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The interaction between the amount and frequencies of nitrogen application has always been a hot issue in improving crop yield and reducing environmental pollution. Photosynthesis and non-structural carbohydrates (NSCs) play an important role in the formation of rice yield. However, the research on photosynthetic characteristics and NSCs under nitrogen fertilizer management on rice yield is still insufficient. This work was a two-year field trial in China’s Hunan Province in 2020–2021. To analyze the photosynthetic characteristics and NSCs of the hybrid rice “Zhu Liangyou 819” (ZLY819), the experiment was set up with N application frequencies, specifically P1 (basal-tiller fertilizer at a ratio of 5:5), P2 (basal-tiller-spike fertilizer at a ratio of 4:3:3), and P3 (basal-tiller-spike-grain fertilizer at a ratio of 4:3:2:1). Additionally, three distinct amounts of N applications were utilized: N1 (90 kg ha−1), N2 (150 kg ha−1), and N3 (210 kg ha−1). The findings indicated that under the same N application amount, N2 increased the effective spike by 9.32–17.80% and the number of grains per spike by 12.21–13.28% compared with N1. Under the same N application frequency, P3 had the highest effective number of spikes and number of grains per spike, which were 320.83 × 104 ha−1 and 113.99–119.81, respectively. Under the same N application amount, the SPAD and photosynthetic rate (Pn) of N2 at the heading stage were increased by 5.61–5.68% and 11.73–13.81%, respectively, compared with that of N1; and at the maturity stage, the SPAD of N2 was increased by 14.79–17.21%. At the same N application frequency, SPAD and Pn were 5.40–6.78% and 4.70–12.85% higher in P3 compared to P1, respectively, at the heading stage. At maturity, SPAD showed 14.59–15.64% higher values in P3 compared to P1. The photosynthetically active radiations (PAR) and radiation use efficiency (RUE) of ZLY819 obtained the highest values under N2 or N3 as the differences between these both were nonsignificant. PAR and RUE tended to increase with the increase in the application frequency. NSC accumulation, output, and contribution rate to grains all exhibited a pattern of initial increase followed by a subsequent decline in response to escalating nitrogen application, i.e., it was highest under N2 treatment. A statistically significant positive correlation was observed between rice yield and effective number of spikes, number of grains per spike, SPAD, Pn RUE, output of NSCs, and contribution rate to grains. Appropriate amount and frequency of N application (P3N2) can significantly improve photosynthetic characteristics and NSCs of rice, thus increasing rice yield.

Optimizing nitrogen fertilizer management can effectively improve soil ecology, promote agricultural production, and increase the income of farmers and workers. Nitrogen fertilizer is an important factor in the growth and development of rice, and it is important to find out the optimal amount and frequency of fertilizer application for the super-hybrid early rice ‘Zhu LiangYou 819’ in Hunan Province, to give full play to its high quality and high yield characteristics. Various N fertilizer application frequencies (P1, basal–tiller fertilizer = 5:5; P2, basal–tiller–spike fertilizer = 4:3:3; P3, basal–tiller–spike–grain fertilizer = 4:3:2:1) and N application amounts (N1, 90 kg ha−1; N2, 150 kg ha−1; N3, 210 kg ha−1) were applied to the hybrid rice ZLY819. The results show that, under the same frequency of N application, ZLY819 had the highest yield, agronomic efficiency, and physiological utilization rate of N fertilizer with the N2 treatment, averaging 7.53 t ha−1, 18.10 kg kg−1, and 34.34%, respectively, with the yield under N2 being 19.38% higher than that under N1. For the same amount of N application, the yield, agronomic efficiency, partial factor productivity of N (PFPN), N contribution to seed, and N use efficiency (NUE) increased with an increase in the frequency of N application, mainly in the order of P3 > P2 > P1, whereby the yield of P3 was 10.11% higher than that of P1. According to the regression equation, the yield is higher when the amount of nitrogen application is 202.15 kg ha−1 and the fertilization frequency is four times. Appropriate N fertilizer management (P3N2) improved the rice growth characteristics, dry matter accumulation, crop growth rate, dry matter transport rate, dry matter contribution rate, and NUE, thus promoting an increase in the rice yield and efficient use of nitrogen.

