Abstract

In acids soils, the use of ammonium fertilizers accelerates the acidification processes and decreases both the production and persistence of the pastures. Nitrogen (N) is the most limiting factor for plant growth in most agricultural systems, but also it is one of the major environmental issues worldwide. In Chile, neither systematic studies in the first period of pasture growth and nor N leaching losses associated have been reported. The aims of this study were: (i) to evaluate the effect of the N sources at increasing rates of application on the yield and quality of ryegrass (Lolium perenne) and (ii) to quantify in the short-term the N potential leaching losses and the pH changes throughout the soil profile under field conditions. A field experiment was conducted during the spring-summer season 2000-2001 on an Andisol of Southern Chile under irrigation. Urea and sodium nitrate were applied at rates of 0, 150 and 300 kg N ha-1. The pasture was cut thrice and dry matter (DM) production and shoot N concentration were determined. The soil pH and concentration of NH4+-N and N0(3)-N were also determined for the 0-10 cm, 10-20 cm and 20-40 cm depth, and the maximum N potential losses by leaching were estimated. Dry matter production increased by 128% as N supply increased from 0 kg N ha-1 to 300 kg N ha-1. The N source did not show any effect on yield. Urea and sodium nitrate induced higher shoot N concentration as the N rate increased and non acidifying effect of urea on DM production and pasture quality were observed. The application of sodium nitrate and urea (300 kg N ha-1) produced the highest N0(3)-N and NH4+-N concentration in the deepest soil layers. For the 20-40 cm of depth, the estimated maximum N potential losses downward the soil profile were about 90 kg N ha-1 and corresponded to the period of the lower rate of pasture growth