Application of the STICS crop model to predict nitrogen availability and nitrate transport in a tropical acid soil cropped with maize

Oxisols have a high likelihood of NO3− leaching which may strongly reduce N availability for tropical crops. The aim of this work was to evaluate the N and the water submodels of the STICS crop model for its ability to estimate N availability in N-fertilised field maize crops on two oxisols in Guadeloupe (French West Indies) with and without Al toxicity: a non-limed plot (NLI, pHKCl 3.9, 2.1 cmol Al3+ kg−1), and a limed plot (LI, pHKCl 4.5, 0 cmol Al3+ kg−1). An uncropped plot (UC, pHKCl 4.5, 0 cmol Al3+ kg−1) was used in order to fit some model parameters for soil evaporation, nitrification and NO3− transport. The model was modified in order to describe nitrification as a partially inhibited process in acid soils, and to take into account NO3− retention in oxisols. Nitrification was described as the result of the multiplicative effects of soil acidity, temperature and soil water content. Soil moisture and NO3− and NH4+ content up to 0.8 m soil depth, above-ground biomass and N uptake by crops, and their leaf area index (LAI), were measured from sowing to the beginning of grain filling. The model described correctly the changes in soil water content during the moist and the dry periods of the experiment, and there was some evidence that capillary rise occurred in the dry period. Nitrogen mineralization, nitrification in UC, NO3− transport and plant uptake were satisfactorily simulated by the model. Because of the effect of Al toxicity on plant growth, LAI at flowering was three times higher in LI than in NLI. Some discrepancies between observed and simulated data were found for the distribution of NO3− and NH4+ in the cropped plots. This was probably due to the change of the ionic N form absorbed by the crops as a function of soil acidity and available P in the soil. No leaching was observed below 0.8 m depth and this was associated with NO3− retention in the soil. The results showed that partial inhibition of nitrification and NO3− retention should be taken into account by crop models to obtain realistic estimates of N availability for plants in tropical acid soils.