Effects of compound microbial fertilizer combined with potassium fulvic acid on soil physicochemical properties, microbial community structure, and cotton productivity in saline-alkali soil

Salinization is a serious challenge for agriculture in arid and semi-arid regions, severely restricting crop production and threatening global food security. It is estimated that the global land area affected by salinization exceeds 950 million hectares and continues to grow at about 10% per year¹. Saline-alkali soil typically exhibits high salinity, abnormal pH, and low nutrient availability^1,2. These factors jointly inhibit the normal growth and development of plants. Therefore, the development of efficient and environmentally friendly saline-alkali improvement strategies is of great significance for achieving sustainable land use and ensuring food security.

In recent years, compound microbial fertilizer has emerged as a research hotspot in agricultural ecological improvement due to its comprehensive role in regulating soil ecological functions, promoting crop growth, and enhancing stress resistance^3,4. This kind of fertilizer is mainly composed of plant-promoting rhizospheric bacteria (PGPR), arbuscular mycorrhizal fungi (AMF) and other functional microorganisms^5,6 By constructing a synergistic micro-ecosystem, soil nutrient transformation and crop physiological response are systematically regulated: on the one hand, soil nutrients are activated and their bioavailability is improved through metabolic activities such as nitrogen fixation, phosphorus and potassium dissolution; on the other hand, it can directly promote crop growth and inhibit soil-borne diseases by secreting plant hormones (such as auxin, cytokinin) and antibacterial substances, thereby enhancing crop adaptability to saline-alkali and other stresses⁵. Studies have shown that strains such as Lactobacillus acidophilus and Corynebacterium glutamicum have outstanding performance in saline-alkali improvement⁷.

In view of the deterioration of the physical and chemical properties of saline-alkali soil, microbial agents have shown various beneficial effects².In terms of soil chemical properties, microbial agents can neutralize alkalinity through microbial metabolites (such as organic acids) and reduce soil pH⁷, electrical conductivity, and water-soluble salt content⁸. At the same time, through microbial residues and metabolic processes, soil organic matter^7,8 and available nitrogen, phosphorus, potassium, and other nutrient content^3,9,10,11 were systematically improved, thereby realizing the systematic improvement of the chemical properties of saline-alkali soil^2,7,12,13. In terms of soil’s physical and biological characteristics, microbial agents can secrete extracellular polymers, promoting the formation and stability of soil aggregates². At the same time, it can increase the activity of key enzymes such as soil urease and phosphatase¹ enhance the function of the biochemical cycle, and create a favorable growth environment for crop roots.

In addition, microbial fertilizer can enhance plant salt tolerance by regulating rhizosphere microecology. The application of microbial agents can increase the number of soil microorganisms and biomass carbon¹⁴, significantly increase bacterial and fungal diversity^13,15 and change the microbial community structure¹³. It is usually manifested as an increase in the abundance of beneficial bacteria, such as Bacillus and Pseudomonas, which, in turn, helps plants resist salt stress and promote nutrient absorption⁵. Some studies have further revealed the mechanisms by which microbial agents promote plant growth by regulating microbial communities. For example, the combination of compost and soil conditioner can improve the versatility of the saline-alkali land ecosystem¹⁶. Planting Spartina can increase bacterial diversity and soil organic matter¹⁷. Arbuscular mycorrhizal fungi (AMF) can improve maize yield and soil microbial community⁶. In addition, the synergistic use of biochar¹⁸, humic acid, and earthworm manure [2] with microbial agents significantly improves soil quality and crop yield on saline-alkali land.

As an important economic crop, cotton (Gossypium hirsutum L.) is crucial to the global textile industry and farmers ‘ income. However, in the main cotton areas, such as Xinjiang, saline-alkali soils severely limit cotton productivity¹⁹. Under saline-alkali stress, cotton often shows low seedling emergence rates, slow plant growth, decreased yield, and reduced fiber quality. Especially in saline-alkali cotton fields using’ dry sowing and wet emergence’ technology, soil quality degradation further aggravates seedling emergence obstacles⁴. Therefore, it is very important to establish a technical system to effectively improve cotton seedling emergence rate and growth in saline-alkali land for the sustainable development of the cotton industry.

