The demonisation of N

Certainly, past use was more gung-ho than is now the norm, and rivers have suffered. Undoubtedly, the extra people on the globe - supported by the use of synthetic nitrogen (estimates are 40-50% of the current global population) are having an environmental impact.

But strategic use of synthetic nitrogen, using precision agriculture in New Zealand is a different story.

Nitrogen (and superphosphate) application have been important in increasing the organic matter, which is approximately 58% carbon, in our soils. Every tonne of carbon is associated with 80-100kg nitrogen (N), 20kg of phosphorus, and 14 of sulphur, as well as other nutrients. These nutrients are part of the value of soil organic matter. Micro-organisms decompose organic matter as their source of energy, releasing nutrients and carbon dioxide as they do so. Some of the nutrients are used to make more micro-organisms, but some are lost, in rainfall for example, before used.

A research paper by Plant and Food scientists – released at the end of last year – reported that after 13 years, fallow plots on the Canterbury Plains had 19-22 t/ha less carbon than pasture plots. The fallow plots were kept plant-free but were not cultivated during that time.

The implication is that soil micro-organisms decomposed organic matter at approximately 1.5 tonnes of carbon a year. And the organic matter was not being replaced by the plants in the constant cycle of growth, litter, death.

In addition, approximately 170 kg/ ha per year of N was mineralised and lost, probably through leaching.

Keeping plant cover actively growing, perhaps with irrigation to overcome soil moisture defects, reduces potential losses significantly. But keeping the cycle going while harvesting material (milk, meat, crops, vegetables… whatever) means replenishing the nutrient supplies to cover the removals in the harvest.

The 190 kg/ha N cap therefore has implications in terms of food production and soil organic matter.

Research published in 2016, based on New Zealand systems, is helpful in considering the changes. Lead author Professor Tony Parsons, now retired from Massey University, used a model based on processes (photosynthesis and decomposition, for instance) and factors such as temperature and moisture.

Moving from 150 kg/ ha N input per year to 15 would, over time, halve the amount of food produced per hectare and the amount of carbon in the soil would decrease by approximately 20 tonnes (27%).

Neither the decrease in food production nor the loss of carbon is desirable.

In 2019, AgFirst economists calculated that the knock-on effects of removing synthetic N from the New Zealand primary sector would mean a decrease in gross output by $19.8 billion, a decrease in value add (GDP) by $6.7 billion and a reduction in employment by 73,760.

Of course, there are theories about using organic sources of N, but the way N reacts in the soil (and in response to rainfall) is independent of source.

The research has been done. Just as it has on food production.

A decrease in food production will mean increased food prices. Given the media flurry over inflation, this should be pointed out everywhere. A 4% increase in food prices in the past year, and 4.6% increase in ready-to-eat and restaurant food – both reflecting increased minimum wage and the challenges associated with Covid – appeared to make more of an impact than a 22% increase in fuel prices.

Food is regarded as a right. Efficient food production, with precision agriculture ensuring the genetic potential of animals and crops is achieved, maintains soil organic matter and optimises yields.

Removing N from the system would mean increasing the area under agriculture in order to feed people. This would not be good for biodiversity; the research has been done.

Keeping agricultural land productive and sustainable requires ever advancing technology and uptake.

New Zealand agriculture – industry and farmers – leads the way.

Jacqueline Rowarth is a soil scientist, Adjunct Professor Lincoln University, and farmerelected director for Dairy NZ and Ravensdown. The analysis and presented here are her own. This email address is being protected from spambots. You need JavaScript enabled to view it..