【Objective】The study was carried out to study the effects of nitrogen application rate and transplanting density on dry matter production and nitrogen use efficiency of double cropping hybrid rice.【Method】Zhuliangyou 819 was used as early rice material and Taiyou 398 was used as late rice material. Three nitrogen application levels (N1, N2, N3 at 0, 120, 150 kg/hm2 for early rice and N1, N2, N3 at 0, 150 and 225 kg/hm2 for late rice, respectively) and three transplanting densities(M1, M2, M3 at 13.3 cm×16.7 cm, 13.3 cm×20.0 cm, 16.7 cm×20.0 cm for early rice and M1, M2, M3 at 16.7 cm×20 cm, 16.7 cm×23.3 cm, 16.7cm×26.7cm for late rice, respectively).【Result】Increasing nitrogen fertilizer can significantly improve the accumulation of dry matter quality of rice, and the effect of transplanting density on dry matter quality is small. The dry matter quality of early and late rice is the highest in N3M1 treatment at maturity, in which the dry matter quality of early rice is 8.81%- 110.24% higher than that of other treatments, and that of late rice is 9.12%-117.12% higher than that of other treatments. The nitrogen uptake and utilization efficiency of early rice N2M2 treatment was up to 60.73%, and that of late rice N2M3 treatment was up to 63.20%. Increasing nitrogen fertilizer could not significantly improve the contribution rate to yield, and had little effect on the harvest index of late rice; Increasing nitrogen fertilizer application in early and late rice could significantly increase the total nitrogen accumulation of rice, with an increase of 67.50%-77.03% in early rice and 73.18%-115.12% in late rice; Each fertilization treatment of early rice had little effect on the agronomic and physiological utilization efficiency of nitrogen fertilizer. The increase of agronomic and physiological utilization efficiency of nitrogen reduction treatment of late rice was 52.66% and 59.00%; Nitrogen reduction in early and late rice can improve the partial fertilizer productivity of rice, and the growth rates of early and late rice are 15.54% and 48.58% respectively. The contribution rates of transplanting density to the yield of early and late rice were 18.52%-47.22% and 23.42%-35.42% higher in M3 treatment than in M1 and M2 treatment, respectively; It had little effect on total nitrogen accumulation, harvest index, agronomic utilization rate and partial fertilizer productivity; Low planting density can improve the physiological utilization rate of early rice, with an increase of 52.19%-144.14%, but it has little effect on the physiological utilization rate of late rice.【Conclusion】Therefore, N2M2 and N3M3 treatment can maintain high nitrogen use efficiency and dry matter accumulation of double cropping hybrid rice, and N2M2 treatment can save fertilizer and maintain high nitrogen use ability.

Water and nitrogen are the key factors affecting the yield and nitrogen recovery efficiency of double-cropping rice, but information about optimizing nitrogen fertilizer and irrigation management to achieve high yield is still limited. The purpose was to study the effects of different nitrogen application rates (D1, D2, D3, D4 (0, 112.5, 150.0, and 187.5 kg ha−1 for early-season rice and 0, 135, 180, and 225 kg ha−1 for late-season rice)) and irrigation conditions (G1, Alternate wetting and drying irrigation, G2, flooding irrigation) on rice yield and nitrogen recovery efficiency. Field experiments were carried out in the early and late seasons of the subtropical environment in Heshan County, Hunan Province, China in 2018 and 2019. The results showed that the yield was increased by the comprehensive action of reasonable irrigation mode and nitrogen management. Under G1D4 and G1D3 treatments, the maximum grain yield of early-season rice was 7.42 t ha−1, which was 0.8~35.9% higher than other treatments, and that of late-season rice was 8.20 t ha−1, which was 13.3~67.0% higher than other treatments. The increase of yield in G1D4 and G1D3 treatments was due to the increase in dry matter accumulation, effective Panicles number, and Spikelets per panicle, whose increase was due to an increase in photosynthesis and nitrate reductase activity. Compared with other treatments, late-season rice G1D3 treatment achieved a higher yield with less nitrogen fertilizer, while early-season rice G1D4 treatment achieved the highest yield, but the nitrogen recovery efficiency decreased. We should focus on more effective nitrogen fertilizer and irrigation management to further improve the yield potential of double-cropping rice.