Seasonal Accumulation and Partitioning of Nitrogen‐15 in Rice

Knowledge concerning the seasonal accumulation and partitioning of fertilizer and total N in direct‐seeded rice ( Oryza sativa L.) is very limited. A field study was conducted on the semidwarf rice cultivar Lemont and a standard‐statured rice cultivar Lebonnet to assess N accumulation and partitioning throughout the season. Fertilizer N as 15 N‐labeled urea was applied at 134 kg N ha −1 either in a single preflood or split applications of 67 kg N ha −1 at preflood and panicle differentiation (PD). A third treatment involved split application of nonlabeled N fertilizer at preflood and 15 N fertilizer at PD. Total N and fertilizer 15 N uptake as well as the N distribution among leaf blades, leaf sheaths, stems, and panicles were measured biweekly prior to heading and weekly after heading until maturity. Cultivar differences in N uptake and distribution among plant parts were minor. In general, the leaves contained 70% of the plant N at flooding and 60% just prior to heading. The percentage of plant N in the stems and sheaths showed a steady increase from flooding to heading. Soon after heading, the panicle became the plant part with the largest N accumulation and steadily increased in N at the expense of other plant parts. By maturity, the panicle contained two‐thirds of the total plant N. Translocation of fertilizer N to the panicle was slower when all of the N was applied preflood, compared with split applications. Maximum fertilizer 15 N uptake occurred by 55 d after emergence when all of the N was applied preflood (79% of applied N), and by 69 d after emergence when N was applied in split applications (72% of applied N). Plant samples taken at maturity underestimated fertilizer 15 N recovery by 14 to 25%, compared with fertilizer 15 N recovery measured by sampling plants 2 to 4 wk before heading.