Do you know the fate of nitrogen (N) fertiliser that’s applied to cropping paddocks? How much ends up in the crop, soil or lost? Victorian researchers have been working with a traceable N fertiliser to find out. The standard assumption is the crop takes up half the fertiliser N in the year it’s applied. Crop uptake across a range of farming systems showed closer to one third in the plants and grain.
Tracking the fate of nitrogen
Several field trials have used N fertiliser with a specific amount of the isotope N-15. Researchers can track where the fertiliser ends up – in the straw, grain or soil. N-15 occurs naturally in soil-crop systems as 0.36% of total N. N-15 levels higher than background levels show the fate of the fertiliser.
How much nitrogen could be recovered?
Soil, grain and straw analysis showed where the N fertiliser ended up. Not all the applied N was recoverable in any of the N15 trials. The amount varied between trial sites and application methods in one trial (see figure). In another project, 71% of fertiliser N was recovered from the crop and soil.
Caption: The recovery of fertiliser N in the grain, straw and soil. The difference between total recovered and applied represents N losses from the system. Source: Mid-row banding nitrogen fertiliser in-season.
Some of the applied N couldn’t be recovered from grain, straw or soil. We presume this N moves out of the cropping system. That’s a cost to growers and the environment. It doesn’t look like the losses happen through leaching. Measuring N15 through the profile didn’t show much downward movement of N.
How much fertiliser nitrogen is in the crop?
Most of the recoverable fertiliser-N ended up in grain. More fertiliser-N in grain and straw meant less remained in the soil. Average uptake of fertiliser N by the crop was low. Growing season rainfall influenced uptake more than soil N status. There were differences N in fertiliser uptake between farming systems:
- 42% in irrigated
- 32% in low/medium rainfall
- 30% cent in high rainfall.
In the mid-row banding trials site and application methods also made a difference to N uptake.
What affects the fate of nitrogen?
Application method caused the most variation in fertiliser fate in Victoria. Placing N fertiliser in the soil (compared to topdressing) will reduce losses via volatilisation. With mid-row banding (MRB), crops took up about 20% more N compared to surface methods. MRB showed the most fertiliser-N ending up in the grain.’
Timing of application affected fertiliser-N uptake. For MRB, August applications followed by low rainfall produced the greatest grain uptake. Banding N below the soil surface might reduce volatilisation in low rainfall conditions.
Researchers recovered less N fertiliser as soil moisture increased above optimal. Between 2014 and 2016, average N recovery from soil + crop was 78%, 75% and 63% in low, medium and high rainfall zones. Note the lowest recovery was in the high rainfall zone. With waterlogging, denitrification can cause significant N losses.
What can you do?
The best strategies to reduce fertiliser costs and N losses are to:
- Predict soil N status with deep soil testing before sowing
- Match N applications to crop demand.