Modeling the water transport and nitrogen dynamics in irrigated salad crops

The use of N fertilizers in agriculture is crucial, and agricultural techniques need to be implemented that improve significantly N fertilizer management by reducing downward movements of solutes through the soil. To achieve this, it is essential to develop and test models against experimental conditions in order to improve them and to make sure that they can be applied to a broad range of soil and climatic conditions. 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, and 28.6% fine and coarse sand). Salad vegetables (Cichorium endivia, Lactuca sativa) were cultivated during two consecutive periods (spring and summer crops). The crops were planted on punched and permeable plastic mulching bands. The field was irrigated with a sprinkler watering system. Local measurements were made combining a neutron probe, tensiometers, and ceramic porous cups to estimate NO3-N concentrations. The model is one-dimensional and is based on Richards' equation for describing saturated-unsaturated water flow in soil. At the soil surface, the model is designed to handle flux-type or imposed-pressure boundary conditions. In addition, provision is made in the model, for example, to 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 based on the convection-dispersion equation. Nitrate uptake by the crop is modeled assuming Michaelis-Menten kinetics. Nitrogen cycle modeling accounts for the following major transformations: mineralization of organic matter, nitrification of ammonium, and denitrification. The results showed that the overall trend of the water potential in the soil profile was correctly described during the crop seasons. Mineralization 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 denitrification, were smaller (1.9, 1.4, and 0.2 kg NO3-N day–1 ha–1, respectively). For the summer crop, intensive denitrification was found in the soil layer at 0.15–0.90 m (5.7 kg NO3-N day–1 ha–1), while the 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 denitrification rates were found in the literature under intensive irrigated field conditions.