Denitrification losses from irrigated crops in central Spain

Denitrification losses from irrigated sandy soil in Central Spain were studied through laboratory and field experiments. In both cases, the acetylene inhibition technique was applied. In the laboratory, potential denitrification was estimated by varying soil nitrate and carbon availability as well as oxygen depletion through the soil water content at a constant (25°C) temperature. Potential denitrification was evaluated within a range of 6–12 kg N ha−1 day−1. When extra soil nitrate and carbon contents were not supplied, observed potential denitrification ranged from 2 to 6 kg N ha−1 day−1 depending on soil water content. The effect of temperature was also studied with the aim of calibrating the influence of the day–night cycle on denitrification. A simple experiment was carried out indicating a N2O+N2 emission four times greater in the case of treatment at a constant temperature than for treatment with a fluctuating temperature, but with the same average temperature. Field experiments were carried out for 2 years on two crops with different cycles: oats, grown in winter and spring with a short irrigation period, and maize, an intensively irrigated summer crop. Moreover, three N treatments were tested: two controls with different N levels and one fertilized with urea. Denitrification was periodically estimated, and soil temperature, water and nitrate content simultaneously measured. Daily denitrification rates (kg N ha−1 day−1) ranged from 0 for all treatments to 1.2 for the urea-treated plot. Maximum emission was observed with the urea treatment (40.7 kg N ha−1) during the whole period of the experiment; 10% of N applied with urea fertilizer was estimated to be lost, mainly during the maize period, when high temperatures and soil N and water contents coincide. Finally, a prediction model was evaluated with acceptable results. This makes it possible to evaluate agronomic practices in relation to their effect on denitrification losses.