Instead, they preferred to outline research and scientific progress, identify risk areas and gaps in understanding and provide options for retaining nitrogen fertilisers and managing soil losses.
An audience of 150-plus people listened to a range of speakers at the NZ Institute of Agricultural & Horticultural Science’s (NZIAHS) Canterbury Forum — Nitrogen: Friend or Foe?
NZIAHS vice-president Professor Jon Hickford said the subject was selected as a rhetorical question because nitrogen was neither friend or foe — it was more complex than that.
He said the take-home message was not to be too prescriptive or make hurried assumptions.
‘‘What we were trying to get at is the jury is still very much out in terms of our understanding around the role that nitrogen plays in life. It’s an essential nutrient and things don’t grow without nitrogen and you can’t make DNA or protein, and therefore the very essence of life, without nitrogen. But in the wrong place and wrong time it can cause problems with eutrophication [nutrients accumulating in water] and these sorts of things so we need to understand it.
‘‘The problem we have at the moment is that we have rendered the discussions around nitrogen down to those simplistic friend or foe, black or white, good or bad type arguments and it’s doing the whole area a huge disservice.’’
The large turnout was because people appreciated the polarisation and simplification of the arguments and the need for a greater understanding of a hot topic, he said.
‘‘The speakers learnt something on the day too. It’s not just about the audience listening and sponging up information — it’s about a uniformity of purpose to get everyone to understand the challenges that reside.’’
Among the snippets he took home was from Environment Canterbury science director Dr Tim Davie that the focus should not solely be on nitrates on waterways.
Water health was more complex with shading and temperature rises contributing factors, he said.
Prof Hickford said the observation was well made during the forum for the need to have repeated measuring of nitrate levels in wells and waterways to get an accurate picture of a problem.
Canterbury hotspots for nitrates were there before the dairy boom and while dairying could not be let off the hook for its impact, there were preceding contributors, he said.
He said another insight was from University of Otago department of surgery research Associate Professor Jacqui Keenan, who pointed out that before getting too excited about the possibility of nitrates in water influencing high incidences of colorectal cancer, other factors such as sugary drinks, alcohol and takeaway food shoud be considered.
Vegetable growing is estimated to have the highest nitrate concentrations in drainage water, followed by mixed cropping with its autumn cultivation and winter grazing and then dairy with its well fed animals, high protein intake and large urine deposits. These land-use systems are followed by beef, sheep and forestry for concentrations.
Lincoln University Emeritus Professor Keith Cameron said vegetables were a high-value crop, but they had shallow roots and needed higher rates of N fertiliser so their risk of leaching was high.
‘‘I love vegetables and if you think about vegetables it’s a good feeling of eating and enjoying them. But some people have suggested actually we should displace some of this [pointing to livestock farming] and have more of this [vegetables] on the Canterbury Plains. I think it would make things worse.’’
He said the scientific community was spending ‘‘hours and hours’’ of their lives to develop options to reduce nitrate-leaching loss.
They included using cool season active plants such as italian ryegrass, although it was an annual and a winter-active perennial ryegrass was needed to be developed by plant breeders.
Scientists were working to reducing N concentrations in urine with plantain, chicory and other feed options and putting in a lower input system, he said.
‘‘As many of you know, a lot of work has gone into checking plantains and chicory to dilute the N deposits in patches and this has been taken from intensive laboratory to intensive field and now into commercialisation on a farm.
‘‘There’s always a challenge of getting out of that valley of death and out the other side, but progress is being made to help farmers to reduce nitrate leaching losses by using alternative pasture species.’’
Other options were feeding farm animals better and perhaps reducing the stocking rate in autumn when leaching was high.
He said new nitrification-inhibitor technology was being developed for the future to slow down the production of nitrates and reduce leaching losses in soils by more than half.
The challenge was getting them approved which took a long time, he said.
‘‘Many other countries are still using DCDs, but New Zealand could not.’’
Prof Cameron said factors affecting nitrate leaching from the soil included rainfall and drainage, soil types and depth, farming systems, fertiliser and effluent rates and their timing, plant uptake of nutrients, irrigation and stocking rates.
A free-draining shallow stony Lismore soil loses nitrate at a greater rate and amount (30kg of nitrogen a hectare a year), than a slow-draining deep Wakanui silt loam (leaching at 10kg/N/ha).
However, nitrate leaching was a more complex process than this as they did not move through soils in a band partly because diffusion and dispersion processes tended to spread out concentrations in the nitrate profile, with some going ahead and some staying behind. They also followed through effluent and plant channels during heavy rainfall, he said.
Prof Hickford said it would be easy to blame dairying and vegetable growing for high nitrate levels in Christchurch rivers and estuary, but domestic impacts such as garden fertiliser could contribute to leaching.
‘‘It’s about bringing all things to the table and understanding that we probably have to work catchment by catchment and, dare I say it, square metre by square metre, and look at land-based systems, be it housing, or farming or horticulture or whatever to come up with better ways of managing this as it’s not a one-size-fits-all solution to try and minimise the adverse consequences of nitrate loss.’’
Lifetime achievement awards were presented by the institute to Dr Trish Fraser, Professor John Hampton and Professor Jacqueline Rowarth at the forum.
