N-fertilising rainfed wheat and barley to intensify cereal production in the Mediterranean basin

In the Mediterranean basin, water scarcity is the most critical yield-limiting factor. Nitrogen fertilisation has been proposed as a tool to increase yield by enhancing WUE. However, farmers are still reluctant to fertilise rainfed cereals because they expect no responses, or even yield penalties, in most years. This assumption is rooted on the Liebig's 'law of the minimum'. However, the concept of this 'law' has been challenged from empirical and theoretical grounds. Therefore, we tested in a multi-location, multi-year study in four different areas of the Mediterranean Basin whether durum wheat and barley may respond positively to nitrogen fertilisation when grown in rainfed conditions and, therefore, whether fertilisation in water-stressed crops may become a tool for intensifying cereal production. The study involved 16 experiments, including different genotypes of durum wheat and barley sown at Morocco, Jordan, North-eastern Spain, and Southern Italy, with different nitrogen doses applied at sowing or early in crop development over four growing seasons. Averaging across all conditions (years-locations-species) yield increased 26% when fertilised. Most importantly, the average did not mask a cross-over response and, therefore, even under the lowest-yielding conditions (cases most limited by soil water) there was a generalised positive response to fertilisation (and naturally the response was greater under less water-limited conditions). Grain number per unit area, and not grain weight, was the main component related to grain yield. In addition, barley and durum wheat responded similarly to nitrogen fertilisation, against the widespread belief that barley responds less.