Alternate wetting and moderate soil drying irrigation counteracts the negative effects of lower nitrogen levels on rice yield

Purpose Alternate wetting and moderate soil drying irrigation (WMD) and nitrogen fertilizer at panicle initiation are widely used in rice production in China. This study investigated the effects of panicle fertilizer optimization on rice root growth, soil properties and grain yield under continuous flooding (CF) and WMD. Methods A factorial combination of two irrigation regimes (CF and WMD) and two N fertilizer rates at panicle initiation (normal N rate [NN] and low N rate [LN]) was designed in both field and pot conditions in 2018–2019. Results Grain yield declined in the order: WMD + NN > CF + NN = WMD + LN > CF + LN. WMD increased grain yield by notably increasing the spikelet number per panicle (SNPP) and could counteract the negative effect of LN. WMD enhanced root morphology and physiology, which was remarkably correlated with SNPP. The urease activity, oxygen content, and the abundance of ammonia-oxidizing archaea/bacteria in rhizosphere soil were significantly increased under WMD compared to CF but were decreased in LN relative to NN under the same irrigation regime. The rice root length, root dry weight, root diameter, root oxidation activity, and soil nitrification rates under WMD + LN were similar to those under CF + NN. Rice root morphology and activity promoted by the enhanced nitrification process and decreased NH 4 + /NO 3 – ratio under WMD facilitated a higher grain yield. Conclusion Combining WMD with LN at panicle initiation could maintain the rice root growth, leading to comparable grain yields but higher nitrogen use efficiency than the farmers’ practice.