The use of nitrogen in a sprinkler-irrigated district in Monegros (Northeast Spain)

• Hydrological properties determine the flow and NO3− concentration in drainage waters.
• Flow regulation by the aquifer leads to a constant NO3− concentration in drainage.
• Basins on less permeable Tertiary materials export 12 kg N ha−1 year−1 by drain water.
• The more permeable Quaternary terraces export 64 kg N ha−1 year−1 by drainage water.
• 26% of N fertilizer in the studied sprinkler irrigated area is lost by drainage water.

Irrigation is essential for agricultural production in semi-arid areas, but it is also an important consumer and contaminator of water. Therefore, it is necessary to improve the efficiency of water and nitrogen use. This paper analyzes the relation between nitrogen fertilization practices, irrigation and the nitrate concentration in the drainage waters of the sprinkler-irrigated LASESA district (LD) in Northeast Spain. The methodology is based on developing water and nitrogen balances in the seven main basins that drain LD (9980 ha) from November 2008 to October 2010, covering the agricultural campaigns of 2009 and 2010. The results show that the hydrological properties of each basin determine the evolution of the flow of and nitrate concentration in drainage waters. In basins with soils located on Quaternary terraces, the drainage fraction (DF) and the concentration of nitrate are 22% and 120 mg L−1, respectively, and the annual mass of nitrogen exported is 64 kg ha−1. In those with soils located on Tertiary marls and sandstones, the lower DF and nitrate concentrations (9% and 90 mg L−1) lead to an annual mass of exported nitrogen which is five times lower (12 kg ha−1). The excess of irrigation and fertilizer use in corn make this crop the main source of nitrogen pollution. In recent years, corn fertilization has been reduced. Nevertheless, 26% of the nitrogen applied as mineral or organic fertilizer is still lost in the drainage water.