Abstract
Because of the higher nitrogen (N) recovery efficiency (NRE) of panicle-stage fertilization compared with basal and tillering fertilization, increasing the proportion of N topdressing at the booting stage (panicle-N) is recommended and commonly practiced in parts of China. To investigate the effects of increasing panicle-N on grain yield and N use efficiency (NUE) and the relationships of the increase and the rice cultivar and soil fertility status, we increased the percentage of panicle-N from 20% to 40% by correspondingly reducing the N amount applied only at the tillering stage in both high- and low-fertility blue clayey paddy fields in 2018 and 2019. Four indica cultivars with diverse panicle types were used, and their grain yield, dry matter accumulation, and NUE were compared. In high-fertility soil, increasing topdressing panicle-N from 20% to 40% reduced tillering ability and reduced the effective panicle numbers of the multi- and medium-panicle cultivars Huanghuazhan (HHZ), C Liangyouhuazhan (CHZ), and Tianyouhuazhan (THZ). These cultivars gave the greatest yield when 30% of N was supplied as panicle fertilizer, whereas the yield, NRE, N agronomic efficiency (NAE), and nitrogen physiological efficiency (NPE) of the heavy-panicle inbred cultivar Yangdao 6 (YD6) continued to increase, resulting in improved dry matter accumulation and grain filling in the late growth stage. The yield, NAE, NRE, and NPE of YD6 peaked when the panicle-N constituted 40%. While in low-fertility soil, the multipanicle cultivar HHZ showed the greatest yield when 30% of fertilizer-N was applied once at the panicle initiation (PI) stage, while the medium-panicle cultivar CHZ showed the greatest yield when the panicle-N percentage was 40%. Our results suggest that the percentage of panicle-N fertilizer should not exceed 30% for multipanicle cultivars, while can be appropriately increased to 40% for heavy-panicle indica cultivars. The effect of increasing topdressing panicle-N on the yield of medium-panicle cultivars was related to soil fertility. The optimum panicle-N percentage was 30% in the high-fertility soil and 40% in the low-fertility soil.
