Biochar-based molybdenum slow-release fertilizer enhances nitrogen assimilation in Chinese flowering cabbage (Brassica parachinensis)

Low molybdenum (Mo) bioavailability in acidic soil obstructs vegetable nitrogen assimilation and thus increases the health risk of vegetable ingestion due to nitrate accumulation. Constantly providing available Mo in acidic soil is a challenge for decreasing nitrate accumulation in vegetables. In this study, three Mo application methods, including biochar-based Mo slow-release fertilizer (Mo-biochar), seed dressing, and basal application, were investigated to enhance Mo bioavailability in acidic soil and nitrogen assimilation in Chinese flowering cabbage (Brassica parachinensis). The results showed that Mo-biochar constantly and sufficiently supplied Mo nutrients throughout the growing period of Brassica parachinensis, as evidenced by the soil available Mo, plant Mo uptake, and Mo values. The improved Mo supply was attributed to the alleviation of acidic soil (pH from 5.10 to 6.99) and the slow release of Mo adsorbed on biochar. Mo-biochar increased the nitrate reductase (NR) activity by 238.6% and glutamate dehydrogenase activity by 27.5%, indicating an enhancement of the rate-limiting steps of nitrogen assimilation, especially for nitrate reduction and amino acid synthesis. The increase in Mo-containing NR could be directly ascribed to the high level of Mo in Brassica parachinensis. Compared with the control, the nitrate content of Brassica parachinensis decreased by 42.9% due to the nitrate reduction induced by increased NR. Additionally, Mo-biochar was beneficial to vegetable growth and quality. In contrast, the transformation from NO₃⁻ to NH₄⁺ was blocked with Mo seed dressing and basal application because of low Mo bioavailability in the soil, resulting in a high nitrate content in Brassica parachinensis. Conclusively, Mo-biochar can slowly release Mo and improve the neutral environment for Mo bioavailability, which is an effective strategy to mitigate the high nitrate accumulation of vegetables planted in acidic soil.