The Yellow River Delta (YRD), located in the warm-temperate zone, is gradually being degraded as one of the important deltaic agricultural economic zones in China (Fang et al., 2005; Luo et al., 2016; You et al., 2021). The saline-alkali land of the YRD is approximately 2,400 km², which is about half of the area. Restricted by salinity stress and nutrient deficiencies, the vegetation structure in this area is simple and biodiversity is low (Luo et al., 2016; Li et al., 2019). Soil microorganisms play an important controller for plant growth and stress tolerance and are often used as an indicator of soil quality within evaluations (Salas-González et al., 2020; Wang et al., 2021). Consequently, appropriate amendments need to be selected which can improve the soil physicochemical and biological properties in order to realize sustainable usage of saline-alkali soil in the YRD.

Organic fertilizers are a positive option for regulating soil properties and plant growth in degraded soil (Le et al., 2016). Application of organic fertilizers in saline-alkali soil offers a better option for increasing organic carbon content in soil and crop yields by providing essential plant nutrients and organic materials (Demelash et al., 2014). Many searches have indicated that the addition of organic fertilizers to soil enhances organic carbon content, soil structure, cation-exchange capacity, and nutrient quality (Le et al., 2016; Gu et al., 2023). Organic fertilizers are also known to stimulate soil microbial biomass (Zhao et al., 2020; Liu et al., 2021), enzyme activities (Plaza et al., 2004; Ge et al., 2009; Insam et al., 2015), and to promote changes in community structure and abundance (Zhao et al., 2016). Nevertheless, organic fertilizers application does not always enhance soil microbial diversity, their impact depends on the duration of application, the source and nature of the fertilizer, soil type, and tillage conditions (Xu et al., 2021). Thus, the impacts of different quantities of organic fertilizer on the soil microbial community should be investigated using organic fertilizers with the goal of reclaiming saline-alkali soils in the YRD.

Biochar has attracted tremendous attention for its soil ameliorating effects, and it enhances carbon storage, soil fertility and quality, and contaminant immobilization and transformation (Zhu et al., 2017). Biochar is an alkaline material, rich in recalcitrant carbon and surface functional groups (Lehmann et al., 2011; Zhou et al., 2019). Because of these reasons, biochar has also been considered as a popular and promising soil conditioner that can improve crop yields (Kang et al., 2018; Zhang et al., 2019). In most researches, biochar has been able to increase the biomass of microorganisms in the soil, with significant changes in microbial community composition possibly explain the biogeochemical impacts of biochar on element cycles, pathogenic bacteria, and crop growth (Lehmann et al., 2011). Several studies have demonstrated that biochar indeed affects soil microbial communities (Chen et al., 2013; Liu et al., 2017; Zhang et al., 2018), and have reported possible effects of biochar on microbial community abundance and structure (Sun et al., 2013). These conflicting results are mainly due to variations in soil type, biochar variety, biochar production conditions and duration, application rate, and time (Dangi et al., 2019). Thus, understanding the soil microbial community and its reactions to diverse biochar amendments will inform a new approach for alleviating poor soil physicochemical and biological properties, and thus provide a practical management approach for sustainable agricultural production in saline-alkali soils.

Wheat (Triticum aestivum) is a leading staple crop and is cultivated all over the world, with approximately 2.1 million km² under cultivation (Meng et al., 2019). Previous study observed that the use of organic fertilizer and biochar could improve soil quality and performance of wheat (Meng et al., 2019). Researches have also indicated that rhizosphere bacteria and fungi can contribute directly to plant growth (Compant et al., 2010; Lehmann et al., 2011). However, in saline-alkali soil, the reaction of the rhizosphere microbial community after organic fertilizer and biochar amendment applications has not been well investigated. Therefore, in this study, the YRD saline-alkali soil was used as the research target to determine the effects of different amounts of organic fertilizer and biochar additions on (i) the nutrient profile and physicochemical characteristics of the soil; and (ii) the rhizosphere bacterial and fungal communities of wheat in the target site. We hypothesized that moderate amounts of organic fertilizer and biochar amendment would enhance soil physicochemical properties and create soil bacterial and fungal communities which alleviate stress induced by saline-alkali soil in the YRD.

Although many researches have explored the effects of biochar and organic fertilizer in agricultural fields, only a few have conducted saline–alkali field experiments (Yao et al., 2017; Gu et al., 2022a, 2022b, 2023). Biochar amendment and application of organic fertilizer generally changes soil quality, such as pH, soil organic carbon, and nutrient content (Lehmann et al., 2011; Luo et al., 2014; Yao et al., 2017). In reports from Yao et al. (2017), the addition of high concentrations biochar in soil has been shown to significantly improve pH and TN. During this research, we found that the biochar and organic fertilizer caused no significant changes in pH and TN, while the TOC content showed significant change, this is like the findings of Gao et al. (2021). This was due to the high background pH of the soil (7.95) for which biochar application resulted in only a slight increase (8.05–8.07), but this result was non-statistically significant (p > 0.05). We found in this research that the EC value of the soil decreases with the addition of biochar and organic fertilizer. Previous study has also showed a similar decrease in soil Ec values when various organic additives were used to remediate the soil (Tejada et al., 2006). Soil AP and AK improved with the addition of biochar, which agreed with Oladele et al. (2019). This may be because some nutrients are produced by the pyrolysis of the biochar during its manufacture, and because the porous structure of the biochar causes it to adhere to more functional groups, which increases its ability to adsorb soil nutrients (Ameloot et al., 2013; Zhang et al., 2019).Organic fertilizer is known to contain nitrogen, phosphorus, and potassium. However, for this research, the addition of organic fertilizer only increased the content of AK, even with a reduced AP content. It is well known that biochar and organic fertilizers can induce variations in the soil environment, such as soil nitrogen, potassium, phosphorus levels, and pH (Sun et al., 2020). The differences between study results may be due to a variety of factors, such as the climate, soil properties, fertilizer, and plants grown (Assefa and Tadesse, 2019).Therefore, many factors need to be considered when analyzing the changes in soil properties.

