CATION AND NITRATE LEACHING IN AN OXISOL OF THE BRAZILIAN AMAZON

High rates of N fertilizers are often necessary to achieve yield goals in the humid tropics, where subsoil acidity prevents deep crop rootin g. However, leaching of fertilizer nitrate may accelerate the leaching of bases from the crop rooting zone, leading to an acidification of t he topsoil and a reduction in crop yields. Our ojective was to investi gate the influence of urea and legume green manure sources of N on cro p yields, leaching of cations, and the fertility of the plow layer of a clayey Oxisol (Typic Acrudox) of the central Amazon basin. We establ ished a split-plot field experiment near Manaus, Brazil where main plo ts received 2 levels of lime (0 and 4 Mt/ha CaCO3) and sub-plots were cropped with (i) a legume green manure (Canavalia ensiformes L. or Muc una aterrima L.) followed by maize (Zea mays L.); (ii) maize receiving 300 kg ha-1 of urea-N, or (iii) left bare-fallow with an application of 300 kg ha-1 of urea-N. Plots were periodically sampled to 1.2 m dur ing three cropping seasons. The field site received 4265 mm of rain du ring the experiment (16 mo). Legume crops accumulated between 142 and 280 kg ha-1 of N. The distribution of NO3 in the soil profile changed in a pattern consistent with leaching. All treatments lost Ca and Mg f rom the plow layer during the experimental period. Losses were greates t (500-1000 kg ha-1 for Ca and 50 kg ha-1 for Mg) in plots treated wit h urea and lime. Leaching of bases and the generation of acidity decre ased base saturation in the plow layer of all treatments, but was mini mized in plots receiving legume green manure N, perhaps because less i norganic N was applied and/or the legume crops recycled leached bases. Unlimed plots receiving urea, had the highest increase in acidity in the 0 to 30-cm layer and a corresponding 44% reduction in grain yield between the first and third maize crops.