Abstract
Polymer-coated urea (PCU) has been traditionally used for broadcast and/or incorporated application of nitrogen (N) fertilizers. To improve N use efficiency (NUE), there has been an increase in sub-surface banded application of this fertilizer technology. However, there is little information on the release and supply of N from PCU granules when applied in a band. This research aimed to investigate the spatial distribution of key N transformations around PCU bands in soils of contrasting physico-chemical properties, and the implications for NUE. Two experiments, consisting of a 60-day diffusion study and a 91-day incubation, were conducted in a Vertosol and Dermosol, with PCU granules banded at a rate equivalent to 150 kg N ha −1 and band spacing of 1.8 m. Compared to standard urea, PCU provided a sustained release of urea-N to soil solution and the lower urea-N concentrations minimized the toxic conditions associated with rapid hydrolysis of urea-N. Nitrogen release from banded PCU was quicker in the Vertosol ( cf . Dermosol), possibly due to a higher volumetric water content and/or greater soil particle surface contact, facilitating rapid water imbibition into granules. However, the proximity of PCU granules to each other in a band restricted the diffusive release of urea-N from PCU granules cf . dispersed application in both soils. Furthermore, the relatively mild chemical conditions in the PCU band ( cf. standard urea) resulted in oxidisation of larger proportions of PCU-derived mineral N. Banded application may extend the duration of N release from PCU granules, confounding efforts to predict N availability. Soil characteristics influenced N release and dynamics from banded PCU, although further investigation is required. Higher rates of nitrification of N derived from PCU bands suggest there could be increased risk of N-loss via denitrification or leaching pathways ( cf. standard urea bands) if release dynamics are not optimally synchronised with crop demand. This study provides the first mechanistic insights into the impact of application method and soil physico-chemical properties on the efficacy of PCU.