Efficiency of nitrogen-15-labelled fertilizers for rice and rye-grass cultivated in an Ultisol of Brazilian Amazonia

Abstract

 The objective of this study was to determine the efficiency of two N fertilizers, (NH₄)₂SO₄ and urea, for rice (Oryza sativa L.) and rye-grass (Lolium multiflorum L.) cultivated in an Ultisol of central Amazonia using ¹⁵N as a tracer. Rice was cultivated in the field, while rye-grass was grown in a phytotron. Fertilization with (NH₄)₂SO₄ caused a 16% increase in the yield of rice grains and urea a 36% increase. In both crops total N uptake and N use efficiency of the fertilizers were higher for urea than for (NH₄)₂SO₄. The low values for N derived from fertilizer showed that the fertilizers contributed little to the total N absorbed by the plants. The "priming effect" or positive added N interaction (ANI) between the fertilizer N and soil organic N was observed, especially with urea. Immobilization by soil microorganisms was greater in the presence of urea, while losses were always higher with the (NH₄)₂SO₄ treatments. These losses were significant, and their reduction should allow more efficient use of this N fertilizer. It is possible that the N use efficiency was higher for urea due to a pH increase, caused by urea hydrolysis, which in turn may have favoured the activity of nitrifying bacteria in this extremely acid soil.