Abstract: Promoting the uptake and assimilation of nitrogen fertilizer is crucial for improving crop yield and quality. However, the effect of low-molecular-weight organic matters on soil nitrogen utilization by plants remains unclear. Here, we investigated the effect of industrial vitamin C production-residue after evaporation (RAE), which is mainly composed of small-molecule organic acids, on the utilization of soil nitrogen by non-heading Chinese cabbage (Brassica campestris ssp. chinensis). The results showed that RAE application increased nitrogen fertilizer agronomic efficiency, nitrogen fertilizer partial productivity, nitrogen fertilizer physiological efficiency, and nitrogen fertilizer apparent efficiency by 171.2%, 57.9%, 46.1%, and 86.1%, respectively. Plant growth was significantly promoted, with an increase of total nitrogen accumulation and biomass of leaves by 60.7% and 57.9%, respectively. Meanwhile, chlorophyll content was increased by 16.7%, and soluble protein and free amino acid contents were increased by 65.4% and 15.1%, respectively. Furthermore, the expression levels of nitrite reductase, glutamine synthetase, and glutamate synthase were up-regulated, indicating that photosynthesis was enhanced and inorganic nitrogen assimilation was significantly promoted. In conclusion, RAE could improve nitrogen use efficiency, yield and quality of non-heading Chinese cabbage. This study provided a scientific basis for resource utilization of RAE as a potential organic synergist for improving nitrogen fertilizer utilization.

Key words: organic acid, non-heading Chinese cabbage, nitrogen fertilizer, assimilation, nitrogen use efficiency