Nitrous oxide emission and denitrification nitrogen losses from soils treated with isobutylenediurea and urea plus dicyandiamide

The aim of this study was to quantify the effects of two N compounds commonly used in controlled release fertiliser (CRF) and bio-inhibitor-amendment fertiliser formulations on denitrification N losses and N2O emission from several soil types at different soil moisture levels. The compounds tested were the slightly soluble isobutylenediurea (IBDU), and urea mixed with the nitrification inhibitor, dicyandiamide (DCD). Unfertilised soils and soils treated with urea alone served as controls. A significant variation in N2O emission and denitrification rates was observed between the fertiliser treatments. This variation was found to be attributable to the nature of the chemical compounds and not to changes in NH4+ or NO3- concentrations. The diminished denitrification rate over time for all the fertiliser treatments was probably associated with the decay of denitrifying microbes. NO emission could generally be correlated with the denitrification rate and the contribution of nitrification was estimated to be low. The addition of the nitrification inhibitor DCD to the urea showed no appreciable effect on denitrification compared with urea alone but did affect N2O emission. Generally, the IBDU treatment gave rise to the greatest denitrification N loss, while losses due to N2O emission were lower than control values in many of the trials. The ratio denitrification loss:N2O emission increased with the soil moisture and clay content of each type of soil, and generally, this ratio was highest for soils treated with IBDU. The soil property that most strongly influenced denitrification and N2O emission was water-filled pore space.