Deep drainage nitrogen losses from banana cropping
The Digital Agronomy Insights team tackle questions relating to the story of nitrogen to help farmers and their advisors identify management practices that maintain productivity and reduce the risk of losing nitrogen off farm.
In our work on the Cassowary Coast of Queensland we have used sensors to measure climate and soil conditions to measure nitrogen loss in deep drainage below the banana root zone. Below we show some of that data and explain when and how nitrogen can be lost.
Nitrogen in cropping systems
Nitrogen is dynamic, that is it can exist in many different ‘forms’ or types. For example, it is important for nitrogen to be in either the nitrate or ammonium form for crop uptake.
Nitrogen is often applied in the urea form. Once urea is in the soil, the nitrogen quickly changes into the nitrate form. Not only is nitrate important for crop uptake, but nitrate is highly soluble in water – that is, nitrate is dissolved in the soil water.
The implication of nitrogen being in the nitrate form is that when the water moves in the soil, nitrate moves with it. Nitrate moves with soil water when the plant takes up water, and nitrate will move with water when water drains from the soil.
Nitrogen deep drainage – heavy rainfall
In the chart left we can see a period of time when there was heavy rainfall at our East Palmerston experimental site in December 2023 associated with Cyclone Jasper.
Between the 12th and 18th December 995 mm of rain fell.
This was the first significant rainfall event of the 2023/24 ‘wet season’. Up until this time the crop had been fully irrigated and fertigated regularly with nitrogen.
Nitrogen deep drainage – evidence from sensors
Monitoring soil moisture and soil Electrical Conductivity (EC) at 120 cm depth with soil sensors allows us to observe when water and nutrients are passing below the root zone.
Soil EC can be used as a proxy for nutrients, as soil EC is influenced by water and salts such as nitrate.
We can see to the right the soil moisture at 120 cm rose rapidly during the heavy rainfall event (starting around December 13), remaining high for the duration of the rain, and then dropped again after the event – indicating a well drained soil.
When water is detected at the 120 cm depth there is a rapid rise in soil EC too. The EC values reduce before the sensed soil moisture values, indicating there are nutrients in the first flush of water moving past the 120 cm deep sensor. Nitrogen deep drainage occurs with the first flush of water moving through the soil profile.
A detailed explanation of how nitrogen deep drainage occurs can be found in our video insights.
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