The purpose of this study was to systematically explore the effects of compound microbial fertilizer (with Lactobacillus acidophilus and Corynebacterium glutamicum as the main components) on cotton seedling emergence rate and soil microbial diversity in saline-alkali land. The effects of different ratios of bacterial fertilizer on cotton emergence under salt stress were evaluated in a field plot experiment. High-throughput sequencing technology was used to analyze the changes in soil microbial community structure and diversity. At the same time, the study will explore the regulation of microbial agents on the cotton rhizosphere microenvironment and on soil physical and chemical properties, to clarify the mechanism by which they promote seedling growth. The research results can provide a theoretical and technical basis for the application of compound microbial fertilizer in cotton production on saline-alkali land and also offer new ideas for the sustainable utilization of saline-alkali land resources and the improvement of crop productivity.

Experimental material

The tested cotton variety was the locally main-cultivated cotton variety 43,541 (the cotton cultivar used in this study was 43541, a major locally cultivated variety in Xinjiang Uygur Autonomous Region, China). The Xinjiang Academy of Agricultural Sciences, Urumqi, China, provided cotton seeds. The test site is in a cotton field on 11 continuous saline-alkali land in the 29th Regiment of Tiemenguan City, Xinjiang. The soil type of the test site is salinized loam. The main physical and chemical properties are as follows: pH 8.11, water-soluble salt content > 10 g/kg, soil organic matter content 21.33 g/kg, total nitrogen 0.38 g/kg, available phosphorus 57.80 mg/kg, available potassium 586.73 mg/kg, alkali-hydrolyzable nitrogen 48.93 mg/kg, soil cation mainly Ca²⁺ and Na⁺, and anion mainly SO₄²⁻and Cl⁻.

Zhejiang Senjing Biotechnology Co., Ltd, provided the compound microbial fertilizer. The main components contained probiotics such as Lactobacillus acidophilus and Corynebacterium glutamicum (effective viable count ≥ 0.5 × 10⁸CFU/mL, N+P₂O₅+K₂O = 6.0%) and metabolites, organic matter, etc. Potassium fulvic acid is a commercially available product.

Experimental design

This experiment was carried out in the saline-alkali cotton field of 29 regiment in Tiemenguan City, Xinjiang in 2023–2024 for two consecutive years. The experiment was conducted in a randomized block design with 7 treatments and 3 replicates. The plot area was 0.033 hm², the plant spacing×narrow row spacing×wide row spacing was 10 cm×10 cm×66 cm, and protective rows were set around each plot. The cultivation and management measures of all plots are completely consistent : sowing on April 18–22 and harvesting on September 25–30 ; the total irrigation amount was 4200 ~ 4500 m³·hm^− 2 during the whole growth period, and the total irrigation was 8 ~ 10 times. The irrigation time and irrigation amount of each plot were exactly the same. The same base fertilizer and topdressing were applied uniformly in all plots, as shown in Table 1.

Test method

Cotton sowing and emergence rate, yield statistics

Sowing at the local conventional sowing date, the emergence of cotton was observed regularly after sowing, the number of seedlings in each plot was counted at the seedling stage, and the emergence rate was calculated (emergence rate=actual number of seedlings/planned number of sowing×100%).

After cotton matures, 15 cotton plants were randomly selected by the five-point sampling method in each plot, and the number of bolls per plant was counted and recorded. During the harvest period, four normal bolls were randomly selected from each sampling point (a total of 20 plots). After removing impurities from cotton bolls, the single-boll weight was determined using an accurate (to 0.01 g) electronic balance. According to the actual yield of the plot, the yield increase rate (yield increase rate= (treatment group yield-control group H yield)/control group H yield×100%) was calculated.

