Low recovery efficiency of basal fertilizer-N in plants does not indicate high basal fertilizer-N loss from split-applied N in transplanted rice

Increasing amounts of N fertilizer at panicle initiation or heading, and decreasing amounts at earlier stages, is recommended and commonly practiced in parts of China. The practice is based on the higher recovery efficiency (RE) of fertilizer N from later application than at earlier stages. However, this approach does not consider the amount of fertilizer N remaining in the soil ('residual soil N') from each application time, and the fact that a considerable amount of the N uptake by the plants, even in heavily-fertilized crops, comes from the soil N. Therefore, the uptake, residual soil N, loss and recovery efficiency of fertilizer N applied to transplanted rice at different growth stages was investigated in a field experiment conducted in 2013 and 2017. Nitrogen was applied as urea at 210 N kg ha−1 in three equal splits at transplanting (basal), tillering and panicle initiation (PI), and 15N-labeled urea was used to trace the change of fertilizer N concentration in the floodwater and soil and the recovery of fertilizer-N in the plants and soil. The fertilizer N recovery efficiency (RE) in the plants was 21-36%, and 15% of the applied 15N remained within the 0-20 cm soil depth at maturity. The RE of N applied at PI (panicle-RE) (33-58%) was greater than the RE of N applied at transplanting (basal-RE) (20-36%) and at tillering (tiller-RE) (13-29%). However, the residual soil N derived from basal fertilizer accounted for more than 90% of the total fertilizer N remaining in the paddy soil at the time of harvest. At harvest time, 70% of the basal-15N, 29% of the tillering-15N and 46% of the panicle-15N, was recovered in the plants and soil, demonstrating that the low basal-RE in the plants does not indicate high basal fertilizer-N loss. Basal-15N still accounted for more than 1% of the total soil N after heading, and all the fertilizer-15N uptake in plants after heading was derived from basal-N.