About 2.5 million hectares of artificially drained farmland is both a challenge and an opportunity for farmers wanting to stem nitrogen losses.
On certain soils those drains can be a superhighway for nitrogen release into larger waterways but scientists are also learning how controlling flow from those drains can help stem nitrogen losses.
While the Massey University project is studying nitrate losses through drains and how to control water levels and drain losses in Rangitikei and Tararua farms it also encompasses understanding the science behind nitrogen attenuation.
That is the ability of highly soluble and mobile nitrates to be converted to benign gases in the right conditions. When it occurs below the soil it is known as subsurface denitrification.
“What our work through Rangitikei and Tararua is showing us is that you can be standing in a dairy farm paddock and below you the conditions are ideal for denitrification.
“Meantime. on the same farm in a different paddock it could be the opposite and nitrogen is ending up being flushed through,” Singh says.
“It is a very spatial occurrence, some places subsurface denitrification happens, some it doesn’t.
“But what we are learning is that it is a relatively stable condition, meaning the degree to which it occurs and where it doesn’t can be mapped.”
The very drains that are designed to move water quickly off farmland can affect potential for denitrification. A slower water flow through subsurface under the right conditions can provide more time for denitrification to occur before the water is flushed through with significantly lower nitrate levels into waterways.
At certain times of year the need to get that water off farmland quickly is greater if major pasture and soil damage is to be avoided.
The conundrum leaves farmers having to flush nitrate-laden water away but also having to feed stock on viable paddocks.
“So, when a lower water table is required we can pass the drain water through treatments that can help convert that nitrate to a gas.”
The Massey work is examining the use of treatments such as wood chip bioreactors – constructed channels containing finely chopped pine wood chips the drainage water flows through with bacteria converting nitrates to nitrogen gas.
Singh, who worked in the United States Mid West before joining Massey, says use of wood chip bioreactors is becoming an increasingly accepted means of nitrate management.
New Zealand has a few trial bioreactors and wood chips as their main component fits well with national goals to increase plantation forestry.
At the drier times of the year having knowledge on a soil’s nitrogen attenuation potential means farmers could confidently keep water tables higher on soils where denitrification rates are greater.
Other techniques are also being tred by the group, studying the effectiveness of recycling farm drainage water for irrigation.
So far, as a proof of concept, Singh and his colleagues have done a preliminary map of Tararua and Rangitikei districts identifying the nitrogen attenuation ability of the soils.
“From this we can determine what dairy farms are on what level of nitrogen attenuation potential land.
“We roughly estimate in the two districts we could have the same number of cows and cut down the nitrogen loading into waterways by 25%, simply by putting the right number of animals in the right places on farms at the right time of the year and using these drainage management techniques.”
Linking that knowledge to naturally existing and built-in new nitrogen attenuation systems they are trialling has big implications for how farmers respond to tightening regional rules on nitrogen losses.
“It may well mean that on stony, free-draining soils linked to ground water supplies you won’t be able to put as many stock on.
“But for areas that are not draining straight into ground water and are relying on drainage systems to take water to larger waterways it could be you don’t end up spending thousands on facilities like herd houses.
“Taking the attenuation knowledge you have for your farm and using these relatively lower-cost techniques means you avoid that major capital outlay.
“You also leave your land use options open – if you are committed to an expensive livestock home, for example, it’s more difficult to change to another land use in the future.”