It was shown that either biochar or fertilizer can change the structure of the soil microbial community and crop performance (Tao et al., 2020; Gao et al., 2021). Biochar can provide a habitat for microbiota through the provision of organic compounds, further mediating changes in microbial population structure and abundance (Gao et al., 2021), and on the other hand, toxic substances such as polycyclic aromatic hydrocarbons (PAHs) present in biochar can adversely affect soil microbial populations, which can lead to alterations in the composition of the fungal community and reductions in its diversity (Zheng et al., 2016; Sheng and Zhu, 2018). In this study, the richness of soil bacterial and fungal communities were not significantly different after biochar treatment compared to CK, but the overall trend showed an increase both in richness and diversity of soil bacterial and fungal communities with the biochar applied (Table 2), and there was a significant increase (p < 0.05) in diversity of soil bacterial community compared to CK under the BM, which were consistent with Yao et al. (2017) and Gao et al. (2021). Tao et al. (2020) showed that the application of organic fertilizer had a significantly higher effect on bacterial richness and diverstiy than CK, but reduced the richness and diversity of fungal species. Sun et al. (2020) also found the same fungal results as Tao et al. (2020). Chen et al. (2021) discovered that organic fertilizer application significantly enhanced the richness and diversity of fungal species, but it had no significant impact on bacteria. Our research results on bacteria were consistent with Chen et al. (2021), while fungi were consistent with Tao et al. (2020). Differences in the effects of organic fertilizer application on bacteria and fungi may be caused by differences in crops, soil conditions, types of organic fertilizer and application methods.

Effects of biochar application on the composition and structure of soil microbial communities have been frequently reported (Zheng et al., 2016; Gao et al., 2021). In this study, the predominant phyla of bacterial community in the wheat soil were Proteobacteria, Actinobacteria, Acidobacteria, Bacteroidota, and Chloroflexi, which fitted to the findings by other researchers also observed in wheat soil (Meng et al., 2019). However, the dominant fungus in this study is the Ascomycota, which is different from the Zygomycota reported by Meng et al. (2019), but consistent with Chen et al. (2021) and Zheng et al. (2016). Yao et al. (2017) found that relative abundances of Acidobacteria were decreased, while the abundances of Chloroflexi were increased with biochar addition. Khodadad et al. (2011) reported that biochar amendment decreased the bacterial diversity but increased the relative abundance of bacterial phyla Actinobacteria and Gemmatimonadetes. Our results indicated that Acidobacteria, Chloroflexi, and Gemmatimonadetes increased with biochar addition, especially BH, whereas Actinobacteria decreased. Wolna-Maruwka et al. (2021) discovered that organic fertilizer increased the abundence of Actinobacteria and Firmicute, but decreased Acidobacteria, which is partially consistent with our results that Gemmatimonadetes and Firmicutes increased with organic fertilizer addition. Acidobacteriota was significantly positively correlated with TOC (p < 0.05) content, it can also partially explain the significant increase in TOC of biochar application compared to CK. Both Ascomycota (copiotrophic) and Basidiomycota (oligotrophic) play important roles in plant growth by improving nutrition and defense against pests (Meng et al., 2019). Yao et al. (2017) reported that biochar addition increased the relative abundance of Ascomycota and decreased the relative abundance of Basidiomycota compared to CK. In this study, the abundance of Ascomycota increased with addition content of biochar and organic fertilizer, but this has not achieved the remarkable level. Basidiomycota, which was significantly positively correlated with the TC (p < 0.05), pH (p < 0.05), AP (p < 0.05), and AK (p < 0.01), also increased with biochar addition and decreased with organic fertilizer addition without statistically significant. This may be one of the reasons why the TC, AP, AK of biochar, especially BH, is higher than those of organic fertilizers. Gao et al. (2021) found that biochar addition significantly increased the relative abundance of Mortierellomycota when compared to CK. In contrast, in this study, whether applying biochar or organic fertilizer, Mortierellomycota significantly decreased, especially when applying organic fertilizer. Differences above may be caused by differences in environment and soil conditions, or types of organic fertilizer and biochar.

