The experimental area was set at Taixing Agricultural Technology Co., Ltd. Vegetable Production Base, Dali City, Xiangyun County, Yunnan Province (25°12’05” N,100°25’22” E). The experimental area is located in northwest central Yunnan Province. The area has a northern subtropical monsoon climate of the plateau, with an average annual temperature of 14.7°C, a frost-free period of 228 days, an annual rainfall of 810 mm, and an annual sunshine duration of 2,624 h. Initially, an analysis of soil physicochemical properties was conducted, the relevant index are as follows: the contents of soil pH (pH), organic matter (OM), alkaline hydrolyzable nitrogen (AH-N), available phosphorus (AP), and available potassium (AK) are 7.60, 26.59 mg/kg, 90.30 mg/kg, 27.97 mg/kg, respectively.

The tested crop was cabbage [Brassica pekinensis (Lour.) Rupr.], and the seeds were supplied by the Taixing Agricultural Technology Co., Ltd. The fertilizers applied, sources and their nutrient content are summarized in Supplementary Table 1 . The BC treatment was formulated using a 3:2 mass ratio of BC to lignite, and the biological agents were granular PGPR ( Supplementary Table 2 ).

Six treatments and one control were designed in this experiment ( Table 1 ). A completely randomized experimental design was adopted, repeated three times, with a plot area of 14 m², totaling 21 residential areas. The planted density of Chinese cabbage was 25 cm × 25 cm, totaling eight rows (28 plants per row) and 28 lines (eight plants per line).

Two-season cabbage was planted to avoid the influence of temperature and light conditions across different seasons. Biochar, lignite, and PGPR were sprayed and plowed evenly before planting cabbage. The fertilization schemes for each treatment are detailed in Supplementary Table 3 . The control and conventional fertilization amounts of nitrogen, phosphorus, and potassium were 347.70 kg/hm², 86.25 kg/hm², 123.00 kg/hm². The nitrogen, phosphorus, and potassium amounts in the nitrogen reduction treatment were 174.75 kg/hm², 79.20 kg/hm², 173.10 kg/hm². All treatments used a water fertilization mixture, and the reduction ratios were respectively 50%, 8%, and -41%.

Field experiment and laboratory test data were preliminarily organized and drawn using Excel 2013. Using SPSS 19.0, conducted variance analysis and significance tests on soil physicochemical properties and cabbage yield. GraphPad Prism 8.0.2 and R software were used for beta diversity analyses and graphic analysis.

Based on the Usearch software (http://drive5.com/uparse/), the UPARSE algorithm was utilized for OTU clustering at a consistency level of 97%. The sequence with the highest frequency within each OTU was selected as the representative sequence. Taxonomic annotation analysis was conducted using the UCLUST classification method in conjunction with the SILVA database (Release_123 http://www.arb-silva.de/). Representative sequences underwent multiple sequence alignment via PyNAST. Additionally, a phylogenetic tree was constructed using FastTree. Use R language for various data conversions and use ggplot2 package for bacterial community composition analysis.

Alpha diversity analysis was performed using R language. Wilcoxon rank sum test and Kruskal-Wallis rank sum test were performed, and multiple comparisons were also performed. Beta diversity analysis was performed using R language. The Unifrac distance, Bray-Curtis and Jaccard distances were calculated. PCA analysis was performed using vegan package.

Adding BC and PGPR increased the AH-N content. The BC application in the E treatment significantly increased the AP content at 160 days, reaching 19.49 mg/kg. In contrast, the improvement effect was not significant among the other treatments. Treatments D, B, and F increased pH that reached 7.78, indicating that BC and PGPR have an upward-regulating effect on pH. The F treatment reached the highest AK content at 60 days (389.95 mg/kg). The F and G treatments reached the highest content after 160 days. This result indicated that the PGPR application or a combination of BC and PGPR enhanced the AK content. Treatment D reached the highest content at 115 days, and treatment G had the highest content at 160 days, reaching 42.59 g/kg. The results showed that BC and PGPR applications could increase OM content. Still, the effect of applying PGPR alone was smaller than that of applying BC alone or BC in combination with PGPR ( Table 2 ).

In conclusion, adding BC and PGPR positively impacted soil physicochemical properties, improving soil health and fertility.

