Understanding how nitrogen management affects climate and water quality
Use of nitrogen in agriculture can impact both water quality and quality of climate and air. Delegates at the Farming for a Better Future Open Day, taking place in Teagasc, Johnstown Castle, today, June 10 heard that awareness of loss pathways is essential to reduce these losses and increase nitrogen use efficiency.
Through a paper produced for the event, Dominika Krol, Owen Fenton, Cathal Somers and Mark Plunkett reminded detailed nitrogen management as specifically how nitrogen is lost to both the air and water.
How Nitrogen enters farmland
Nitrogen enters farmland from organic and inorganic sources: urine patches from grazing animals, slurry, synthetic fertilisers and mineralised soil organic matter after ploughing. In soil it occurs in many forms, but plant-available inorganic nitrogen is mainly ammonium and nitrate. These can be lost to the environment as nitrate leaching to groundwater, ammonia emissions to air, and nitrous oxide, a potent greenhouse gas. Nitrate is the mobile form that percolates vertically to groundwater, then to surface waters and estuaries, linking field management to water quality.
Most susceptible soils to leaching
Free- and moderately-draining soils are most susceptible to nitrate leaching. Leaching requires two simultaneous conditions: nitrate present in the root zone and downward drainage. In pasture systems, urine patches are the dominant source of nitrogen loss: cows excrete roughly 70–95% of ingested nitrogen, mostly in urine, and single patches can contain 200–1,000 kg Nitrogen/ha— far exceeding plant uptake and creating hotspots for leaching and gaseous losses.
Leaching risk is highest when urine falls on free-draining soils shortly before rain; nitrous oxide losses are largest on wet, poorly-draining soils. These risks peak in late autumn and early winter due to high drainage and low plant uptake.
In the below video, Dominika Krol tells us more:
Losses from synthetic fertilisers
Synthetic fertilisers vary in loss profiles. Calcium ammonium nitrate (CAN) contains roughly equal ammonium and nitrate; nitrate is readily lost with drainage while ammonium is more soil stable. Protected urea converts to ammonium in soil, reducing leaching risk and ammonia emissions compared with unprotected urea; nitrous oxide emissions are generally higher from CAN than from protected urea. Nitrogen released after ploughing also poses leaching risk, so tillage timing and rapid re-establishment of plant cover matter.
Learnings from research
Mitigation research includes reducing dietary crude protein via low-N supplements in autumn and winter to lower urinary N concentrations, though increasing stocking rates can negate benefits. Housing or off-paddock systems let farmers collect and store excreta for application when soils are drier and plants demand N, reducing high-risk losses.
Key take home messages:
- Know your farm landscape and how your soils react to nitrogen inputs
- Be aware how time of year and weather conditions can affect losses associated with nitrogen management