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 (1995–1998 and 2006–2008) 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 1995–1998 and 332 kg N/ha in 2006–2008) 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 1995–1998 to 20% in 2006–2008. NO3− concentrations and NO3-N loads in the IRF decreased from 40 mg/L and 106 kg N/ha in 1995–1998 to 21 mg/L and 22 kg N/ha in 2006–2008, 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.

► N balances were performed in La Violada irrigation district in 1995–1996 and 2006–2008.
► Crop pattern changed from corn to alfalfa/barley, decreasing N fertilization by 56%.
► Mean NO3− concentrations in return flows decreased from 40 mg/L to 21 mg/L.
► N loads in return flows were five fold higher in the 1990s.
► 31% (1990s) and 20% (2000s) of the applied fertilizer N was lost in the return flows.