Simulation of nitrogen dynamics in an alluvial sandy soil with drip fertigation of apple trees

In order to optimize the management of the N-fertilizer inputs with drip fe rtigation on sandy-silt soil under apple tree orchard cultivation, we obser ved is situ: (i) the N and water soil transfers, (ii) the N levels in all l eaves, fruits and annual shoots, and (iii) the root distribution. Then we u sed a mechanistic one-dimensional model (WAVE,Vanclooster ed al, 1994) to q uantify the annual parameters of the water and nitrogen balance on a daily basis. The horizontal heterogeneity along the row of the tree-soil-dripper system has been treated with two adjacent compartments: one under the dripp er and receiving fertigation and the other outside this zone. N transfers i n the tree make it impossible to estimate directly N uptake by roots over t ime. The simulated N losses were due to equal amounts of N leaching below 0.9 m deep (9 g N tree(-1)year(-1) and denitrification (7 g N tree(-1)year(-1). T he simulated losses of gaseous N were localized predominantly in the compar tment under the dripper and showed a higher rate of leaching during the per iod of N input when the wet conditions and the high NO3- concentrations wer e favourable to denitrification. The N-leaching at 0.9 m depth was greatest outside the growing season and was caused by the extension of the N-inputs after the harvest date. This practice, based on the objective to store nit rogen before the period of dormancy does not seem to be justified.