Effect of increased plant density with reduced nitrogen on yield formation and nitrogen use efficiency of hybrid rice under high temperature and high humidity conditions

The effects of increased plant density with reduced nitrogen (N) application rate on yield formation and nitrogen use efficiency (NUE) of hybrid rice were studied to provide a theoretical basis for optimum nitrogen fertilizer management and plant density under high temperature with high humidity conditions. Field experiments were conducted in Luzhou City from 2018 to 2019. The high yield and high quality hybrid rice variety ‘Nei6you107’ was grown under six combinations of plant density and N application rate: 1) locally recommended combination with a plant density of 16.5×10⁴ hills∙hm⁻² and a N rate of 180 kg∙hm⁻² (LDN_ck); 2) combination of a plant density of 16.5×10⁴ hills∙hm⁻² and a reduced N rate by 15% (153 kg∙hm⁻², LDN_−15%); 3) combination of a plant density of 16.5×10⁴ hills∙hm⁻² and a reduced N rate by 30% (126 kg∙hm⁻², LDN_−30%); 4) combination of a increased plant density by about 27% (21.0×10⁴ hills hm⁻²) and a reduced N rate by 15% (153 kg∙hm⁻², HDN_−15%); 5) combination of a increased plant density by about 27% (21.0×10⁴ hills∙hm⁻²) and a reduced N rate by 30% (126 kg∙hm⁻² HDN_−30%); and 6) combination of a plant density of 16.5×10⁴ hills∙hm⁻² and zero N rate (LDN₀). The grain yield, yield components, dry matter, N uptake and NUE were measured. The results showed that the grain yield of hybrid rice was significantly affected by different combinations of plant density and N rate (P<0.01). HDN_−15% and HDN_−30% produced higher grain yields than LDN_ck by 4.3%−4.9% and 2.3%−3.6%, respectively. The higher grain yields under HDN_−15% and HDN_−30% were attributed to improvement in spikelets per panicle, grain filling rate, translocation of dry matter accumulated at heading stage (TDM_HD), translocation percentage of dry matter accumulated at heading stage (TPDM_HD), contribution percentage of pre-anthesis dry matter translocation to grain yield (CPDMTG_HD) and harvest index. The LDN_−15% and LDN_−30% had 2.3%−2.5% and 4.8%−5.0% lower grain yield than LDN_ck, respectively. The yield gap between LDN_−15%, LDN_−30% and LDN_ck was attributed to the difference in effective panicles, total dry matter, dry matter accumulation from heading to maturity, and contribution percentage of dry matter accumulation from heading to maturity stage to grain yield (CPDMG_HD-MA). The HDN_−15% and HDN_−30% had lower nitrogen accumulation from heading to maturity and total N uptake than LDN_ck, whereas the translocation of N accumulated at heading stage (NTGN_HD), translocation percentage of N accumulated at heading stage (TPN_HD), contribution percentage of pre-anthesis N accumulation translocation to grain N accumulation (CPNTGN_HD), N use efficiency for biomass production (NUEBP), N use efficiency for grain production (NUEGP) and N harvest index under HDN_−15% and HDN_−30% were higher than those under LDN_ck. Consequently, HDN_−15% and HDN_−30% had lower N requirements to produce 100 kg of grain (NRPG) than LDN_ck by 6.8%−8.4% and 9.0%−9.9%, respectively. HDN_−15% enhanced the agronomic efficiency of applied N (AE_N) by 36.7%−37.4%, partial factor productivity of applied N (PFP_N) by 22.8%−23.5% and recovery efficiency of applied N (RE_N) by 5.6%−12.0% over LDN_ck. The HDN_−30% produced higher AE_N, PFP_N and RE_N than LDN_ck by 55.5%−60.4%, 46.3%−48.2% and 17.0%−20.0%, respectively. The rational combination of plant density and N rate can improve panicle number per unit area, grain filling, TDM_HD, TPDM_HD, NTGN_HD, TPN_HD and harvest index, which further increasing the grain yield and NUE. The optimum combination is plant density of 21.0×10⁴ hills∙hm⁻² plus N rate of 126−153 kg∙hm⁻² in high temperature with high humidity condition.