Soil sample collection and microbial diversity sequencing

After cotton emergence, rhizosphere soil at 0–20 cm depth in each plot was collected using the five-point sampling method. After removing plant residues, stones, and other impurities, they were placed in sterile collection bags and transported back to the laboratory. Some soil samples were stored at -80℃ in a refrigerator for microbial diversity analysis. The other part was used to determine the soil’s physical and chemical properties after natural air drying.

Soil microbial genomic DNA was extracted using the Tiangen Biochemical DP336-02 soil genomic extraction kit, in strict accordance with the kit instructions. After the extracted DNA was tested for purity and integrity, it was sent to Shanghai Meiji Biomedical Technology Co., Ltd. For high-throughput sequencing. Bacterial sequencing used 16 S rRNA gene V3-V4 region primers (338 F: 5’-ACTCCTACGGGAGGCAGCAG-3’, 806R: 5’-GGACTACHVGGGTWTCTAAT-3’); fungal sequencing was performed using ITS1 region primers (ITS1F:5’-CTTGGTCATTAGAGGAAGTAA-3’, ITS2R: 5’-GCTGCGTTCTTCATCGATGC-3’).

Determination of soil physical and chemical properties

Soil pH was measured using a potentiometric method at a soil-to-water ratio of 1:5. Water-soluble salt content was determined by the gravimetric method. Soil anions (HCO₃⁻, Cl⁻, SO₄²⁻) were analyzed by ion chromatography, and cations (Ca²⁺, Mg²⁺, K⁺, Na⁺) were determined using an atomic absorption spectrophotometer. All soil samples were air-dried and sieved through a 2 mm mesh. All results were expressed on an oven-dried soil basis. Standard reference materials and strict quality control were applied, and all analyses were performed in laboratory duplicates. Electrical conductivity (EC), sodium adsorption ratio (SAR), exchangeable sodium percentage (ESP), and exchangeable Na⁺ were not determined in this study.

Data analysis

After the sample sequence was aligned to the minimum sequence number, low-quality sequences and chimeras were removed using QIIME 1.9.1 to obtain the effective sequence. The operational taxonomic units (OTUs) were clustered by Usearch 11 software, and the clustering standard was nucleotide sequence similarity > 97%. The representative sequences of OTUs were annotated using the Silva database (http://rdp.cme.msu.edu/), and the confidence threshold was 70%. The Alpha diversity indices (Shannon, Chao, and Coverage) were calculated using Mothur 1.30.2. A Venn diagram and a bar diagram were drawn using R language (version 3.3.1), and RDA analysis and PCA analysis were performed using R language (version 2.4.3).

The data on cotton emergence rate, yield, and soil physical and chemical properties were statistically analyzed using SPSS 22.0. The differences between treatments were compared using one-way analysis of variance (ANOVA), and the Duncan multiple-comparison method was used for the significance test (P < 0.05).

The effects of treatment, soil depth, year, and their interactions on soil physicochemical properties were analysed using two-way ANOVA. Beta diversity was assessed through principal coordinate analysis (PCoA) based on Bray–Curtis distance, while PERMANOVA was conducted to evaluate the impact of treatment on microbial community structure. Redundancy analysis (RDA) was utilised to elucidate the relationships among microbial communities, soil properties, and cotton yield. Dominant microbial taxa underwent a differential abundance analysis. Pearson correlation analysis was carried out to examine the associations between key genera and soil physicochemical properties, as well as cotton growth and yield indices. Normality was assessed prior to ANOVA using the Shapiro–Wilk test, and homogeneity of variance was evaluated using Levene’s test. All data met the assumptions of normality and homogeneity of variance. The independence of observations was ensured through a randomized block design and independent plot sampling. Percentage data, such as emergence rate, were not transformed as they satisfied the requirements of normality and homoscedasticity.

