WATER-BALANCE AND NITRATE LEACHING IN AN IRRIGATED MAIZE CROP IN SW SPAIN

During 3 consecutive years (1991-1993) a field experiment was conducte d in an intensively irrigated agricultural soil in SW Spain. The main objective of this study was to determine the water flow and nitrate (N O3) leaching, below the root zone, under an irrigated maize crop and a fter the growing season (bare soil and rainy period). The experiment w as carried out on a furrow-irrigated maize crop at two different nitro gen (N)-fertilization rates, one the highest traditionally used by far mers in the region (about 500 kg N ha(-1) per year) and the other one- third of the former (170 kg N ha(-1) per year). The aim was to obtain data that could be used to propose modifications in N-fertilization to maintain crop yield and to prevent the degradation of the environment , The terms for water balance (crop evapotranspiration, drainage and s oil water storage) and nitrate leaching were determined by intensive f ield monitoring of the soil water content, soil water potential and ex traction of the soil solution by a combination of neutron probe, tensi ometers and ceramic suction cups. Nitrogen uptake by the plant and NO3 -N produced by mineralization were also determined. The results showed that, in terms of water balance, crop evapotranspiration was similar at both N-fertilization rates used. During the irrigation period, drai nage below the root zone was limited. Only in 1992 did the occurrence of rainfall during the early growing period, when the soil was wet fro m previous irrigation, cause considerable drainage. Nitrate leaching d uring the whole experimental period amounted to 150 and 43 kg ha(-1) i n the treatments with high and low N-fertilization, respectively. This occurred mainly during the bare soil and rainy periods, except in 199 2 when considerable nitrate leaching was observed during the crop seas on due to the high drainage. Nitrate leaching was not so high during t he bare soil period as might have been expected because of the drought during the experimental period. A reduction of N-fertilization thus s trongly decreased nitrate leaching without decreasing yield.