Abstract

High N-NH₃ losses are expected when conventional urea is applied to the soil surface. In order to reduce it, urea granules could be coated with different materials to decrease fertilizer dissolution rate or to stabilize N-NH₄⁺ by acidification. In this study, we investigated the effect of a polymer-coated urea and powdered S0 added to urea, in the presence or absence of a S-oxidizing bacterium (Acidithiobacillus thiooxidans), on soil pH, SO₄^2- availability, NH₄⁺, and NH₃ volatilization. Applying S⁰ before urea and the inoculation with bacteria have promoted the highest S⁰ oxidation rates. The greater decrease in soil pH occurred when S⁰ was applied before urea at a higher dose, which also decreased NH₃ volatilization by 83% up to 4 days after urea application. However, the decrease in soil pH did not increase the concentration of NH₄⁺, nor did it decrease the accumulated amount of volatilized NH₃ over time. The inoculation of A. thiooxidans accelerates S⁰ oxidation process, but it was insufficient to counteract the H⁺ consumption by urea hydrolysis. Therefore, the S⁰ application with urea did not offer chemical protection against NH₃ loss, but a physical barrier in the controlled-release urea had less dissolved urea in soil and reduced NH₃ losses.

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