Effects of compound microbial fertilizer on the emergence rate and yield of cotton

Cotton emergence rate is a key index to reflect the initial state of cotton growth in saline-alkali land. As shown in Fig. 1 the application rate of compound microbial fertilizer was positively correlated with the emergence rate of cotton. Among them, the emergence rate in the HT3 treatment group (80.56%) was significantly higher than that in the control H treatment group (76.67%). The emergence rate in the HT5 treatment group was 85%, which was significantly higher than those of the control H treatment group (78.56%) and the S treatment group, indicating that an appropriate amount of compound microbial fertilizer could effectively alleviate the inhibitory effect of saline-alkali stress on cotton emergence and improve emergence quality.

Cotton yield and its components (boll number per plant and boll weight) are the core indicators for measuring the application effect of compound microbial fertilizer. From Table 2 it can be seen that after the application of compound microbial fertilizer, the number of bolls per plant and single boll weight of cotton increased to varying degrees, and the number of bolls per plant of S treatment group and HT5 treatment group was significantly higher than that of control H treatment group; the single boll weight of the S, HT1, HT2, HT3 and HT4 treatment groups was significantly higher than that of the control H treatment group. In terms of yield, each treatment group showed an increasing trend compared with the control H treatment group, but the increase was different: the S treatment group increased by 9.35% compared with the H treatment group; HT4 treatment group had the best yield increase effect, with a yield increase rate of 13.31%. The increase rate of HT5 treatment group was 3.97%. The results showed that a single application of the compound microbial fertilizer or a combined application with potassium fulvic acid could effectively increase cotton yield, and the effect of a reasonable combined application was better.

Analysis of soil bacterial diversity

OTU number and alpha diversity index of soil bacterial community

The total number of OTUs reflects the species richness of soil bacterial communities. As shown in Fig. 2, compared with the control group H, the total number of bacterial OTUs in the HT1, HT3, and HT4 treatment groups decreased slightly, whereas those in the HT2 and HT5 treatment groups increased slightly. However, the overall change was small, indicating that the compound microbial fertilizer had a relatively mild effect on soil bacterial species richness.

The alpha diversity index is an important measure of the diversity of the soil microbial community. The Shannon index reflects community diversity, and the Chao index characterizes species richness; the Simpson index reflects community diversity through the concentration of dominant species (the larger the index value, the lower the community diversity). As shown in Fig. 3 compared with the control group H, the Shannon and Chao indices of HT2 and HT5 increased slightly, while those of HT1, HT3, and HT4 decreased slightly, but there was no significant difference between any treatment group and the control group. The Simpson index results showed that, compared with the H group, there was no significant fluctuation in the HT1 and HT2 groups; the index in the HT3 group was significantly increased (corresponding to a decrease in community diversity). In contrast, the Simpson index in the HT4, HT5, and S groups decreased slightly (indicating an increase in community diversity), suggesting that when the application amount of compound microbial fertilizer reached a specific level, its effect on community diversity was more pronounced. Overall, there was no significant difference in Shannon and Chao indices between all treatment groups and the control group, and only some treatment groups showed slight fluctuations, further indicating that the effect of compound microbial fertilizer on soil bacterial diversity was relatively gentle.

Analysis of soil bacterial community composition

After applying the compound microbial fertilizer, the microbial community composition of each sample changed. At the phylum level, the soil bacterial community of each sample was mainly composed of Proteobacteria, Chloroflexi, Actinobacteriota and Myxococcota. The cumulative relative abundance of these four types of bacteria accounted for more than 70% of the total bacterial community, making them the dominant group in saline-alkali soil. At the family level, the soil bacterial community was mainly composed of Bacillaceae, Micrococcaceae, and Pseudomonadaceae. Using the OTU abundance of the family ≥ 500 as a threshold analysis, it was found that the bacterial community composition changed selectively after the application of compound microbial fertilizer: the relative abundance of Pseudomonadaceae, Planococcaceae, and Flavobacteriaceae in the HT4 and HT5 treatment groups decreased significantly; the relative abundance of Sphingomonadaceae, Vicinamibacterales, and JG30-KF-CM45 increased slightly (Fig. 4).

