Rhizosphere environments play a vital role in the nutrient cycling of crops and soil organic nitrogen mineralization. Sugar beet is a highly nitrogen (N)-demanding crop, and it is necessary to explore the relationship between the sugar beet root exudates, the microbial community, and nitrogen utilization. In this study, a special separation method was employed to create rhizosphere (H3) and non-rhizosphere (H2 and H1) environments for sugar beet. After 50 d of cultivation in nearly inorganic-free soil, the microbial diversity and its correlation with root metabolites and N were examined. The results showed that in H3, the inorganic N content was over 23 times higher than in H1 and H2, with a 13.1% higher relative abundance of ammonia-oxidizing bacteria compared to H2 and a 32% higher abundance than H1. The relative abundance of nitrite-oxidizing bacteria was also 18.8% higher than in H1. Additionally, a significant positive correlation was observed between inorganic nitrogen content and serine (Ser) and isoleucine (Ile). The organic nitrogen content exhibited positive correlations with glycine (Gly), alanine (Ala), and tyrosine (Tyr) but displayed negative correlations with certain amino acids, organic acids, and glucose. The co-linearity network indicated that the microbial composition in H3 also exhibited higher node connectivity. It can be inferred that under the influence of sugar beet root exudates, the changes in the rhizosphere’s microbial diversity were more intricate, thereby benefiting soil nitrogen cycling and inorganic N accumulation. These findings provide profound insight into sugar beet soil organic nitrogen mineralization and contribute to the sustainable and environmentally friendly development of modern agriculture.

Sugar beet is a nitrogen (N)-sensitive crop, and its N regulation and utilization are critical for enhancing productivity. Sugar beet seedlings at the two-true-leaf-pair stage were hydroponically grown in an artificial climate chamber. Leaves and roots from three seedlings per treatment were sampled at 10, 20, 25, and 30 days after exposure to N treatments (N5: 5 mmol/L, N10: 10 mmol/L, N15: 15 mmol/L, and N20: 20 mmol/L) to assess the effects of N supply level on growth, photosynthesis, and carbon and nitrogen metabolism. The results revealed a time-dependent dynamics in beet biomass accumulation, with N20 inducing chlorosis and necrosis symptoms by 10 days post-treatment (DPT), resulting in the lowest biomass. While N15 significantly promoted root biomass by 30 DPT, showing a 23.70% (root dry weight, RDW) increase over N20; chlorophyll content and gas exchange parameters-net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) exhibited significant N dependence, with N15 maintaining high chlorophyll level (0.78 mg/g) and photosynthetic rate (220.33 μmol/(m²·s). Nitrogen assimilation, as indicated by glutamine synthetase and glutamate synthetase activity (GS and GOGAT), was stronger under N15, promoting amino acid synthesis and root growth, whereas N20 inhibited enzyme activity. Carbon metabolism analysis revealed that N15-driven sucrose synthesis significantly increased root sucrose content, sucrose phosphate synthase and sucrose synthase activity (SPS and SS), optimizing source–sink allocation. Correlation analysis showed a positive relationship between leaf and root biomass (r = 0.91), and root sucrose content was positively correlated with GOGAT activity (r = 0.90), emphasizing the synergistic regulation of C/N metabolism. On the contrary, N20 led to disrupted C/N metabolic homeostasis, inhibited enzyme activity, and C/N distribution. These results indicated that the photosynthetic output, enzyme efficiency, and sucrose distribution were coordinated by nitrogen optimization, and the growth of sugar beet seedlings was optimized.

Global crop production faces serious threats from soil salinization. Microbial resources are often exploited to be used as fertilizers or seed coatings to address this issue. Parametarhizium changbaiense, as a novel beneficial microorganism, has been discovered to be capable of assisting limited crops such as mung bean in resisting salt–alkali stress. To investigate the effects of P. changbaiense on sugar beet under salt–alkali stress, the salt (NaCl:Na₂SO₄, molar ratio 9:1) and alkali (NaHCO₃:Na₂CO₃, molar ratio 9:1) stress were set on sugar beet germplasm 780016B. Results demonstrated that P. changbaiense improved the phenotypic characteristics of sugar beet seedlings under salt–alkali stress. The biomass parameters such as plant height and fresh weight significantly increased by growth-promoting effect. The elevated antioxidant enzyme activity could help protect plants from ROS damage induced by stress. Relative electrical conductivity and MDA content decreased with inoculation, thereby mitigating membrane lipid peroxidation and improving membrane system stability. The higher content of soluble sugar could maintain cell turgor pressure and alleviate osmotic stress. Inoculation with P. changbaiense enhanced chlorophyll content, fluorescence, and photosynthetic capacity. The more superior root vitality and architecture were suitable for the functions of metabolism and absorption. P. changbaiense could promote the growth and physiological characteristics under salt–alkali stress, so it has practical application value in agricultural production.