Nitrogen use efficiency

Nitrogen use efficiency (NUE) refers to the proportion of nitrogen applied to the soil — whether from mineral fertilisers, livestock slurries or organic amendments — that is actually absorbed by the crop and contributes to its growth. This agronomic indicator is generally expressed as a percentage of total nitrogen applied. It differs from apparent efficiency, which merely compares the yield difference between a fertilised plot and an unfertilised control. Low efficiency signals substantial losses to the atmosphere or water bodies, whereas a high value indicates good alignment between supply and plant requirements.

Agronomic determinants

The level of efficiency depends first and foremost on the nitrogen source and cropping practices. Mineral fertilisers typically show higher recovery rates than organic materials: the ammoniacal nitrogen and nitric nitrogen from synthetic fertilisers enable more rapid uptake by roots. Conversely, liquid manures (slurries) and farmyard manures exhibit lower efficiencies owing to ammonia volatilisation and the slow mineralisation of organic nitrogen compounds^[1].

Several levers can improve this recovery. Matching application rates to the yield potential of the crop is the foremost. It is also necessary to ensure uniform plant establishment and optimal plant density, then to correct any deficiencies in other nutrients to guarantee balanced nutrition. Splitting applications into several passes — in increments of 25 to 40 kg N/ha — proves particularly beneficial on coarse-textured soils and for long-cycle varieties. The number of splits should be increased accordingly^[2].

Comparison of nitrogen fertiliser forms

Nitrate fertilisers stand out for their superior efficiency compared with urea-based products. According to trials conducted on cereals in the United Kingdom, maintaining equivalent yields required a markedly greater quantity of nitrogen with urea than with ammonium nitrate. At identical nitrogen dose, grain protein content and overall yield were significantly lower with urea^[3]. This difference is explained by the volatilisation losses inherent to urea, which necessitate compensatory applications.

Nitrate-containing fertilisers — defined as those with at least 50% of their nitrogen in nitric form — also offer better control of nitrate leaching and a lower carbon footprint over the life cycle than urea-based products^[4][5]. Calcium ammonium nitrate (CAN), ammonium nitrate (AN) and nitrate-based NPK fertilisers fall into this category, whereas urea, ammonium sulphate and UAN solutions are excluded or only partially included.

Environmental challenges and evolving practices

Nitrogen use efficiency and environmental losses vary inversely. Poor recovery by the crop results in nitrous oxide (N₂O) emissions to the atmosphere and nitrate discharges to surface or groundwater^[2]. The EU Nitrates Directive precisely regulates these risks by imposing spreading prohibition periods and nitrogen fertilisation ceilings in vulnerable zones.

Integrating nitrogen within overall water management and implementing balanced fertilisation — accounting for all nutrients — form the pillars of more efficient agriculture^[2]. The adoption of nitrification inhibitors or urease inhibitors, the use of controlled-release fertilisers and the development of green and blue ammonia illustrate current pathways for progress. On the industrial side, reducing energy consumption in ammonia manufacture via the Haber-Bosch process has achieved substantial gains over the past century, although technical progress is now less decisive than energy prices for the competitiveness of production units^[6][7].