derived nitrate (NO3) losses as well as the greenhouse gas emission, nitrous oxide (N20).

Other management decisions could include the timing of fertiliser applications, grazing and cut-and-carry to potentially mitigate excess N and associated N losses.

In Southland, where tile drainage is often used, they found dissolved organic N increased after major rainfalls while nitrates (NO3) remained stable. Dr Baisden says that suggests the ongoing breakdown of soil organic matter releases N, which should be considered in farm and catchment N budgets.

He says much of the work being used in farming and catchments focuses on best practice, such as the Overseer model, but best practice does not always occur and there are sources in catchments that affect N measurements on farms. Models are needed, but he says they need additional tools.

Tracers are tools that will be useful in raising issues within catchments by delving into the detail of what is happening and helping people on the ground better observe how the catchment’s plumbing works.

Dr Baisden says most nitrogen sources have unique signatures which identify what it is and what it isn’t, such as urea, urine and fertiliser – that are easy to reduce. Or it may look like soil nitrogen, which indicates that pastures and soils are working well and retaining nitrogen.

Though scientists have been working on isotope tools for the past decade, Dr Baisden says there has been disappointing interest in the work until now. He also says the focus has been mainly on nutrient accounting to support policy without sufficient interest in confirming sources and processes to support farmers and catchment groups to identify and mitigate their nitrogen losses.

Now there is growing interest in the research and that is coming from the likes of Landcare Trust and farmers.

Landcare’s Nelson-Marlborough regional coordinator, Annette Litherland, became interested in tracking sources of nitrate from work in Golden Bay. Dairy farmers in the Motupipi catchment are low-intensity dairy farms that have carried out extensive environmental work to improve the river quality, but the nitrate levels remain high in the river. The river is also spring-fed, with water brought from far outside the immediate river edge and she says it is not clear where the nitrate is coming from. Similar cases can be found in springs and aquifers in Tasman.

She says the methods to reduce sediment, phosphate and E.coli are easy to understand as they all involve transport over the ground. But reducing nitrate levels is more difficult. Finding a tool that could identify sources would enable the most appropriate cost-effective mitigation methods to be applied.

Deciding it was time for more discussion on the subject, she organised an online discussion between scientists, industry and farmers from across the country. It was also aimed as an opportunity for farmers to communicate with scientists on the gaps in knowledge around nitrate leaching.

Dr Baisden says the farming industry and wider catchments need the science now to address the issue of nitrate losses, whether that is to improve waterways or to retain nitrogen in soils for farming. He says it is a win-win if farms can retain productive nitrogen and avoid impacts in water.

The science is still expensive, but he says it has begun to look cost effective compared with the limit-setting costs being imposed on farmers. He thinks there will be more collaboration between farmers and councils to find solutions and he expects that will be driven by farmers and their catchment groups.

Farmers and catchment groups wanting to track nitrate sources using the isotope tracers can work with an isotope scientist to collect samples along with their regular water samples and send them to a laboratory at GNS Science. A catchment coordinator or similar can then work with a scientist to interpret those results so they can be used for practical solutions.