Nitrogen use efficiency in cereals production under Mediterranean conditions

Abstract

Nitrogen use efficiency in cereals production under Mediterranean conditions Mário carvalho, Universidade de Évora, Instituto de Ciências Agrárias e Ambientais Mediterrânicas (ICAAM), Portugal. mjc@uevora.pt

The efficient use of nitrogen for winter sown cereals is particularly difficult under Mediterranean conditions, due to climatic and edaphic constraints. Cereals are usually sown in November. Tillering and spikelets differentiation takes place during December and January, so nitrogen has to be available for the crop at that time. However, an excess of precipitation over evaporation in winter increases N loss, owing to leaching and volatilization. The total amount of N to be applied depends on rainfall, particularly that falling between November and February, as it influences both yield potential of the crop and N loss. However, the considerable climatic variability of the region acts as a second constraint to nitrogen use efficiency (NUE). Soils of the region commonly contain only 1% organic matter (SOM). Consequently, availability of N to a crop is dependent on the application of mineral N. To respond to climatic variability the management of N fertilizer application has to be flexible and appropriate to the existing rainfall pattern. The classic approach of using a yield goal and an estimation of N losses can lead to gross errors. We developed a model relating rainfall, expected yield and N required to adjust the amount of N fertilizer applied at first and second top dressing depending on N losses for the season. This model can reduce mistakes with N fertilizer application and contribute to enhanced NUE. The contribution of ammonium fertilizer together with a nitrification inhibitor (3,4-dimethyl pyrazole phosphate - DMPP) was assessed. The results indicate no benefits over splitting the same amount of N into two applications. The benefits of using legumes in the crop rotation were investigated. The total amount of soil mineral N at seeding for wheat is greater if the preceding crop is a legume rather than sunflower, although forage legumes (hay) leads to a greater increase than grain legumes. However, the N recovered by the cereal was at the most 24 kg N/ha after the forage legumes - a very small fraction of the N left in the soil by the legume. The effect on NUE for winter cereals of increasing SOM was also investigated in a long term study. Increasing SOM from 1 to 3% raised NUE of the applied N from 20 to ~100 kg of wheat per kg of applied N.