INTERNAL DRAINAGE AND NITRATE LEACHING IN A TYPIC HAPLUDOX UNDER A MAIZE-BRACHIARIA-MAIZE SUCCESSION AT DIFFERENT NITROGEN FERTILIZER RATES

The ideal N fertilization should be defined to satisfy the crop needs, but with minimum environmental risk. For this purpose, N fertilizer recommendations must be as accurate as possible. This study was carried out in a sand-clay soil (22 degrees 42 ' 30 '' S; 47 degrees 38 ' 00 '' W; 546 m asl) and the objective was to evaluate the internal drainage and nitrate leaching during the cycles of three crops in succession under no-tillage at a soil depth of 0.80 m, using different doses of N-15-labeled ammonium sulphate. The crop succession consisted of: maize in 2006 followed by brachiaria plus fallow and finally maize in 2007. Labeled N fertilizer rates of 60, 120 and 180 kg ha(-1) and a control (no N) were tested. The labeled fertilizer was applied to previously defined subplots, only to maize in 2006. Internal drainage was derived from the soil-water flux density calculated by the Darcy-Buckingham equation in which the hydraulic conductivity and total water potential gradient were estimated based on daily readings of Hg tensiometers at depths of 0.70, 0.80 and 0.90 m. The hydraulic conductivity was determined in the field as a function of soil-water matric potential at a depth of 0.80 m, by the instantaneous profile method, using tensiometers and soil-water retention curves. The soil-water flux density was also used, together with nitrate concentration and the N-15 atom % in soil solution, to estimate nitrate leaching of both total and fertilizer nitrate. Internal drainage decreased with the increase of applied N levels to the crop succession, changing from 31.5 to 73.4 % of the total rainfall (97 mm) in the first maize, from 26.1 to 58.1 % of the total rainfall (695 mm) in brachiaria + fallow, and from 56.6 to 87.4 % of the 419 mm of total rainfall in the second maize crop. The leaching of total nitrate (from fertilizer and other sources) was very low in the first maize crop for all applied N levels and significant for the rates of 120 and 180 kg ha(-1) in the periods of brachiaria plus fallow (26.16 for 120 kg ha(-1) and 39.8 for 180 kg ha(-1)) and of the second maize crop (approximately 23 kg ha(-1) for both levels). There was no N leaching from fertilizer in the first maize crop and N leaching was very low in the brachiaria and second maize crop.