Comparison of Urea and Ammonium Nitrate in Long-Term Trials: Synthesis of Ten Years of Experimentation

Keywords: Nitrogen uptake, nitrogen use efficiency, long-term experiments, ammonium nitrate, urea.

From 2002 to 2012, a network of 12 long-term experiments, comparing two nitrogen (N) fertilisers – urea and ammonium nitrate (AN) – were carried out by several partners in a project framework co-ordinated by UNIFA in France. In addition to the traditional comparison of the response to nitrogen fertilisation within single-year experiments (annual effect), the originality of the trial design made it possible to demonstrate the existence of a past effect, depending on which kind of fertiliser has been applied the previous year, and of a ‘system effect’ which is related to the repeated use of the same nitrogen fertiliser each year.

The variability of response to nitrogen fertilisers was high between the trials, with a very important effect of the location and the annual climate conditions. The multiplication of trial sites in terms of location and years, and the convergence of the results made it nevertheless possible to quantify the differences in efficiency between the two fertiliser forms tested. However, as shown in other published studies, these average differences masked an undoubted variability, and did not allow building a statistical model to a priori predict the effect of fertiliser form on the nitrogen use efficiency (NUE). Overall, the main conclusions resulting from this network are as follows:

1. an annual positive effect on crop yields for ammonium nitrate compared to urea, with an average difference of 2%***, which corresponded to a yield difference of 1.2**, 1.4** and 1.3* dt/ha for rapeseed, wheat and barley respectively (crops fertilised at the provisional balanced rate);

2. a positive past effect of fertilisation on yield, in the same order of magnitude as the annual effect of nearly 2%** in favour of ammonium nitrate;

3. a ‘system’ effect on yield, combining the annual effect and the past effect, of 4.1%***which corresponds to a difference of 2.4*** 2.7*** and 3.1** dt/ha for rapeseed, wheat and barley respectively (crops fertilised at the provisional balanced rate). This ‘system’ effect was also significant on the crop nitrogen uptake. For crops fertilised each year with ammonium nitrate, the nitrogen uptake was increased by 27,** 19*** and 15*** kg N/ha for rapeseed, wheat and barley, respectively. The NUE calculated reached 91% with ammonium nitrate compared to 84% with urea.

The lower amount of nitrogen absorbed following urea fertilisation may be related to a higher risk of ammonia volatilisation after spreading. The increased amount of nitrogen uptake when fertilised with ammonium nitrate may increase the nitrogen content in roots and crop residues that are partly mineralised during the next crop cycle.

P. Eveillard¹, M. Lambert², M. Herve³, A. Bouthier⁴, L. Champolivier⁵, S. Marquis⁶, C. Rocca⁷ and D. Roussel⁸.

¹ UNIFA French Fertiliser Industry Association,

² Yara France,

³ Eurochem Agro,

⁴ Arvalis-Institut du Vegetal Applied Research Institute for Cereals,

⁵ CETIOM Applied Research Institute for Oil and Protein Crops,

⁶ Borealis-L.A.T.,

⁷ InVivo Group of French Cooperatives,

⁸ OCI Agro.

19 pages, 7 figures, 2 tables, 13 references.