At the genus level, each sample was mainly composed of Pseudarthrobacter, Vicinamibacterales, and so on. Compared with other treatment groups, the relative abundance of some beneficial bacteria (such as Sphingomonas) in the HT4 treatment group was higher, which may be related to the highest yield in this treatment. From the perspective of time dynamic change (Fig. 5), after 1 year of application of compound microbial fertilizer, the relative abundance of Pseudarthrobacter and Bacillus in the bacterial community was higher; after 2 years of application, the relative abundance of Vicinamibacterales, Actinomarinales, and other groups increased, indicating that the effect of compound microbial fertilizer on the bacterial community had a certain time continuity.

Analysis of soil fungal diversity

OTU number and alpha diversity index of soil fungal community

Compared with the control group H, the treatment group showed slightly different species numbers at each taxonomic level. After analyzing the total number of OTU in each sample, it was found that the total number of OTU in each treatment group after the application of compound microbial fertilizer was significantly higher than that in the control group H (Fig. 6), indicating that the application of compound microbial fertilizer affected the composition of fungal communities in the environment, which could significantly increase the species richness of soil fungi.

As shown in Fig. 7, the Shannon and Chao indices for fungi in each treatment group were higher than those in the control group H. Among them, the increase in HT4 and HT5 treatment was more pronounced, and there was a significant difference between the treatment and control groups. The Simpson index showed the opposite trend. The Simpson index for each treatment group was lower than that of the control group (H), and the decrease in the HT4 and HT5 groups was more pronounced, which was also significantly different from that of the control group. The results showed that the compound microbial fertilizer had a significant promoting effect on soil fungal diversity.

Analysis of soil fungal community composition

After applying the compound microbial fertilizer, the microbial community composition of each sample changed. At the phylum level, the fungal community of each sample was mainly composed of Ascomycota, Mortierellomycota, and Basidiomycota. Among them, Ascomycota was dominant in the microbial community, accounting for 60%-80% of the relative abundance. It should be noted that there is a significant error in the HT3 treatment group. The proportion of Ascomycota in HT3 is about 60%, which is significantly lower than that in other repetitions in the same treatment, while the proportion of Basidiomycota is about 30%, which is significantly higher, suggesting that the heterogeneity of the soil microenvironment may be related.

At the family level, the fungal community is mainly composed of Pseudeurotiaceae, Pyronemataceae, and Microascaceae, with Pseudeurotiaceae dominant. Compared with the control group, the relative abundance of Pyronemataceae and Aspergillaceae decreased significantly in most treatment groups, whereas that of Microascaceae and Mortierellaceae increased slightly (Fig. 8).

At the genus level, the samples were mainly Pseudeurotium, Lophotrichus, Cladosporium, etc. (Fig. 9). Among them, the HT4 treatment group had the highest abundance of Pseudeurotium, and the relative abundance of beneficial fungal groups such as Mortierella was also higher than other treatments, which may be closely related to the best effect of this treatment on improving soil and promoting cotton growth.

Effects of compound microbial fertilizer on soil physical and chemical properties

Effect on soil pH value

Soil pH is a key factor affecting crop growth and nutrient availability in saline-alkali land. The results showed that the pH change trend in the 0 ~ 20 cm and 20 ~ 40 cm soil layers was essentially the same. Compared with the control group H, the soil pH of each treatment group decreased slightly, and the pH of the HT5 treatment group decreased the most, reaching more than 0.1 (Fig. 10). This indicates that the probiotics in the compound microbial fertilizer produce organic acids through reproduction and metabolism, which can effectively improve the weak alkaline soil, improve the availability of soil nutrients, and create a suitable acid-base environment for cotton growth.

Effect on soil water-soluble salt

The tested soil belongs to extremely severe salinization soil (water-soluble salt content > 10 g/kg), which seriously inhibits the growth of cotton roots and shoots. The results showed that the variation trend of water-soluble salt content in 0 ~ 20 cm and 20 ~ 40 cm soil layers was completely consistent. Compared with the control group H, the soluble salt content decreased in most treatment groups (Fig. 11), indicating that the compound microbial fertilizer could effectively reduce soil salinization. The reasons mainly include two aspects: first, the accumulation of bioactive organic matter in soil under the action of microorganisms increases, which enhances the ability of soil to inhibit salt and return salt; second, the improvement of soil nutrient availability, combined with the stimulation of microorganisms and their metabolites, the total amount of nutrients absorbed by cotton increased, which indirectly reduced the accumulation of soluble salt in soil.

