Irrigation management, Nitrogen fertilization and Nitrogen losses in the return flows of La Violada irrigation district (Spain)

Nitrogen (N) pollution induced by irrigated agriculture is a significant environmental problem. The main N inputs and outputs were measured or estimated in the semi-arid La Violada irrigation district (Spain). Data on two periods (1995-98 and 2006-08) were compared and related to observed changes during the decade in cropping patterns and N fertilization and irrigation management. N fertilization exceeded crop N uptake due to over-fertilization of corn (426 kg N/ha in 1995-98 and 332 kg N/ha in 2006-08) and alfalfa (62 kg N/ha). Between the two periods, N fertilization decreased by 56%, primarily due to a change from corn to alfalfa and barley. Accordingly, N losses in the irrigation return flows (IRF) diminished from 31% of the applied fertilizer in 1995-98 to 20% in 2006-08. NO3 concentrations and NO3-N loads in the IRF decreased from 40 mg/L and 106 kg N/ha in 1995-98 to 21 mg/L and 22 kg N/ha in 2006-08, due to lower N fertilization, lower corn area and improved irrigation efficiency. N contamination in the IRF will be minimized by increasing the irrigation efficiency and decreasing the corn area and its N fertilization rates, particularly when supplemental organic N is applied at pre-sowing.