Site-specific effects of variable water supply and nitrogen fertilisation on winter wheat

The plant-available soil water, amount and distribution of rainfall or irrigation are primary factors that may affect yield and quality of winter wheat in heterogeneous fields. The objective of this 2-y study was to vary N application and water supply in order to achieve a more mechanistic insight into the effects of underlying differences in the site-specific productivity on heterogeneous fields. Two N fertilizer rates (120 and 180kg N ha^-1) and three different water supply treatments (rain sheltering, irrigation, rain-fed) were compared on field sites with lower or higher plant available soil water capacities. On the whole, the site, rather than rainfall or N fertilisation, was the primary factor that accounted for variability in grain yield. Rainfall distribution during the growing season affected the overall yield level in a given year. The sites characterised by lower plant available water capacity did not show higher grain yield and improved quality with the increased N rate. This suggests that the reduced N rate should be recommended on these sites to take into account the environmental sustainability of N fertilisation. With respect to the higher N application at sites of high plant available soil water capacity, although the already high yield levels were not increased further, the protein quality was significantly improved in the first season within all treatments and in the second season in the irrigated treatments. Therefore, a higher N-rate proved to be advantageous, especially considering that the residual nitrate levels after harvest were low. The study demonstrated that the response of winter wheat to water shortage or abundance and N fertilisation is site-specific and dependent on the availability of soil water.