The yield of cabbage planted in winter was higher than that grown in summer due to climate differences. During the summer, the yield of all treatments increased compared to control A, with D exhibiting the significantly highest yield. The B, C, and G treatments followed in order of yield. Still, there were no significant differences among the three treatments. The yield of the E and F treatments was the lowest, with no significant difference between them. During the winter, the D treatment reached the highest yield compared to control A, and the E treatment reached the lowest yield during winter ( Figure 5 ). The combined BC and PGPR applications significantly enhanced the yield of Chinese cabbage. At the same time, such a combined application reduced the application of nitrogen fertilizer without decreasing the yield.

The physicochemical properties are the evaluation index of soil quality. Wei et al. (2023) found that the soil pH of the BC application treatment was 7.0% higher than that of the control. This result was also observed here ( Table 2 ). The BC application and the reduction of nitrogen fertilizer with PGPR in this study significantly increased soil pH. The mechanism of BC induce soil pH change depends on the alkaline ions and surface functional groups of it. Additionally, the increase in pH can constrain the mobility of heavy metals, thereby ensuring soil health (Pan et al., 2021). The BC application with reduced nitrogen fertilizer, the sole PGPR application, and the combined BC and PGPR application significantly increased the soil OM content ( Table 2 ). This result is consistent with the research results found by Urooj et al. (2022). Such an increase in OM content occurs because BC is rich in organic carbon. Biochar application adds exogenous organic carbon to the soil, increasing the carbon pool and OM content and affecting soil fertility.

Alkaline hydrolyzable nitrogen can reflect the nitrogen supply capacity of the soil for the current or recent season. Alkaline hydrolyzable nitrogen is a crucial indicator of soil fertility and is critical for balancing fertilization and effectively controlling agricultural non-point source pollution. Both BC and PGPR enhanced the AH-N content in the soil ( Table 2 ). This result is consistent with the research results of Lalay et al. (2022). This result may be attributed to BC enhancing the biological nitrogen fixation capacity and reducing soil N₂O emissions (Harter et al., 2014). Moreover, PGPR can achieve nitrogen fixation, phosphorus solubilization, potassium release, and other functions by producing nitrogenase, phosphatase, nuclease, or organic acids through metabolism, thereby improving the AH-N, AP, and AK contents and soil fertility. The combination of nitrogen fertilizer reduction and PGPR and their combined use with BC significantly increased the AK content in the soil ( Table 2 ). This result may be because the PGPR used in this experiment is a growth-promoting strain with the ability to dissolve phosphorus and potassium, which is conducive to activating potassium in the soil, promoting plant absorption, and thus increasing the content of AK in the soil (Olaniyan et al., 2022). Sui et al. (2022) have noted that adding BC and reducing nitrogen fertilizer application with BC can enhance the AP content in the soil. This result is consistent with this study ( Table 2 ), possibly because BC can directly interact with the applied chemical fertilizers, adsorbing phosphate within its porous structure, resulting in a slow loss of effective phosphorus from the soil (Yang et al., 2021).

The changes of microorganisms in soil can impact plant growth, and the development of plants may also influence the variations in soil microorganisms (Singh et al., 2022). This study utilized Illumina sequencing technology to demonstrate that different fertilizer treatments significantly impacted the structure of rhizosphere microbial communities in cabbage. The distribution characteristics of the dominant bacteria at the phyla and genera levels in the soil samples in this experiment were analyzed. Proteobacteria, Chloroflexi, Firmicutes, Bacteroidetes, and Actinobacteria were the dominant abundant bacteria in each treatment ( Figure 1 ). These microflora organisms have also been identified as dominant populations in several other studies (Jin et al., 2022; Yin et al., 2022). This result indicated that the soil rhizosphere of the different treatments in this experiment had a common composition of dominant soil microbial communities, but the proportions were different.

