The effect of metal oxide coating of urea on mineralization in two contrasting soils

Urease inhibitors are used to delay the conversion of N supplied to soil as urea to better match the mineral N supply to crop demand. Using soil in petri dishes, we compared the spatial and temporal mineral N dynamics close to the site of urea addition for two soils of contrasting texture-sandy soil (2% clay) and clay soil (51% clay)-treated with N fertilizer products that had the potential to inhibit urease. The treatments included trace metal oxide coatings (ZnO, NiO, and CuO) of granular urea fertilizer, N-(n-butyl)thiophosphoric triamide (NBPT; a commercial urease inhibitor) coated urea and uncoated urea. In the sandy soil, NBPT inhibited urea hydrolysis, whereas the metal oxide-coated urea had little effect. Unlike in the sandy soil, the most striking result for the clay soil was the "disappearance" of mineral N in the latter part of the incubation. However, a delay in the disappearance of mineral N, hence a higher recovery, for the NBPT-urea and CuO-urea was observed at day 14 in the clay soil presumably due to the slower hydrolysis of urea compared to other treatments. Overall, the study of both spatial and temporal mineral N dynamics in soils supplied with urea fertilizers suggests that CuO-coated urea could have properties that delay the hydrolysis of urea in certain soils. Given the strongly contrasting outcomes for the two soils, further experiments with soils that span the range of texture between these two extremes are warranted.