Novel Approach to Assessing Root Nitrogen Uptake in Rice Using 15 N.

A novel method was developed to inject ¹⁵N labelled ammonium sulfate into an undistrubed soil profile below a recently established rice plot. Following the establishement of a permanent flood, N was injected into the soil profile at depths of 30, 45 and 60 cm. Plant samples were taken at 50% heading to determine fertizer nitrogen uptake efficiency. The main effect of depth was the only significant factor influencing FNUE and FNUE decreased with increasing depth. As fertilizer placement depth increased, there was a decrease in N recovery with FNUE values of 93, 40 and 9% for the 30, 45 and 60 cm depths, respectively. These findings indicate that roots of rice produced on silt loam soils are able to penetrate and utilize N at depths of 30 to 60 cm. Previous research that indicates a significant amount of potentially mineralizable N at depths up to 60 cm coupled with the data presented here identify the need for modified soil sampling depths during the development of soil-based N tests such as N-STaR.