Background Biochar amendments enhance crop productivity and improve agricultural quality. To date, studies on the correlation between different amounts of biochar in pakchoi (Brassica campestris L.) quality and rhizosphere soil microorganisms are limited, especially in weakly alkaline soils. The experiment was set up to explore the effect of different concentrations of biochar on vegetable quality and the correlation between the index of quality and soil bacterial community structure changes. Methods The soil was treated in the following ways via pot culture: the blank control (CK) without biochar added and with biochar at different concentrations of 1% (T1), 3% (T2), 5% (T3), and 7% (T4). Here, we investigatedthe synergistic effect of biochar on the growth and quality of pakchoi, soil enzymatic activities, and soil nutrients. Microbial communities from pakchoi rhizosphere soil were analyzed by Illumina MiSeq. Results The results revealed that adding 3% biochar significantly increased plant height, root length, and dry weight of pakchoi and increased the contents of soluble sugars, soluble proteins, Vitamin C (VC), cellulose, and reduced nitrate content in pakchoi leaves. Meanwhile, soil enzyme activities and available nutrient content in rhizosphere soil increased. This study demonstrated that the the microbial community structure of bacteria in pakchoi rhizosphere soil was changed by applying more than 3% biochar. Among the relatively abundant dominant phyla, Gemmatimonadetes, Anaerolineae, Deltaproteobacteria and Verrucomicrobiae were reduced, and Alphaproteobacteria, Gammaproteobacteria, Bacteroidia, and Acidimicrobiia relative abundance increased. Furthermore, adding 3% biochar reduced the relative abundance of Gemmatimonas and increased the relative abundances of Ilumatobacter, Luteolibacter, Lysobacter, Arthrobacter, and Mesorhizobium. The nitrate content was positively correlated with the abundance of Gemmatimonadetes, and the nitrate content was significantly negatively correlated with the relative abundance of Ilumatobacter. Carbohydrate transport and metabolism in the rhizosphere soil of pakchoi decreased, and lipid transport and metabolism increased after biochar application. Conclusion Overall, our results indicated that applying biochar improved soil physicochemical states and plant nutrient absorption, and affected the abundance of dominant bacterial groups (e.g., Gemmatimonadetes and Ilumatobacter), these were the main factors to increase pakchoi growth and promote quality of pakchoi. Therefore, considering the growth, quality of pakchoi, and soil environment, the effect of using 3% biochar is better.

The study investigated the dynamics of nitrogen accumulation in mung been under different nitrogen levels and different nitrogen forms. A pot experiment was conducted, including six treatments: no fertilizer applied (N0), 233 g ¹⁵N.plant ⁻¹ (N1), 292 g ¹⁵N. plant ⁻¹ (N2), 350 g ¹⁵N.plant ⁻¹ (N3), 146 g ¹⁵N+146 g organic nitrogen (N _organic), plant ⁻¹ (ON1), and 292 N _organic. Plant ⁻¹ (ON2). The organic nitrogen was provided by the fermented soybean cake. The ¹⁵N was provided in chemical nitrogen form. The yield in N2 was the highest among N0-N3. With ¹⁵N increasing, Rhizobia fresh weight showed downward trend. Compared with N2, both ON1 and ON2 improved the 1000-grain weight, yield and grain crude protein content. According to ¹⁵N-labeled method, mung bean plants got more nitrogen from soil before R1 stage than other growth periods, later the absorbed nitrogen would be dynamically exchanged out of plant through root-Rhizobia interaction, but mung bean again accumulated more nitrogen from soil at R8. N2 had the largest ¹⁵N translocation rate from roots to grains in the experiment; The nitrogen use efficiency increased with chemical nitrogen increasing, the value being 14.0% in N1, 22.5% in N2 and 26.1% in N3. The nitrogen use efficiency of ON1 was 23.0%. The repart coefficient of ¹⁵N ranged from 50.4–56.7% in grains, 3.0–3.1% in shells, 18.7–27.2% in leaves, 14.3–15.4% in stems and 4.9–8.3% in roots. The nitrogen use efficiency calculated based on nitrogen difference was 6.6–31.3% higher than that based on ¹⁵N tracer technique. In sum, appropriate nitrogen fertilizer (N2) is beneficial to the increase of mung bean yield. The application of organic nitrogen fertilizer could alleviate the inhibition effect of nitrogen fertilizer on Rhizobia, reduce the loss of soil nitrogen, and further improve the yield of mung bean.