IS LOW ROOTING DENSITY OF FABA BEANS A CAUSE OF HIGH RESIDUAL NITRATECONTENT OF SOIL AT HARVEST

It was the aim of this study was to evaluate the hypothesis that low r ooting density of faba beans is the major reason for the comparable lo w depletion of N-min-nitrogen from the rooted soil volume during the v egetation period. Therefore a simulation study was carried out using d ata from a two-year field experiment with faba beans and the reference crop oats. Since the nitrate dynamics in the soil is closely coupled with the water budget, the model simulated also the water uptake by pl ants, movement and content in the soil applying a numerical solution o f the Richard's equation. The nitrogen budget part of the model includ es calculation of vertical nitrate movement in the soil, mineralisatio n of nitrate from organic matter and nitrate uptake by the crop. Verti cal nitrate movement was simulated with the convection-dispersion equa tion. Mineralisation was computed from a simple first order kinetic ap proach using only one fraction of mineralisable organic matter. Nitrat e uptake was assumed to be determined either by the nitrogen demand of the crop, which was estimated from a logistic growth equation that wa s fitted to measured data of N-accumulation, or by the maximum nitrate transport rate towards the root surface. The latter was computed from a steady state solution of the diffusion - mass flow equation for cyl indrical co-ordinates. For oats the model calculated a maximum nitrate transport rate towards roots that was quite close to the measured N-u ptake of that crop. For faba beans, however, the calculated maximum ni trate transport towards roots was much lower than total N-uptake and l ower than for oats. Consequently, simulated N-min-contents below faba beans were during the growing season about 20-30 kg N ha(-1) higher th an below oats. This difference matches quite close with the observed d ifferences between the two crops. Therefore it was concluded that low nitrate uptake resulting from low rooting density is the main reason f or higher residual nitrate contents below faba beans at harvest time.