A composite guanyl thiourea (GTU), dicyandiamide (DCD) inhibitor improves the efficacy of nitrification inhibition in soil

This study investigated whether applying dicyandiamide (DCD) and guanyl thiourea (GTU) in conjunction with urea improves the efficacy of nitrification inhibition relative to traditional fertiliser application of urea or urea + DCD. Urea at a rate of 100 mg N kg−1 soil was applied to soil microcosms (high nutrient tenosol and low nutrient hydrosol) which were treated with either no inhibitor (urea-only); 15 mg DCD kg−1 soil or 15 mg DCD kg−1 soil plus 21 mg GTU kg soil−1. Mineral N (NH4+ & NO3−) concentrations, potential nitrification rates (PNR) and abundances of ammonia oxidising bacteria (AOB) were measured over time. After 100-days incubation, ∼73 mg N kg−1 soil was found as NH4+ when urea + DCD + GTU were applied to the tenosol. NH4+ concentrations were lower (11-32 mg N kg−1 soil) when urea or urea + DCD were applied. This suggests that the application of GTU in conjunction with DCD elongated the effects of nitrification inhibition. In both soils, PNRs were faster and AOB abundances (gene copies g−1 soil) were higher when urea was applied without nitrification inhibitors. There were, however, no differences in PNR or AOB abundances in either soil type when 'urea + DCD' or 'urea + DCD + GTU' were applied. The results indicate that the application of GTU with DCD may extend nitrification inhibition in certain soil types. This finding has the potential to improve the efficacy of commercially available and widely used inhibitors such as DCD.