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 NH4+. 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 N-15-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 N-15 recovery as volatilized NH3, mineral N (as exchangeable NH4+, NO3-, and NO2-), and immobilized organic N. The OCh amendment reduced NH3 volatilization by up to 12% but had no effect on urea hydrolysis, NH4+ and NO3- concentrations, NO2- accumulation, or immobilization. In contrast, the use of NBPT to inhibit urea hydrolysis was markedly effective for moderating the accumulation of NH4+, which reduced immobilization and also controlled NH3 toxicity to nitrifying microorganisms that otherwise caused the accumulation of NO2- instead of NO3-. Oxidized charcoal is not a viable alternative to NBPT for increasing the efficiency of urea fertilization.