Nitrogen management in double-annual cropping system (barley-maize) under irrigated Mediterranean environments

Improving nitrogen (N) use efficiency (NUE) is an agricultural necessity, as it can contribute to increasing crop productivity, while decreasing environmental degradation. Double-annual cropping systems can reduce N losses while increasing productivity and profitability per land unit. The objective of this study was to evaluate annual N fertilization strategies in a double-annual cropping system (barley-maize) under irrigated Mediterranean environments. A three-year N fertilization experiment (2013-2016) was implemented with combinations of different N rates applied to barley (0 and 100 kg N ha(-1)) and maize (0, 100, 200 and 300 kg N ha(-1)). Grain and biomass yields and N content, plant N uptake, residual N, N efficiency, soil organic carbon (SOC) and economic return (ER) of the double-annual barley-maize cropping system were determined. The optimum rate of N to achieve maximum yields in the global barley-maize system was 230-240 kg N ha(-1), split between barley and maize. Potential annual yields of the double-annual barley-maize cropping system under irrigated Mediterranean environments could be as high as 20 Mg ha(-1) yr(-1) for grain (6.71 and 13.42 Mg ha(-1) for barley and maize, respectively) and 35 Mg ha(-1) yr(-1) for biomass. The long growing period of the double-cropping systems contributed to promote high recovery of the post-harvest residual N. Barley yield increased with high residual N of the maize, whereas maize yield was not affected by the residual N of the barley. Non-N fertilized barley achieved up to 5.79 Mg ha(-1) when 300 kg N ha(-1) were applied to the previous maize. After three years of the study, SOC did not change in any of the N treatments, even in the treatments with the highest N deficiency (0 kg N ha(-1) yr(-1) applied). Further research is needed to fine-tune the N fertilization strategy over long-term periods for the double-annual barley-maize cropping system.