Objectives We investigated the regulation effect of basal and topdressing ratios of nitrogen fertilizer on photosynthetic efficiency and yield of sugar beet in soda alkali-saline farmland.

Methods The field experiment was conducted in farmland of Tongliao City, Inner Mongolia from 2020 to 2021. Under the total N input of 150 kg/hm², nine basal to topdressing ratio treatments were setup, as: 10:0:0 (N10), 8:2:0 (N821), 7:3:0 (N731), 6:4:0 (N641), 5:5:0 (N551), 8:1.6:0.4 (N822), 7:2.1:0.9 (N732), 6:2.8:1.2 (N642), 5:3.5:1.5 (N552), taking no nitrogen fertilizer as control (CK). The two topdressings were carried out at the rapid growth stage of leaf tufts (V1) and the growth and sugar accumulation stage of root tubers (V2). The photosynthetic indices and aboveground biomass were measured at V1, V2 and V3 (sugar accumulation) stages, and the sugar content, beet and sugar yield were investigated at harvest stage.

Results Compared with CK, all the fertilizer treatments significantly increased the photosynthetic efficiency, dry matter accumulation and yield of sugar beet; The treatments with basal N ratio higher than 70% did not impact the sugar content, and treatments with N basal ratio less than 60% did not increase beet yield; and the fertilizer treatments, except N551 and N552, significantly increased sugar yield. In the two years, treatment N732 achieved significantly higher sugar yield than all the other treatments. Compared with treatment of N10, split fertilizer treatments improved photosynthetic efficiency and dry matter accumulation at V1 stage, and the increase by one-time topdressing treatment was better than by two-time topdressing under the same basal dressing ratio. At V2 stage, all the fertilizer treatments, except basal N rate of N821 and N551 treatments, significantly increased photosynthetic capacity and dry matter accumulation of sugar beet, and the effect of two-time topdressing treatments were better than that of one-time topdressing treatments. Treatment N732 exhibited the highest net photosynthetic rate, intercellular CO₂ concentration, transpiration rate, stomatal conductance and SPAD value of sugar beet, which were averagely 17.20%, 13.33%, 21.34%, 21.01%, and 10.35% higher than the other treatments. Its dry matter accumulation in leaves, petioles, tuber roots and whole plants were 26.02%, 46.69%, 47.96%, and 41.92% higher. Treatment N732 effectively stimulated the distribution and transport of dry matter in sugar beet. In detail, it significantly reduced the proportion of dry matter in above ground, increased that in tuber roots after V1 stage forming a good “source sink” relationship, laying a foundation for yield formation. Through principal component analysis, the yield and quality of sugar beet were positively correlated with leaf net photosynthetic rate, intercellular CO₂ concentration, transpiration rate, stomatal conductance and SPAD value.

Conclusions Reasonable basal and topdressing ratio of nitrogen fertilizer could increase sugar beet yield and sugar yield by regulating the leaf photosynthetic efficiencies in soda alkali-saline farmland. In this study, the basal to top dressing ratio of N732 (N 105 kg/hm² as base fertilizer, 31.5 kg/hm² applied at the rapid growth stage of foliage and 13.5 kg/hm² at tuber growth and sugar accumulation stage) increased the yield by 19.53%, sugar yield by 20.47%, improved the agronomic efficiency and partial productivity of nitrogen fertilizer by 167.48% and19.53%, so is recommended as nitrogen fertilizer management system for high yield and quality of sugar beet in soda alkali-saline farmland.