The Impact of Banding Polymer-Coated Urea on Nitrogen Availability and Distribution in Contrasting Soils

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.