Grain yield and nitrogen use efficiency of an ultrashort-duration variety grown under different nitrogen and seeding rates in direct-seeded and double-season rice in Central China

Nitrogen (N) and seeding rates are important factors affecting grain yield and N use efficiency (NUE) in direct-seeded rice.  However, these factors have not been adequately investigated on direct-seeded and double-season rice (DDR) in Central China.  The objective of this study was to evaluate the effects of various N and seeding rates on the grain yield and NUE of an ultrashort-duration variety grown under DDR.  Field experiments were conducted in 2018 in Wuxue County and 2019 in Qichun County, Hubei Province, China with four N rates and three seeding rates.  The results showed that the grain yield of the ultrashort-duration variety ranged from 6.32 to 8.23 t ha^–1 with a total growth duration of 85 to 97 days across all treatments with N application.  Grain yield was increased significantly by N application in most cases, but seeding rate had an inconsistent effect on grain yield.  Furthermore, the response of grain yield to the N rates was much higher than the response to seeding rates.  The moderate N rates of 100–150 and 70–120 kg N ha^–1 in the early and late seasons, respectively, could fully express the yield potential of the ultrashort-duration variety grown under DDR.  Remarkably higher N responses and agronomic NUE levels were achieved in the early-season rice compared with the late-season rice due to the difference in indigenous soil N supply capacity (INS) between the two seasons.  Seasonal differences in INS and N response should be considered when crop management practices are optimized for achieving high grain yield and NUE in ultrashort-duration variety grown under DDR.