Assimilation of ^sup 15^N Labeled Fertilizer Injected at Various Depths by Delayed-flood Rice

The recently developed Nitrogen-Soil Test for Rice or N-STaR requires a 0- to 45-cm soil sample to accurately predict the N fertilizer requirements of rice (Oryza sativa L.) produced on silt loam soils. Following its release questions were raised concerning the ability of rice to assimilate N from soil depths >30 cm. Therefore, a study was established using ^sup 15^N-labeled (NH^sub 4^)^sub 2^SO^sub 4^ to determine if rice roots were penetrating and actively using N from depths >30 cm. During 2009 and 2010, research trials were conducted on six silt loam soils across the primary rice producing regions of Arkansas to determine if rice roots were assimilating N at soil depths up to 60 cm. A modified fertilizer injection system was developed to inject ^sup 15^N-labeled (NH^sub 4^)^sub 2^SO^sub 4^ at depths of 30, 45, and 60 cm, below the rice crop in an undisturbed soil profile. Statistical analysis indicated that fertilizer placement depth was the only factor significantly influencing the fertilizer nitrogen uptake efficiency (FNUE) (p < 0.0001). 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 use 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. [PUBLICATION ABSTRACT]