Comparison of urease inhibitor N-(n-butyl) thiophosphoric triamide and oxidized charcoal for conserving urea-N in soil

Charcoal-based amendments have a potential use in controlling NH₃ volatilization from urea fertilization, owing to a high cation-exchange capacity (CEC) that enhances the retention of NH $ _4^+ $. An incubation study was conducted to evaluate the potential of oxidized charcoal (OCh) for controlling soil transformations of urea-N, in comparison to urease inhibition by N-(n-butyl) thiophosphoric triamide (NBPT). Four soils, ranging widely in texture and CEC, were incubated aerobically for 0, 1, 3, 7, and 14 d after application of ¹⁵N-labeled urea with or without OCh (150 g kg⁻¹ fertilizer) or NBPT (0.5 g kg⁻¹ fertilizer), and analyses were performed to determine residual urea and ¹⁵N recovery as volatilized NH₃, mineral N (as exchangeable NH $ _4^+ $, NO $ _3^ - $, and NO $ _2^ - $), and immobilized organic N. The OCh amendment reduced NH₃ volatilization by up to 12% but had no effect on urea hydrolysis, NH $ _4^+ $ and NO $ _3^ - $ concentrations, NO $ _2^ - $ accumulation, or immobilization. In contrast, the use of NBPT to inhibit urea hydrolysis was markedly effective for moderating the accumulation of NH $ _4^+ $, which reduced immobilization and also controlled NH₃ toxicity to nitrifying microorganisms that otherwise caused the accumulation of NO $ _2^ - $ instead of NO $ _3^ - $. Oxidized charcoal is not a viable alternative to NBPT for increasing the efficiency of urea fertilization.