Using stable 15N isotopes to trace nitrogen fertiliser recovery and losses in agroecosystems

Stable isotope tracing is one of the most useful tools we have for measuring flows of elements in biogeochemical systems. Our research uses this technique to determine where nitrogen fertiliser goes after it is applied to a crop. To do this, we label fertilisers like urea with enriched ¹⁵N isotope, and measure where the ¹⁵N is recovered in the crop, soil and residues, or lost from the field in gases, runoff and leaching. The part that isn’t recovered can be inferred as losses of nitrogen. The balance between nitrogen recovery and losses is known as nitrogen use efficiency and is a key indicator of the economic costs and environmental impacts resulting from nitrogen fertiliser use in agriculture.

This VRES project will aim to determine the mid-season dynamics of fertiliser nitrogen in grains crops using ¹⁵N tracing techniques. Many studies measure the overall recovery and losses at the end of the season, but there is little understanding of the temporal dynamics of fertiliser N availability or when losses occur during the growing season.

Within this project, there will be opportunities to prepare samples for analysis by IRMS, follow lab protocols to determine ¹⁵N recovery in soil nitrate and ammonium pools, or measure nitrogen gas emissions from soil. All aspects will include data processing and analysis of ¹⁵N recovery data, and background literature reading. Soil and plant samples are available from ¹⁵N recovery field trials in Australian grains cropping systems collected over the past 3 years.

The outcomes of this research will determine how far into the season fertiliser nitrogen remains available to grains crops, giving a more detailed view of the uptake and loss patterns. This will enable more targeted management strategies to prevent nitrogen losses and improve nitrogen use efficiency.