Compared with control H, HT5 significantly decreased soil pH by 0.12 ± 0.03 units (P < 0.05) and reduced water-soluble salt content by 12.5% in the 0–20 cm layer and by 10.8% in the 20–40 cm layer. Similarly, Cl⁻ and Na⁺ contents were reduced by 18.3% and 15.6%, respectively. The order of improvement was HT5 > HT4> HT3 > HT2> HT1 > S > H (P < 0.05).

The effect on soil anions(HCO₃⁻、Cl⁻、SO₄²⁻)

The changes in anion content (HCO₃⁻、Cl⁻、SO₄²⁻) of each sample at different soil depths were analyzed. It was found that the trend of anion content in the 0 ~ 20 cm and 20 ~ 40 cm soil layers was essentially the same. The content of anions in soil is SO₄²⁻ > Cl⁻ > HCO₃⁻. Compared with the control group (H), the concentrations of HCO₃⁻ and SO₄²⁻ in each treatment group did not change significantly. In contrast, the content of Cl⁻ decreased significantly, and the decrease was positively correlated with the application amount of compound microbial fertilizer (Fig. 12). This indicated that the effect of compound microbial fertilizer on reducing soil water-soluble salt was mainly related to the decrease of Cl⁻ content.

Effects of soil cations (Ca²⁺, Mg²⁺, K⁺, Na⁺)

The changes in cation content (Ca²⁺, Mg²⁺, K⁺, Na⁺) for each sample at different soil depths were analyzed, and it was found that the trends at 0 ~ 20 cm and 20 ~ 40 cm were essentially the same. The cation content in the soil is Ca²⁺ > Na⁺ > Mg²⁺ > K⁺. Compared with the control group H, there was no significant change in the content of K⁺ in each treatment group, and the contents of Na⁺, Mg²⁺, and Ca²⁺ changed to varying degrees. When the application rate of compound microbial fertilizer was ≥ 60 L·hm^− 2, the content of Na was significantly reduced (Fig. 13), indicating that the effect of compound microbial fertilizer on reducing soil water-soluble salt was also closely related to the decrease of Na content, which was of great significance to alleviate the damage of cotton salt stress.

Cotton emergence and yield response to microbial fertilizer

Soil salinization and high pH in saline-alkali land are serious challenges for global agriculture, severely restricting the growth and yield of crops such as cotton and threatening food security and sustainable agricultural development^12,20. The soil microbial community is a core component of the soil ecosystem, and its structure and functional stability are directly related to soil fertility, nutrient cycling, and crop health^21,22. In view of this, compound microbial fertilizer is an important eco-friendly strategy for improving saline-alkali land and promoting crop growth²³.

An interesting phenomenon observed in this study is that HT5 showed the highest cotton emergence rate, while HT4 achieved the highest seed cotton yield. This indicates that the highest application rate of compound microbial fertilizer is not optimal for final cotton productivity. A high dose of microbial fertilizer can effectively alleviate saline-alkali stress and improve seed germination and seedling emergence at the early growth stage. However, excessive microbial input may intensify rhizosphere microbial competition, cause nutrient immobilization, or fail to further promote reproductive growth and boll development. In contrast, HT4 (120 L·hm⁻² microbial fertilizer + 15 L·hm⁻² potassium fulvic acid) achieves a better balance among microbial functional activity, soil chemical improvement, nutrient release, and plant uptake. In addition, emergence rate only represents the early growth ability of cotton under stress, whereas the final yield is comprehensively determined by boll number per plant, boll weight, and reproductive efficiency, which further explains the inconsistency between emergence rate and yield among treatments. This result suggests that rational matching of compound microbial fertilizer and potassium fulvic acid is more important than simply increasing the application rate for high-yield cotton production in saline-alkali soil.

