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.