Determining the contribution of nitrogen fertilizer and mineralization to volatilized ammonia through the use of nitrogen-15

Ammonia volatilization (AV) is one of the main pathways of nitrogen fertilizer loss, resulting in reduced crop yields, and a negative impact on the environment. Therefore, reducing AV through proper fertilizer management is essential. We can, however, only provide appropriate management advice when based on accurate measurements, along with understanding the processes involved. For this purpose, the ¹⁵N technique has a unique advantage over other methods to precisely identify the sources of ammonia production.

A field experiment was established at the SWMCN laboratory in Seibersdorf on maize with four replications and 120 kg N ha^-1 was applied through two equal split applications at 20 DAP (Days After Planting) and 34 DAP. Two ¹⁵N microplots inside each main plot were installed. In these microplots, ¹⁵N-labeled urea replaced the unlabeled urea according to the time of fertilizer application. Each microplot for ¹⁵N-labelled urea was 2.5 m by 2.5 m,and the buffer zone between microplots was 1 m to minimize ¹⁵N contamination from adjacent microplot. For these microplots, ¹⁵N-labeled urea was used with an enrichment of 5.23 atom% ¹⁵N excess. The first microplot received ¹⁵N-urea at 20 DAP and unlabeled urea at 34 DAP, the second microplot received ¹⁵N-urea at 34 DAP and unlabeled urea at 20 DAP. Ammonia volatilization was measured with semi-static chambers and chambers were installed inside the ¹⁵N microplots.

The total cumulative NH₃ emissions from urea after the first and second split applications were 13.9 kg N ha^-1 and 18.0 kg N ha^-1, respectively. This calculation is based on the difference in AV between experimental treatments and control treatment, assuming that AV in control plots indicates the amount of AV from the soil source, whereas AV of the fertilized treatments presents AV from soil and fertilizer sources. It also assumes that all nitrogen transformations, i.e., mineralization, immobilization, and other process in the case of nitrogen, are the same for control and experimental plots. Therefore, the amount of AV in urea treatment was subtracted from the amount of AV in control treatment. The cumulative NH₃ emissions from the control treatment (without nitrogen fertilizer) at the same time were 2.7 kg N ha^-1 and 3.6 kg N ha^-1, respectively. Accordingly, about 20% of the ammonia volatilized from the soil source and the rest could be attributed to the added urea fertilizer.

However, using the ¹⁵N labelled fertilizer, it was found that the above assumption shows some flaws. The fraction of nitrogen in the ammonia samples derived from the soil is not constant but changes significantly due to nitrogen fertilizer application. The results show that the nitrogen in the ammonia derived from the fertilizer was 65% and 53% after the first and second split applications, respectively. Therefore, the fraction of nitrogen in the ammonia samples derived from the soil source was 35% and 47% after the first and second split applications. So, the use of the ¹⁵N technique shows that adding nitrogen fertilizer likely increased the rate of mineralization by changing the ratio of carbon to nitrogen.