Particle Size Impacts of Subsurface-Banded Urea on Nitrogen Transformation in the Laboratory

Abstract

Replacing commercial urea granules (0.01–0.02 g) with urea pellets (1.5 g) could improve crop yield and reduce nitrogen (N) losses into the environment. Since urea particle size affects N transformations and, subsequently, N-loss pathways, laboratory studies were conducted to study the effects of subsurface-banded urea particle size or specific surface area (SSA) effects on dissolution, mechanism of dissolved urea movement, and N mineralization (urea hydrolysis and nitrification). To simulate subsurface banding, urea treatments were applied in a plane beneath the soil to Ross loam soil at volumetric soil moisture content (θv) of 31.4% (34 cbars) in all studies. At 50% dissolution, granules dissolved eight times faster than 1.5-g pellets. Molecular diffusion was likely the predominant mechanism of dissolved urea movement in both pellets and granules. Urea hydrolysis was significantly lower by 3.1% in 1.5-g pellets than in granules after 7 d. At 35 d, nitrification rate of the applied-N was 11% (significantly) lower in 1.5-g pellets than granules. Compared with granules, pellets dissolved slower and inhibited both urease and nitrifier activity to a greater extent; however, nitrification inhibition was likely the predominant mechanism that reduced nitrate availability for both uptake and loss. Hence, when granules and 1.5-g pellets are both subsurface-banded in the soil, slower nitrification in pellets could reduce the potential for N losses. However, greater benefits in terms of increased crop yield and N uptake and, potentially, reduced N losses are likely when surface-broadcast urea granules are replaced with subsurface-applied urea pellets.