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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 – January 2020

What Did We Do?

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 developed a catchment-scale model to assess denitrification processes and end products (nitrous oxide and/or dinitrogen) in shallow groundwaters in the Manawatu and Rangitikei river catchments (located in the lower part of the North Island, New Zealand).

This conceptual model could be developed into a method to map and effectively utilise complete benign denitrification capacity at the farm scale, to help achieve environmentally friendly dairy farming across agricultural landscapes.

How Can The Research Be Used?

  • This research found that the capacity of natural 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.
  • Farming the attenuation capacity will be a defining feature of future land uses and a key component of suitability criteria.
  • The results suggest a reduction of greater than 25% in the river nitrate loads from dairy farming areas could potentially be achieved by spatially aligning dairy land with areas of high subsurface nitrate-attenuation capacity, and by managing critical flow pathways using innovative edge-of-field technologies such as controlled drainage, drainage-water harvesting for supplemental irrigation, woodchip bioreactors, and constructed wetlands in the study catchments.
  • There is potential to build on this work to better understand, map and effectively utilise existing natural and new built-in nitrate-attenuation capacity to significantly reduce water-quality impacts from dairy farming across environmentally sensitive agricultural catchments. This knowledge and tools could help farmers close the gap between what can be achieved with current, in-field mitigation practices and the nitrogen-loss allocation imposed by regulatory authorities.
  • Regional councils can use this research to inform policy and the allocation of nitrogen leaching allowances, to achieve better water quality outcomes. 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 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

In Some Places, Intense Land Use Will Be Okay

Our Land and Water blog, 16 April 2019

In areas where natural denitrification processes have a high capacity to reduce nitrogen, research suggests it's possible to both intensify land use and improve water quality – as long as land-use is de-intensified over the low-attenuation-capacity areas in the same catchment.

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Our Land and Water takes on agricultural challenges

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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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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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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Team Snapshot

Research Outputs

Papers

Water-quality issues facing dairy farming: potential natural and built attenuation of nitrate losses in sensitive agricultural catchments

Ranvir Singh and David J. Horne
Animal Production Science, November 2019

We developed a conceptual catchment-scale modelling analysis assessing potential natural and built attenuation of nitrate losses from dairy farming in the Tararua and Rangitikei catchments (located in the lower part of the North Island, New Zealand). This exploratory analysis suggests that a reduction of greater than 25% in the river nitrate loads from dairy-farming areas could potentially be achieved by spatially aligning dairy land with areas of high subsurface nitrate-attenuation capacity, and by managing critical flow pathways using innovative edge-of-field technologies such as controlled drainage, drainage-water harvesting for supplemental irrigation, woodchip bioreactors, and constructed wetlands in the study catchments.

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

Nitrogen Attenuation via Benign Denitrification

David Horne
Our Land and Water Symposium, August 2019

Video: https://vimeo.com/356108131

Benign denitrification in shallow groundwaters

Ranvir Singh, Neha Jha, Marcela Gonzalez, and Andrew McMillan
Land Use and Water Quality Agriculture and the Environment, Denmark, June 2019

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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