EVALUATING IRRIGATION STRATEGIES FOR LETTUCE BY SIMULATION - 2 - NITROGEN BUDGET

A mechanistic water-flow, solute transport and crop growth model was u sed to compare three irrigation strategies for lettuce in French Medit erranean conditions: (1) the current strategy based on the occurrence of rainfall, (2) a strategy based on a simplified modelling of the soi l water balance and (3) a strategy based on tensiometer readings. Wate r and solute instantaneous fluxes versus time were simulated for two g rowing periods (summer and autumn) over 100 years. The climatic data w ere provided by a weather generator. Leenhardt et al. (1998) compared the three strategies with regard to their effects on water flows in th e soil. The present paper deals with their effects on the nitrogen bal ance and particularly nitrate leaching. The three strategies rank simi larly for all processes of the N cycle. Strategy 1 generates soil wate r content conditions favourable to denitrification and nitrate leachin g and less favourable to mineralisation and nitrification, leading to a low NO(3)(- )availability for the crop, and thus a low nitrate uptak e. Strategy 1 minimizes the water stress but induces the maximum nitro gen stress. At the other extreme is strategy 3, whereas strategy 2 has an intermediate position, but closer, however, to strategy 3 than 1. A direct conclusion would suggest avoiding strategy 1 and choosing str ategy 3. (C) 1998 Elsevier Science B.V.