Modelling the fate of nitrogen following pig slurry application on a tropical cropped acid soil on the island of Réunion (France)

A comprehensive field study was conducted to determine the fate of nitrogen in pig (Sus scrofa) slurry applied to an acid tropical andic soil of Re´union with the aim of estimating drainage and nitrogen leaching below the root zone. Water movement and nitrate dynamics were monitored during two successive cropping seasons on a plot (PSP) treated with liquid manure, with an input of 264 kg N ha_1 the first year and 185 kg N ha_1 the second year, in comparison to levels recorded in a unfertilized control plot (CP). The field was cropped with rainfed maize (Zea mays L.) The process-based WAVE fieldscale model was used to simulate water flow and nitrogen transport in the unsaturated zone and highlight the main processes controlling water and N fate. A calibration procedure was performed one year, while the prediction capability of the model was assessed during another cropping year. A sensitivity analysis was performed to address some critical parameters. Due to the high hydraulic conductivities measured in this andic soil, the drainage risk became high when the rain intensity was above 30 mm d_1 and the soil humidity was close to saturation. The time between the first slurry application on PSP and the nitrate onset in the drainage water at 135 cm depth (about 15 months) was attributed to nitrate adsorption on the soil particles (the retardation factor was estimated at 2.6 in the surface layer and 1.5 in deeper layers) and to the fact that the water stored in the 0-135 cm soil layer was slowly displaced. The nitrate migrated in this andic soil at rate of about 50 mm per 100 mm of infiltrated water. The main features of the experimental values of state variables (water content, water pressure head, NO3_ concentration, natural mineralization and nitrification of the pig slurry ammonium at different depths and dates) as well as the water fluxes across boundaries were generally correctly reproduced by WAVE for both plots. The calibrated modelled budget error arising from net mineralization was +15 and +9 kg N ha_1 for CP and PSP, respectively. For the model evaluation, it was estimated at _9 and _13 kg N ha_1, respectively, which was considered as very acceptable. WAVE required refinements in some processes and parameters but was still found to be robust enough to work in conditions for which it was not primarily designed. (Résumé d'auteur