LOSSES OF NITRATE FROM A SANDY LOAM SOIL UNDER CORN: LYSIMETER EXPERIMENT.

Losses of nitrate from an undisturbed sandy loam soil contained in two monolith lysimeters, L₀ and L₁, (1 m diameter, 1.5 m deep) under corn and irrigation (628 mm) were studied. The crop was fertilized with 0 (L₀) and 800 (L₁) kg N ha^-1. Water draining was collected periodically and analysed for nitrate content during the experimental period (20 March 91–23 March 92).

Mean values of nitrate concentration in the drainage water were 3.1 and 41.2 mg NO₃-N 1^-1 in L₀ and L₁ respectively, the latter much higher than the limit imposed by the EC for potable water (11.3 mg N 1^-1).

Three periods were distinguished during the experiment: i) Crop season, in which the total volume of water drained was 310 and 47 mm respectively in the L₀ and L₁, while the nitrate losses were of the same order in both lysimeters (21 and 17 kg N ha^-1, respectively); ii) Dry season, in which there was no water drainage; and iii) Rainy season, in which the total drained water was 157 and 139 mm, and the nitrate losses 5 and 91 kg N ha^-1 in L₀ and L₁ respectively.

The greater amount of water drained in L₀ is due to the lower crop development (grain yield 4.4 and 21.3 t ha^-1; total nitrogen plant uptake 69 and 400 kg N ha^-1, in L₀ and L₁ respectively).

Total nitrate losses in L₁ account for 13% of the nitrogen fertilizer applied. Most of these losses occur in the rainy season.

At the end of the experimental period a decrease of the soil nitrate content was observed in both lysimeters.