Jackie Harrigan

Managing the drainage water out of a block of drained pasture or cropping land can have a huge influence on the nitrogen leached.

Researchers from a team from Massey University, funded by a Sustainable Farming Fund grant, have been working with a Rangitikei sheep, beef and cropping farm and a neighbouring dairy farm to explore how drainage water can be managed to best work for the productive capability of the land and the environment. Extension and project management is run by AgFirst. Additional co-funding has come from Ravensdown and the landowners and is supported by the ACRE (Agricultural Communities respecting the Environment) group.

Hyde Park is an O’Brien Group 1000-cow dairy farm and Waitatapia Station is a mix of beef, sheep, arable, vegetable growing and production forestry. Both are in Horizons Regional Council One Plan sensitive catchments with set nitrogen leaching limits.

In the Rangitikei coastal sand country, irrigation and surface drainage have been important steps in making the land much more productive and profitable, but these flow pathways have increased the loss of nutrients into waterways, Massey University associate professor in environmental science and soil hydrology Ranvir Singh says.

On many farms that require artificial drains due to high water tables and poorly drained soils, most nitrogen is lost through drains rather than leaching into groundwater. New Zealand has 2.5 million hectares of potentially artificially drained farmland and these drains provide a short circuit for nutrients to quickly enter waterways – leaving little time for nutrient reduction in the subsurface environment, Massey associate professor in soil sciences David Horne says.

By spatially aligning intensive dairying land with areas of high nitrogen-attenuation (reduction) capacity, reductions of river nitrate loads of 25% may be possible, says Singh and Horne.

Subsurface denitrification is a key process of nitrogen attenuation (reduction) whereby soil microorganisms denitrify NO3 and turn it into benign dinitrogen. The capacity of soils to denitrify is governed by physical, chemical and biological characteristics in the soil – namely low dissolved oxygen, the presence of denitrifying microbes and the availability of dissolved organic carbon or other suitable electron donors.

The Rangitikei trial looks at two in-field/ edge-of-field technologies to manage the drainage water to reduce loss of nutrients to waterways.

The first is essentially building a dam over the end of an open drain with a single large diameter pipe through it with an end that can be raised and lowered to slow or prevent drainage of the drain. At certain times the pipe will be pulled up to restrict drainage flow and maintain a higher water table in the drainage area. In late spring and early summer, restricting drainage flow holds water in the soil profile and allows more time for the microbes to denitrify the water. Limiting drainage also means more water available for plant growth.

Capillary rise also enhances the forage growth, Singh says.

“Capillary rise is a phenomenon where when the water table is held near the surface the water is attracted up into the root zone and increases grass growth.

“It’s like a sponge or drinking straw pulling the water up like a subsurface irrigation system which has the advantage of lifting the water by up to 500mm and has the potential to save irrigation water,” Singh says.

“Controlling the drainage gives the farmers flexibility to keep the N and water back in the field when he needs it – it’s a good opportunity to manage the nutrient flow in soils where the nitrogen is not travelling down into the ground water.”

When the landowner needs to get on to the paddocks to graze pastures or get tractors in to plant crops, lowering the drainage pipe would quickly drain the length of the drain and lower the water table.

In the Rangitikei trial there are 18 controlled drainage structures on seven drains which will be monitored for two more years, with piezometers installed to track the groundwater levels.

The data will allow researchers and engineers to make recommendations for design protocols for the depth of drains and proximity to fields and waterways.

Waitatapia farm development manager Boots White installed the structures and has been tracking the progress of the research over the past six months across the 35ha catchment for the drains.

He says the research certainly has the potential to hold up the water table in certain seasons.

“I really noticed in May that holding the drainage up held the water table up in the paddocks.”

The first year of the trial involved baseline monitoring of water and nutrient flows in the drains and then bringing in the drainage control structures.

When the drainage water is flowing, the second technology employed in the trial has the water plumbed to travel through a wood chip bioreactor comprising of a one-metre-deep bed of wood chips in a lined pit of 90 cubic metres.

Bioreactors have been studied extensively overseas in places like the American mid-west, Singh says, and are proven technologies for treating and reducing nitrate loads in agricultural drainage waters, along with the use of wetlands.

“The bioreactors provide the carbon source for denitrifying nitrates in the drainage water – depending on the flowrate and time of travel through the bed the nitrates can be reduced by 50-80% in the drainage water.

“The microbes in the bioreactor eating the carbon for growth are using the oxygen in the bed and once the oxygen is depleted they start denitrifying the drainage water –reducing the nitrates.

In the trial the water travels through the bioreactor in five-six hours which at a rate of two or three litres/second the denitrification process is kickstarted. The pit of woodchips is expected to last for 10 – 15 years, and extra low-cost carbon is easily added in the form of more wood chips.

Landowner Hew Dalrymple is excited about the possibility of the work being validated and accepted by the Horizons Regional Council at Waitatapia and Hyde Park after which farmers can access the mitigation options on other farms as well.

The third mitigation strategy being considered is the storage and recycling of the drainage water as irrigation to recycle the nutrients out of the water.