The abundance of norank_f__JG30-KF-CM45 increased with the increase of biochar and organic fertilizer concent, and was positively correlated with TP and TOC in biochar and organic fertilizer treatments, respectively, which suggested that norank_f__JG30-KF-CM45 may be beneficial for saline alkali land remediation. This result was consistent with Shen et al. (2022), who thought that norank_f__JG30-KF-CM45 was beneficial genus that might potentially promote tobacco growth. Yao et al. (2022) reported that biochar addition inhibited nitrification in salt-affected irrigation-silting soil by shifting the community structures of AOB and reducing the relative abundance of dominant functional ammonia-oxidizers, such as Nitrosospira. In this study, the addition of biochar had no effect on the abundance of Nitrosospira while the addition of organic fertilizer significantly increased the abundance of Nitrosospira, which was consistent with the result of Lin et al. (2018). Furthermore, we also found that the relative abundance of Bacillus increased with biochar and organic fertilizer application rate, but only organic fertilizer application reached significant level. Bacillus species are commercially marketed as biopesticides, biofertilizers, and soil amendments (Cao et al., 2011). So the increase abundance of Bacillus with biochar (Yao et al., 2017) and organic fertilizer (Tao et al., 2020) addition would be benefit for soil physical and chemical properties. Organic fertilization reduced the relative abundances of some pathogenic genera detected in our study, such as Alternaria, Gibellulopsis, and Cladosporium. A similar effect of organic fertilizers was reported in Semenov et al. (2022) and Lu et al. (2020). In addition, organic amendments have been proposed as a strategy for the management of plant diseases caused by soil-borne pathogens, the suppressive effect is likely related to an increase of pathogen-antagonistic fungi, since the organic fertilizers may act as an alternative C source for the antagonists (Semenov et al., 2022). Cladorrhinum, a potential biological control agent, increased under long-term application of manure (Semenov et al., 2022), Trichoderma, which are known as biocontrol agents against plant pathogens and opportunistic avirulent plant symbionts, which can be parasites and antagonists of many phytopathogenic fungi, thus protecting plants from disease (Vinale et al., 2008), and Humicola, a potential antagonists for the biological control of plant diseases (Ko et al., 2011), all of above genera increased with biochar and organic fertilizer application in our study. Sordariomycetes, which was positive correlation with TOC under biochar and organic fertilizer application in this study, also increased with biochar and organic fertilizer application, and this result was consistent with Ding et al. (2017) and Yu et al. (2018). In a word, biochar and organic fertilizer application in our study had led to an increase in some beneficial and biological control bacteria, as well as a decrease in pathogenic bacteria, which contributes to soil remediation in saline alkali soils.

As it is well known, soil microorganisms do not exist in isolation, but rather coexist and jointly construct complex ecological correlation networks, leading to different important and complex interactions, including but not limited to competition, commensalism, and mutualism (Duan et al., 2021). Therefore, a comprehensive understanding of the interactions between bacteria and fungi is also crucial for improving soil system services (Zhu et al., 2022). In this study, we found that the interactions among organic fertilizer treatments formed more connections than that of biochar treatments, in the meantime, both of the content of TOC and TC were higher under biochar application compared to the organic fertilizer application, which was consistent with De Menezes et al. (2017), who thought fungi-bacteria correlations were stronger in low soil organic matter soils. Highly connected taxonomic groups in the co-occurrence network typically significantly influence microbial community structure and function irrespective of their abundance across time and space (Banerjee et al., 2018). In our study, the nodes with the highest connections were Sarocladium, Devosia, and Streptomyces in organic fertilizer application, with a degree of 7, and Unclassified_c__Sordariomycetes in biochar application, with a degree of 9, indicating that these genera were relatively active in the interaction network between bacteria and fungi under biochar and organic fertilizers application, and whether they have an impact on soil physicochemical properties and plant growth deserves further study.

This study provides evidence that the application of biochar and organic fertilizer has different effects on soil physicochemical properties and microbial community structure. The application of biochar and organic fertilizer significantly reduced Ec and increased nutrient content of saline-alkali soils of the YRD. pH and TP were crucial contributors in regulating the bacterial and fungal community distribution. Sphingomonas, norank_f__Geminicoccaceae, and Skermanella were the dominant bacteria genera in biochar application, while Arthrobacter and Sphingomonas were the most abundant bacteria genera in organic fertilizer application. Whether biochar or organic fertilizer application, Chaetomium and Gibberella were the most dominant fungal genera. Microbial network analysis showed that bacterial and fungal communities responded differently to biochar and organic fertilizer treatments. Compare to biochar treatments, organic fertilizer treatment increasing the complexity of bacterial communities and decreasing the complexity of fungal communities. The results of the current study provides evidence that biochar and organic fertilizer treatments affect microbial communities differently and provides new insights to remediation of saline-alkali land of the YRD. The combined application of biochar and organic fertilizer and its long-term effects need further study.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/Supplementary material.

ML contributed to the conception of the study and wrote the manuscript. YG and XG also contributed to the conception of the study. CC, HZ, and JL contributed to field investigation and sample acquisition. ZW, XL, and KY performed the experiments and data analyses. NS contributed to the editing and revising of the manuscript. All authors contributed to the article and approved the submitted version.