MODELING THE WATER TRANSPORT AND NITROGEN DYNAMICS IN IRRIGATED SALADCROPS

The use of N fertilizers in agriculture is crucial, and agricultural t echniques need to be implemented that improve significantly N fertiliz er management by reducing downward movements of solutes through the so il. To achieve this, it is essential to develop and test models agains t experimental conditions in order to improve them and to make sure th at they can be applied to a broad range of soil and climatic condition s. A field experiment was carried out in the French department of Gard . The soil was a clay loam (26.7% clay, 44.7% fine and coarse silt, an d 28.6% fine and coarse sand). Salad vegetables (Cichorium endivia, La ctuca sativa) were cultivated during two consecutive periods (spring a nd summer crops). The crops were planted on punched and permeable plas tic mulching bands. The field was irrigated with a sprinkler watering system. Local measurements were made combining a neutron probe, tensio meters, and ceramic porous cups to estimate NO3-N concentrations. The model is one-dimensional and is based on Richards' equation for descri bing saturated-unsaturated water flow in soil. At the soil surface, th e model is designed to handle flux-type or imposed-pressure boundary c onditions. In addition, provision is made in the model, for example, t o account for a mulch plastic sheet that limits evaporation. The model accounts for heat transport by diffusion and by convection, while the modeling of the displacement of nitrate and ammonium in the soil is b ased on the convection-dispersion equation. Nitrate uptake by the crop is modeled assuming Michaelis-Menten kinetics. Nitrogen cycle modelin g accounts for the following major transformations: mineralization of organic matter, nitrification of ammonium, and denitrification. The re sults showed that the overall trend of the water potential in the soil profile was correctly described during the crop seasons. Mineralizati on was high for the spring crop (4.7 kg NO3-N day(-1) ha(-1)), whereas the other sink components, such as root uptake, drainage, and denitri fication, were smaller (1.9, 1.4, and 0.2 kg NO3-N day(-1) ha(-1), res pectively). For the summer crop, intensive denitrification was found i n the soil layer at 0.15-0.90 m (5.7 kg NO3-N day(-1) ha(-1)), while t he mineralization was always an important component (9.2 kg NO3-N day( -1) ha(-1)) and the sink terms were 1.7 and 1.7 kg NO3-N day(-1) ha(-1 ) for root uptake and drainage, respectively. Similar high denitrifica tion rates were found in the literature under intensive irrigated fiel d conditions.