Studies have shown that compound microbial fertilizer can significantly increase cotton emergence rate and yield on saline-alkali land²⁴. This finding is consistent with the general law that microbial fertilizer improves saline-alkali land and promotes crop growth²³.

Improvement of saline-alkali soil chemical properties

Probiotics in compound microbial fertilizers, such as Lactobacillus acidophilus and Corynebacterium glutamicum, play a role through a variety of mechanisms: (1) Functional strains may produce organic acids during metabolism, which could help neutralize soil alkalinity and potentially reduce soil pH^25,26. The decrease of soil pH helps to improve the availability of phosphorus, iron, and other nutrients in the soil, making it easier for plants to absorb and utilize²²; (2) Probiotics may form biofilms during reproduction²⁷, which could potentially contribute to immobilizing or reducing the availability of Na⁺ and Cl⁻ in soil²⁸. Through adsorption, the direct toxicity of these ions to cotton roots can be reduced, thereby increasing the seedling emergence rate and seedling survival rate of cotton^29,30; (3) Probiotics may potentially secrete plant hormones such as auxin and cytokinin, which could contribute to promoting cotton bud differentiation and boll development, increase boll number per plant and single boll weight, and ultimately increase yield³¹. This study found that the yield increase effect was the best in the HT4 treatment group. This shows that the combined application of compound microbial fertilizer and potassium fulvic acid has a significant synergistic effect²⁵. Potassium fulvic acid may help improve soil aggregate structure and potentially¹², enhance soil water and nutrient retention capacity^32,33, and provide a more suitable microenvironment for the growth and reproduction of probiotics³³, thereby enhancing the improvement effect of compound microbial fertilizer. Improving soil aggregate structure is very important for stabilizing soil organic matter and the microbial community^34,35.

Selective response of bacterial and fungal communities

Soil microbial diversity is an important indicator of soil ecosystem health^21,22. This study found that the effect of compound microbial fertilizer on soil bacterial diversity was relatively mild, but it significantly increased fungal diversity, which may be related to fungi’s high sensitivity to soil environmental changes²¹. The greater sensitivity of fungi compared to bacteria may be ascribed to their stronger dependence on organic carbon sources and wider niche adaptation to altered soil pH and salinity. The reduced pH and decreased water-soluble salts provided a more favorable habitat for fungal propagation. Meanwhile, potassium fulvic acid supplied labile carbon substrates that effectively promoted the growth of saprophytic fungi. The enriched beneficial fungi such as Mortierella and Pseudeurotium contributed to nutrient mineralization, nutrient cycling, and root growth promotion. The increased fungal diversity was not directly linearly correlated with cotton yield but indirectly enhanced yield by improving the rhizosphere microenvironment, soil physicochemical properties, and nutrient availability. After applying a compound microbial fertilizer, the relative abundance of beneficial microbial groups, such as Sphingomonas and Mortierellaceae, increased in the soil. These microbial groups have multiple ecological functions, including the degradation of soil pollutants, the secretion of plant growth hormones, and the inhibition of pathogenic bacteria, thereby further improving the soil microecological environment and promoting cotton growth³⁶. For example, soil microbial communities are influenced by environmental drivers, microbial-mediated interactions, and complex relationships among microbial processes. Among them, soil characteristics, climate change, and abiotic stresses (such as salinity) all significantly affect microbial communities. The application of compound microbial fertilizer alters microbial community structure by regulating environmental factors²¹. At the same time, the relative abundance of some groups, such as Pseudomonas and Pyronemataceae, decreased, which may be because the compound microbial fertilizer altered environmental conditions, such as the soil carbon-nitrogen ratio and pH³⁷, thereby inhibiting the growth of these microorganisms²². In addition, with the extension of application time, the composition of the soil microbial community continues to change, indicating that the compound microbial fertilizer has a long-term effect on regulating the soil microecological system, which is of great significance for maintaining soil ecological balance and sustainability³⁸.

