To evaluate the effects of various rotation systems on rice grain yield and N use efficiency, a paddy–upland cropping experiment (2013–2016) was conducted in southeastern China. The experiment was designed using six different rice––winter crop rotations: rice–fallow (RF), rice–wheat (RW), rice–potato with rice straw mulch (RP), rice–green manure (Chinese milk vetch; RC–G), rice–oilseed rape (RO), and rice–green manure crop (oilseed rape with fresh straw incorporated into soil at flowering; RO–G) and three N rates, N0 (0 kg N ha⁻¹), N1 (142.5 kg N ha⁻¹), and N2 (202.5 kg N ha⁻¹). Average rice yields in the RF (5.93 t ha⁻¹) rotation were significantly lower than those in the rotations with winter crops (7.20–7.48 t ha⁻¹) under the N0 treatment, suggesting that incorporation of straw might be more effective for increasing soil N than winter fallow. The rice yield differences among the rotations varied by year with the N input. In general, the grain yields in the RP and RO–G rotations –were respectively 11.6–28.5% and 14.80–37.19% higher than those in the RF in plots with N applied. Increasing the N rate may have tended to minimize the average yield gap between the RF and the other rotations; the yield gaps were 18.55%, 4.14%, and 0.23% in N0, N1, and N2, respectively. However, the N recovery efficiency in the RF was significantly lower than that in other rotations, except for 2015 under both N1 and N2 rates, a finding that implies a large amount of chemical N loss. No significant differences in nitrogen agronomic efficiency (NAE) and physiological efficiency (NPE) were found between the rotations with legume (RC–G) and non–legume (RO and RW) winter crops, a result that may be due partly to straw incorporation. For this reason, we concluded that the return of straw could reduce differences in N use efficiency between rotations with and without legume crops. The degree of synchrony between the crop N demand and the N supply was evaluated by comparison of nitrogen balance degree (NBD) values. The NBD values in the RP and RW were significantly lower than those in the other rotations under both N1 and N2 rates. Thus, in view of the higher grain yield in the RP compared to the RW under the N1 rate, the RP rotation might be a promising practice with comparable grain yield and greater N use efficiency under reduced N input relative to the other rotations. The primary yield components of the RF and RP were identified as number of panicles m⁻² and numbers of kernels panicle⁻¹, respectively. The NAE and NPE were positively correlated with harvest index, possibly providing a useful indicator for evaluating N use efficiency.

Compared to drought-susceptible rice cultivars (DSRs), drought-resistance rice cultivars (DRRs) could drastically reduce the amount of irrigation water input and simultaneously result in higher grain yield under water-saving irrigation conditions. However, the mechanisms underlying these properties are unclear. We investigated how improved agronomic traits contribute to higher yield and higher water use efficiency (WUE) in DRRs than in DSRs under alternate wetting and drying (AWD). Two DRRs and two DSRs were field-grown in 2015 and 2016 using two different irrigation regimes: continuous flooding (CF) and AWD. Under CF, no statistical differences in grain yield and WUE were observed between DRRs and DSRs. Irrigation water under the AWD regime was 275–349 mm, an amount 49.8%–56.2% of that (552–620 mm) applied under the CF regime. Compared to CF, AWD significantly decreased grain yield in both DRRs and DSRs, with a more significant reduction in DSRs, and WUE was increased in DRRs, but not in DSRs, by 9.9%–23.0% under AWD. Under AWD, DRRs showed a 20.2%–26.2% increase in grain yield and an 18.6%–24.5% increase in WUE compared to DSRs. Compared to DSRs, DRRs showed less redundant vegetative growth, greater sink capacity, higher grain filling efficiency, larger root biomass, and deeper root distribution under AWD. We conclude that these improved agronomic traits exert positive influences on WUE in DRRs under AWD.