Page 38 - Green knowledge 2025

Photo: Morten Günther, edited.
Effective measures against nitrogen runoff
Balancing fertiliser use, sowing catch crops and avoiding autumn tillage are among the
most important measures farmers can adopt to reduce nitrogen losses to watercourses. For
the Oslo Fjord, which receives more nitrogen than it can tolerate, such steps could make
a substantial difference.
The Oslo Fjord faces serious environmental chal-
lenges, due to excess nitrogen and phosphorus from
agriculture, wastewater and industry. Nutrient losses
from agricultural areas is one of the largest sources.
“First and foremost, it is about practising good
agronomy and managing nitrogen resources prop-
erly,” says Dr Marianne Bechmann.
She explains that phosphorus follows soil particles
during erosion, while nitrogen in the form of nitrate
is water-soluble and is mainly lost through drainage
systems – regardless of erosion risk.
A fertiliser surplus relative to the crop yield increases
the risk of leaching. Therefore, split fertiliser applica-
tions adjusted according to growing conditions is an
important measure.
“GPS-controlled precision fertilisation also provides
better distribution and less surplus,” says Dr Bech-
mann.
Catch crops such as ryegrass absorb nitrogen after
harvest. The effect depends on avoiding soil tillage in
autumn. Ley in the crop rotation and buffer zones
have a similar effect. Overwintering in stubble can
reduce nitrogen losses by 20–40 per cent.
Early autumn ploughing combined with mild
autumn conditions result in a long period of nitrogen
mineralisation and increased losses. If ploughing is
necessary, it should be done as late as possible.
In Denmark, wetlands and treatment systems for
drainage water are being tested.
“Preliminary results are promising, but we need
more knowledge about Norwegian conditions,” she
says.
Nitrogen is a valuable resource. With the right meas-
ures, we can balance food production and environ-
mental considerations – and secure a healthier
future for the Oslo Fjord.
Purpose: Contact: Provide an overview of measures that can help reduce nitrogen runoff from agricultural land.
Research Professor Marianne Bechmann, Division of Environment and Natural Resources.
Email: marianne.bechmann@nibio.no | Phone: +47 412 19 506.
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