Integrated soil–microbe–plant mechanism

The compound microbial fertilizer has a significant effect on improving the physical and chemical properties of saline-alkali land. It can effectively reduce the content of water-soluble salt in soil, especially the removal effect of Na⁺ and Cl⁻ is obvious, and the improvement effect is positively correlated with the application amount^12,39, which is mainly attributed to the metabolic activity of probiotics in compound microbial fertilizer. The organic acids produced by probiotics can not only neutralize the alkalinity of the soil, but also combine with Na⁺ in the soil to form soluble sodium salts²⁵. These soluble sodium salts infiltrate into the deep soil under the action of irrigation leaching, reducing the accumulation of salt in the surface soil²⁰. In addition, the increase of organic matter content improves the adsorption capacity of soil colloid and enhances the fixation of harmful ions in soil⁴⁰, thus reducing the degree of soil salinization⁴¹. It is worth noting that the improvement effect of compound microbial fertilizer on 0 ~ 20 cm and 20 ~ 40 cm soil layers is basically the same, which shows that it has good diffusivity in the soil profile, can comprehensively improve the soil environment of the cotton root distribution area, and provide more healthy growth space for roots²⁶.

Limitations and future research

Although compound microbial fertilizer has shown great potential for improving saline-alkali land and promoting cotton growth, it still has some shortcomings. and limitations in this study. Specifically, no blank control group (CK: 0 microbial fertilizer + 0 potassium fulvic acid) was established in this experiment. And this study has not yet explored the effects of compound microbial fertilizer on soil microbial functional genes, lacks dynamic monitoring of physiological and biochemical indicators during cotton growth, and has a relatively short test cycle. Future research can further conduct long-term positioning experiments, combined with advanced technologies such as metagenomics and transcriptomics, and in-depth analysis of the molecular mechanisms of compound microbial fertilizer to improve saline-alkali land. This will help clarify how changes in microbial community structure and function affect soil nutrient cycling and plant physiological metabolism, providing more comprehensive and in-depth theoretical support for the optimal application of compound microbial fertilizer and the improvement of saline-alkali land.

In this study, the effects of compound microbial fertilizer on the emergence rate, yield, soil microbial diversity, and the physical and chemical properties of cotton in saline-alkali land were investigated using a field experiment system. The following conclusions were drawn: (1) The application of compound microbial fertilizer can significantly improve the emergence rate and yield of cotton in saline-alkali land. The emergence rate of HT5 treatment was the highest (85.00%), and the yield increase effect of HT4 treatment (compound microbial fertilizer 120 L·hm^− 2 + potassium fulvic acid 15 L·hm^− 2) was the best (yield increase rate 13.31%), and the compound microbial fertilizer and potassium fulvic acid had synergistic effect. (2) Compound microbial fertilizer can effectively improve the physical and chemical properties of saline-alkali soil, reduce soil pH value, water-soluble salt content and Cl⁻ and Na⁺ contents, and the improvement effect is positively correlated with the application amount, and the improvement effect of 0 ~ 20 cm and 20 ~ 40 cm soil layer is consistent; (3) The compound microbial fertilizer has a selective regulation effect on soil microbial diversity: it has a mild effect on bacterial diversity and only causes a slight change in the relative abundance of some families and genera; however, it can significantly improve fungal diversity and species richness, and promote the growth of beneficial fungal groups such as Pseudeurotium and Mortierella. (4) By improving soil physical and chemical properties and regulating soil microbial community structure, compound microbial fertilizer creates a suitable soil environment for cotton growth, thereby alleviating the damage of saline-alkali stress to cotton and improving cotton production efficiency. Therefore, the compound microbial fertilizer serves as a promising technical approach to boost cotton growth and yield in Xinjiang saline-alkali farmland. The combination of 120 L·hm⁻² compound microbial fertilizer and 15 L·hm⁻² potassium fulvic acid shows preferable performance in this study. Further multi-year and multi-location trials are needed to confirm its applicability before large-scale field promotion.