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Future Landscapes

Benign Denitrification in Groundwaters

Developing a cost-effective technique to assess denitrification processes and end products in shallow groundwaters

PROJECT DETAILS

Challenge funding: $170,000

Research duration: January 2017 – December 2018

What Are We Doing?

Denitrification is the natural process of soil bacteria converting nitrate back into atmospheric nitrogen gas.  This process could reduce (attenuate) the environmentally damaging impacts of nitrate leached from farms to freshwater.

It is important that the denitrification process is complete, because incomplete denitrification can release nitrous oxide (N2O, a harmful greenhouse gas) rather than dinitrogen (N2, a harmless gas making up 78% of the atmosphere).

Benign Denitrification in Groundwaters research will develop a cost-effective and rapid technique to accurately assess denitrification processes and end products (nitrous oxide and/or dinitrogen) in shallow groundwaters in the Manawatu and Rangitikei river catchments. This cost-effective technique will be used to measure and map complete benign denitrification hotspots across New Zealand’s agricultural catchments.

How Can The Research Be Used?

  • The research has found that the capacity of natural denitrification processes to reduce nitrogen varies a great deal spatially, even within the same catchment. Some parts of a catchment may therefore contribute disproportionately to river contamination of nitrogen, while areas with high nitrogen attenuation capacity may be suitable for strategic intensification of land use.
  • Researchers in the Benign Denitrification in Groundwaters project are working collaboratively with Horizons Regional Council and Waikato Regional Council, having agreed that the denitrification measurement tools will be used by the Councils.
  • The first end users will include Massey University’s Fertilizer and Lime Research Centre (FLRC), Horizons Regional Council, Waikato Regional Council, and Our Land and Water researchers and partners.

In the Media

Irrigation NZ News

Autumn 2019 (page 36)

The Massey University-led research analysed the likely result of strategically intensifying land use in more than 83,000ha of high nitrogen attenuation capacity areas in the catchment – while de-intensifying land use over about 10,000ha

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Science omnivore takes on the tough questions

Dairy Farmer, 18 February 2019

Because the leached nitrogen flowed through aquifers where a lot of denitrification occurred, most of the leached nitrogen was converted to gaseous forms and lost to air. The result was that the load in the river would have likely decreased by 6%

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Rural Delivery

TVOne, 2015

“We found that nitrogen reduction capacity varies quite a bit from sub-catchment to sub-catchment,” says Ranvir Singh. “That is very interesting because we did not know that this reduction underground was that variable, and we are not utilising it properly”

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Troubled Waters

NZ Geographic, July-August 2017

This capacity to attenuate nitrogen is highly variable across a catchment, Singh said. “This leads you to conclude that some parts of a catchment are contributing disproportionately to river contamination”

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Team Snapshot

Research Outputs

Conference Papers

Efficacy of subsurface denitrification to attenuate nitrate in shallow groundwaters

Jha, N., Singh, R., and McMillan, A
Fertilizer and Lime Research Centre workshop 31, February 2018

Our preliminary results indicate occurrence of complete ‘benign’ denitrification in the reducing shallow groundwater sites with lower dissolved oxygen (DO) contents (DO < 1mg L-1), as compared with the oxidizing shallow groundwater sites (less-reducing) with higher DO contents (DO > 1mg L-). This study is continued with further monitoring and experiments for examination of processes and regulatory factors driving subsurface denitrification in shallow groundwaters in the Manawatu and Rangitikei River catchments.

Molecular Approaches to Identify Benign Denitrification in Shallow Groundwaters

Jha, N., Singh, R., McMillan, A. and Gonzalez, M.
NZ Soil Science Society Conference, December 2018

We measured lower dissolved N2O and higher denitrifier genes abundance in the groundwater samples collected from reducing (anoxic) groundwaters sites, as compared with the groundwater samples collected from non-reducing (oxic) groundwater sites. This highlights the potential of microbial measurements (denitrifier genes abundance) as a proxy marker, in combination with the groundwater chemistry measurements, to measure and map benign ‘complete’ denitrification across NZ agricultural catchments.

Predicting Land-Based Nitrogen loads and Attenuation in the Rangitikei River Catchment – the Model Development

Ranvir Singh, Ahmed Elwan, Dave Horne, Andrew Manderson, Maree Patterson, Jon Roygard
Fertilizer and Lime Research Centre workshop 30, 2017

We investigated and developed a simple model to account for the influence of different soil types and underlying geology on the transformation of nitrogen (N) in the Rangitikei River catchment. The main soil and rock types of the catchment were classified into low, moderate and high N attenuation capacities then assigned nitrogen attenuation values in order to predict soluble inorganic nitrogen (SIN) loads to the river. The prediction of SIN loads in the river was improved by incorporating the spatial effects of both of the different soil types and underlying geologies on N attenuation in the subsurface environment of the Rangitikei River catchment.

Predicting Land-Based Nitrogen loads and Attenuation in the Rangitikei River Catchment – the Implications for Land Use

Dave Horne, Ahmed Elwan, Ranvir Singh, Andrew Manderson, Maree Patterson, Jon Roygard
Fertilizer and Lime Research Centre workshop 30, 2017

Where both the intensity of land use in high N-attenuation areas was increased and the intensity of land use in low N-attenuation areas was decreased, N loss from agricultural land was greater but, importantly, N load to the River was lower. The approach described here is useful to identifying the potential (so-called) ‘head space’ for increased N leaching from farms within catchments and more efficiently allocating land use intensity to contrasting landscapes.

CONFERENCE PRESENTATIONS

Denitrification in shallow groundwaters – an ecosystem service or a pollution swap?

Gonzalez, M., Singh, R., Jha, N., and McMillan, A.
Fertilizer and Lime Research Centre workshop 32, February 2019

Dissolved gases as indicator of denitrification in shallow groundwaters in agricultural landscape

Gonzalez, M., Singh, R., Jha, N., and McMillan, A.
NZ Soil Science Society Conference, December 2018

Benign denitrification in the subsurface environment

Ranvir Singh, David Horne, Uwe Morgenstern, Andrew McMillian, Abby Matthews, Jon Roygard, Mike Hedley
Our Land and Water Symposium, April 2017

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