ENVIRONMENT DRAINAGE
Left: Massey University associate professor Ranvir Singh talks the crowd through the process of water being piped into the bioreactor and the microbes denitrifying the water as it travels through the woodchips.
flowing into deeper groundwater. Where dissolved oxygen is low this means that the microbes are working, eating the carbon and splitting the O2 out of the nitrate and releasing the nitrogen as gaseous N into the atmosphere. A Further PhD project will investigate if there are ways of further enhancing denitrification in groundwater?
CONTROLLED DRAINAGE
Down the drain...or not? Words by: Jackie Harrigan
T
wo years into a Sustainable Farming Fund project the results are looking promising for the innovative drainage management technologies employed on two Rangitikei farms with artificial drainage. The project has investigated how the nitrates present in drainage water could be attenuated (lessened or reduced) through controlling the drainage and the drainage water. More than 2.5 million hectares of New Zealand farmland is artificially drained, being both a challenge (providing a short circuit to surface waters for pollutants) and also an opportunity (for farmers to lower the water table and maximise growth for pastures and crops). Reducing the time of the drainage water spent in the soil profile, however, reduces the time for subsurface denitrification, when the bacteria in the soil convert water-soluble nitrate into harmless nitrogen gas.
GROUNDWATER MONITORING The first part of the trial work has been monitoring the shallow groundwater to investigate the extent of nitrate 60
denitrification. Sampling water from tubes and screens set into the soil was sampled at levels 1-2.5 metres below the soil surface, 2-3m below the surface and 3.4-4.4m below the soil surface. Analysis of the water for nitrates and dissolved oxygen has shown that nitrate levels dropped from around 9ppm to less than 1ppm between 1m below the surface down to 3.5m below soil surface - showing that more denitrification is happening at the lower levels and nitrates are not
Controlling the speed of water in surface drains can slow the movement of surface water, holding the water in the soil profile making it accessible for plants and allowing more time for natural denitrification. Alternatively speeding up drainage removes more water to facilitate field operations. Putting in a weir and mechanism to speed up or stop the drainage in 2019, the trials have tested the nitrate levels in uncontrolled drainage water at 11g/ cubic metre NO3-N and the water from the controlled drain at 1.41g/cu m NO3-N, proving that denitrification is enhanced by intercepting H2O and slowing the movement of it through the soil profile to drains. Massey University associate professor of soil science Dave Horne estimated with a simple control structure costing $1000, the
Building the bioreactor, filling with woodchips.
Dairy Exporter | www.nzfarmlife.co.nz | December 2020
