Seasonal Uptake Patterns of Fertilizer Nitrogen Applied in Split Applications to Rice

When rice ( Oryza sativa L.) is grown in a direct‐seeded, delayed‐flood system as used in the southern USA, split topdress applications of N fertilizer have proven to be one of the most effective methods for achieving relatively high grain yields and efficient fertilizer‐N uptake. Knowledge concerning the seasonal uptake patterns of fertilizer N applied in split applications is limited. Consequently, a field study was conducted to measure the seasonal uptake of N fertilizer from split applications made at preflood, 1.3‐cm internode elongation (IE), and 14 d after IE (IE + 14 d). To evaluate each fertilizer application, 15 N‐labeled urea (2.42 atom% 15 N) was applied at preflood (10 g N m −2 ), IE (5 g N m −2 ), or IE + 14 d (5 g N m −2 ). Fertilizer N contained in the soil as NH 4 , NO 3 , and organic N were determined periodically after each 15 N application, along with the fertilizer and total N in the rice roots, shoots, and panicles. Plant uptake of fertilizer N from the preflood application was essentially complete within 21 d after application (63.2% of the applied N). Nitrogen uptake was essentially complete within 3 d after application of the midseason treatments. Three days after IE, 63.3% of the applied N had been taken up by the plant whereas the plant had taken up 70.1% of the applied N 3 d after the IE + 14 d N application. Approximately 20% of the fertilizer N was recovered in the soil organic fraction at maturity regardless of the time of fertilizer application. Fertilizer N incorporated into the soil organic fraction reached a maximum within 21 d after the preflood treatment.