Research has shown that Proteobacteria are primarily facultative or obligate anaerobes and heterotrophic microorganisms, and that Sphingomonas can remove refractory pollutants (Wang et al., 2024), Bradyrhizobium has a bioremediation function on soil (Harter et al., 2017). The relative abundance of Proteobacteria in the BC and PGPR combined applications was the highest in this study ( Figures 2 , 3 ). This result indicated that beneficial microbial communities in the rhizosphere soil were greater than in the other treatments. The ability to remove soil pollutants and repair soil was also the strongest, and environmental friendliness was also the highest (Saeed et al., 2022). Actinomycetes can decompose cellulose and lignin, and abundant Actinomycetes are beneficial to decomposing plant organic residues in soil (Zhang et al., 2021). In this study, the relative abundance of Actinomycetes in the combined BC and PGPR application was the highest ( Figures 2 , 3 ), indicating that the OM content in the rhizosphere soil was high, and the soil fertility was the strongest under this treatment condition. This confirmed that the addition of BC and PGPR can indeed improve soil fertility. In addition, some Firmicutes can repair their environments, degrade or transform pollutants, and reduce the impact of soil pollution on the ecological environment. Bacillus in Firmicutes are a widely used biocontrol bacteria with long survival periods and strong resistance. Many strains of these genera have attracted widespread attention from researchers due to their nitrogen-fixing ability, resistance to pathogenic bacteria, and growth-promoting effects on plants (Li et al., 2020; Sun et al., 2022). The relative abundance of Firmicutes in the single PGPR application in this study was the highest, followed by the combined BC and PGPR application with reduced nitrogen fertilizer application ( Figures 2 , 3 ). This result indicated that both treatments have high environmental friendliness, strong resistance to pathogens, and nitrogen-fixing capability. Bacteroidetes participate in the nitrogen cycling process in soil, influencing the content of mineralized nitrogen through its activities. The abundance of Bacteroidetes is also high in environments with a high OM content in the soil. In addition, the microbial Bacteroidetes community may help enhance plant defense capabilities and reduce disease occurrence (Pan et al., 2023). The relative abundance of Bacteroidetes in this study was highest in the single BC or PGPR application ( Figures 2 , 3 ), indicating that the two treatments had strong nitrogen-fixing ability and increased soil OM content and disease resistance. In summary, the application of BC and PGPR can effectively increase the relative abundance of beneficial microbial communities in soil, especially biocontrol bacteria. They are direct indicators of root diseases in Chinese cabbage. For example, cabbage clubroot disease caused by Plasmodiophora brassicae (Javed et al., 2023). It implied that the application of BC and PGPR could reduce the occurrence of root diseases in Chinese cabbage, but more evidence is needed.

The alpha and beta diversity analyses showed differences in the richness and diversity of rhizosphere soil bacterial communities under different treatments ( Table 3 ; Supplementary Table 8 ; Figure 4 ). Compared to conventional fertilization, the application of single BC, single PGPR, and a combination of BC and PGPR significantly increased both the Shannon and Simpson indices of bacteria in the soil ( Table 3 ), indicating that the microbial diversity and species dominance of these three treatments were significantly increased, which was consistent with the results of previous studies (Gulnaz et al., 2017; Di Salvo et al., 2018). The differences in the microbial community structure may be attributed to significant differences in environmental factors, as soil physicochemical properties have significant impacts on the microbial community structure. For example, Wang et al. (2021) found that soil physicochemical properties were determinant factors driving changes in the number of soil microorganisms of Gastrodia elata. Another reason may be that different fertilizations lead to different degrees of enrichment of microbial communities. The above showed that soil physicochemical properties and fertilizers were the key factors that produce differences in the microbial community structures.

Dry matter is the final form of crop photosynthetic products, and it guarantees an increased crop yield. Dry matter accumulation is closely related to nitrogen supply at different growth stages. The application of nitrogen fertilizer is irreplaceable in crop production and yield enhancement. The cabbage yield in the three treatments with a 50% nitrogen reduction did not significantly decrease compared to the control ( Figure 5 ). This result is consistent with previous research results. Fan et al. (2024) showed that reducing nitrogen between 54% and 80% will not significantly impact the yield of vegetables grown in Southwest China. Therefore, optimizing nitrogen fertilizer application strategies and technologies can maintain or increase vegetable yield, significantly improve nitrogen use efficiency (NUE), and reduce crop nitrogen loss. Our research also showed that BC addition could increase the yield of Chinese cabbage ( Figure 5 ), consistent with the results of Tang et al. (2022). At the same time, PGPR has been proven to promote plant growth (Vejan et al., 2016). In this experiment, the application of PGPR also promoted increased cabbage yield, consistent with previous research findings (Gul et al., 2023).

Our study confirmed that a co-application of BC and PGPR can positively affect the Chinese cabbage yield, and even a 50% nitrogen reduction did not lower the Chinese cabbage output. Therefore, more extensive exploration of the optimal application scheme of BC and PGPR at different scenarios and crops needs to be further studied.