Nitrogen fertilizer leaching in an Oxisol cultivated with sugarcane

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Abstract
Nitrogen (N) leaching below the crop-rooting zone represents not only a valuable loss of nutrients for the plant, but also a potential pollution source of groundwater. The objective of this work was to quantify leaching losses of native N and that derived from fertilizer in an Oxisol that was cultivated with sugarcane (Saccharum officinarum) during the crop plant cycle. The sugarcane was planted and fertilized with urea in the planting furrow, with 120 kg ha-1 of N. In order to determine the fate of the fertilizer - N, four microplots with 15N enriched fertilizer were installed. Input and output of N at the depth of 0.9 m were quantified from the flux density of water and the N concentration in soil solution. During the evaluation period the rainfall was 141 mm less than the historical average (1,315 mm), and the climate was drier than normal in January. The average concentration of mineral N in soil solution was 1.8 mg L-1. The abundance of 15N was very high at the beginning (first week) of the assessment period and remained approximately constant (0.453 atom% of 15N) until the end of the period. The internal drainage was 91 mm of water and the N leaching loss was 1.1 kg ha-1 of N, with only 54 g ha-1 derived from fertilizer. Therefore, under high demand of N by the crop in a system without burning before planting, the leaching of N was not considerable, mainly because the surplus of water between the months of December and March was lower than expected and also because the extraction of nitrogen by the crop was high.