Rice Nitrogen Uptake As Affected By Different Nitrogen Application Depths

Subsoil nitrogen (N) can contribute to rice (Oryza sativa L.) nutrition and affect its response to N fertilization. The N recovery by depth for flooded rice using N-15 was evaluated in three undisturbed soils (varying in their physical and chemical properties) in a greenhouse experiment. Rice was grown after N-15-labeled ammonium sulfate application at the soil surface and at 0.10, 0.20, 0.30, and 0.45 m depths. The N located at greater depths was absorbed over time, resulting in a similar N-15 concentration in leaves among treatments at rice heading. The N-fertilizer application at the near soil surface resulted in higher dry matter production for all soils, with a constant decrease with depth. Although, N-15 recovery and N derived from fertilizer were greater with N application at 0.10, 0.20, or 0.30 m depth, probably due to N dilution effect caused by soil N and greater N loss at the soil surface. The intensity of N-15 recovery from 0.45 m was related to soil properties, such as hydraulic conductivity. Even later in the season, rice plants effectively used N from subsoil layers, therefore greater soil depths should be considered in studies regarding correlation and calibration of soil-based N tests for N-fertilizer recommendations.