3.1 Physical and chemical properties of compost
The temperature variations in the study are depicted in Fig. 1a. The ambient temperature range during the composting process is 16.1–36.1°C. In all composting treatments, the temperature of the compost material passes through three typical phases: mesophilic, thermophilic, and curing. At the beginning of composting, the temperature of the compost increases dramatically due to intense microbial degradation of organic matter and rapid microbial multiplication, and all treatments enter the thermophilic phase (50°C) on days 5 and 6, with the PN-VR treatment reaching a peak temperature of 57.9°C on day 9. The thermophilic phase should be maintained for at least 3 days to ensure that the compost product is sterilized, and in this phase, the PKN-VR treatment lasted for a maximum of 6 days and the CK treatment lasted for a minimum of 4 days. This indicates that microbial metabolic activity varies in response to changes in temperature. However, the temperatures of the treatments begin to decrease after reaching the peak and reach the ambient level, entering the stabilization stage at 35 days. This indicates that the degradable organic matter is almost exhausted. At the end of composting, the cumulative temperature of the PKN-VR treatment (2061.3°C) is 5.82% higher than that of the CK group (2458.1°C). Additionally, the temperature of the PKN-VR treatment is significantly higher than that of the other treatments in the curing stage (Fig. 1b). As shown in Fig. 1c, which illustrates the change in moisture content during the composting process, the moisture content follows a similar pattern in all treatments. Initially, during the first 0–3 days of composting, there is a rapid decrease in moisture content. This decrease helps to speed up the compost maturation time. Then, from days 3–7, the moisture content decreases quickly due to the significant growth of microorganisms and the thermophilic stage of composting. After the 14th day, the rate of moisture content decline slows down as microbial activity decreases. By the end of the composting process, the moisture content stabilizes at around 30% in all treatments. Figure 1d shows the color change during composting of vinegar residue waste, which generally turns darker due to the evolution of humic substances from the decomposition of organic matter. It can be seen that the vinegar residue composting starts with yellow color, on the 3rd day of composting the vinegar residue shows the appearance of white mycelium, which indicates that the microorganisms are multiplying in large quantities, and then it gradually changes to brown, and then to dark brown after 28 days, which indicates that the organic matter has been decomposed in large quantities, and there is no significant change thereafter.
Throughout the composting process, the pH of the six treatments changes drastically with an increasing trend in pH, which gradually stabilizes after 28 days (Fig. 2a). Upon completion of composting, the final pH of the six composts ranges from 7.40–7.70. At the completion of composting, the final pH range of the six composts is 7.40–7.70. Figure 2b shows the electrical conductivity (EC) during composting. The EC values of the six treatments show a similar trend. A partial decrease in EC values is shown on days 0–3, which is mainly attributed to the volatilization of ammonia as well as to mineral salts and may be partly attributed to the production of leachate. Finally, the EC values (0.65–1.89 dS/m) of the compost products of all six treatments are below the recommended threshold (4 dS/m), indicating that there is no phytotoxicity in the final product, and all of them meet the compost maturity criteria.
As shown in Fig. 2c, the changes in GI are similar in different treatments, and all of them show an increasing trend. According to the newly revised Chinese organic fertilizer standard (NY525-2021), which stipulates the requirement of GI ≥ 70%, the toxicity of the compost material is relatively low when the GI value is > 50%, and when the GI value is > 90%, it indicates that the compost is mature and almost non-phytotoxic (Yang et al., 2021a). At the end of the composting process, GI values in all treatments ranged from 103.5–115.0%. The addition of microorganisms to the treatments results in higher GI values compared to the CK treatment. This may be due to the accelerated biodegradation of toxic substances in the composting material by microorganisms. The E4/E6 ratios of all treatments decrease at the beginning of composting due to the production of organic acids, tend to increase when entering the thermophilic phase, and then gradually decrease until the compost matures (Fig. 2d). Throughout the process, the highest E4/E6 ratio is observed in the NK-VR treatment, indicating a low degree of humification in this compost. The lowest E4/E6 ratio is observed in the PKN-Car-VR treatment, which indicates a high degree of condensation of humic aromatic nuclei. The E4/E6 ratios of all treatments are in the range of 3.81–4.53 at the beginning of the composting but decreased to 2.92–3.47 at the end of the composting (an average decrease of 23.4%), indicating that the composted products have a certain degree of humification or putrefaction.
As shown in Fig. 2, the changes in NH4+-N and NO3−-N content follow a similar trend in all treatments. At the beginning of composting, the concentrations of NH4+-N and NO3−-N gradually increase. The volatilization of ammonia at high temperatures as composting progresses leads to a gradual decrease in NH4+-N and NO3−-N content, which stabilizes at the end of composting. At the end of composting, NH4+-N and NO3−-N contents decrease to 0.005–0.035 g/kg and 0.0398–0.0521 g/kg, respectively. The decrease in nitrogen content indicates that several treatments showed signs of good composting and maturation process. In this process, the NH4+-N content of PKN-VR treatment is higher than other treatments in the pre-composting stage, and the lowest in CK group. At the end of composting, the highest NH4+-N content was found in the CK treatment and the lowest in the NK-VR treatment, which indicates that microbial activities are favorable to promote the decomposition of organic matter and accelerate the volatilization of ammonia.
As shown in Fig. 3a, the organic matter content of each treatment gradually decreases during the composting process, especially the loss of organic matter was higher in the PKN-VR and PKN-Car-VR treatments, while it is lowest in the CK treatment. As composting progresses, most of the small organic molecules are decomposed so that the rate of organic matter degradation gradually tends to stabilize, while the difficult-to-degrade substances will also be further decomposed and utilized by microorganisms during the cooling and decaying process. At the end of composting, the content of organic matter remaining in each treatment group is stable at 56.55–63.54%, with a reduction of 26.26–35.25%, and the NK-VR group has the highest degradation rate of 35.25%, followed by PKN-VR at 34.852%. Total nutrient content is a key parameter in assessing the fertility and application value of compost (Fig. 3b). At the end of composting, the concentrations of total nitrogen (TN), phosphorus (TP), and potassium (TK) reach 4.06–6.30%, 0.275–0.552%, and 1.693–2.301%, respectively, in each group. Although nitrogen loss and inorganic nitrogen production during the composting process reduce the absolute content of TN, the N, P, and K nutrient contents of the compost products in all groups are significantly higher compared to those before composting. Compared to CK, the concentrations of TN, TP, and TK are increased by 24.1–55.2%, 22.1-100.99%, and 7.46–35.93%, and the total nutrients (Σ (N + P2O5 + K2O)) increased by 21.35–49.85% in all groups of compost, respectively.
3.2 Effect of vinegar residue biofungal fertiliser on wheat seedlings
The present study verified the effect of vinegar residue-based biofungal fertilizer on growth traits of wheat seedlings (Fig. 4). The mean stem length of wheat seedlings in all treatments ranged from 16.54 to 25.99 cm, with the S4 treatment having the largest mean stem length, followed by the S3 and S5 treatments (25.56 cm, 25.8 cm). Compared with the CK group (6.65 cm), the S6 treatment shows longer root performance (11.54 cm), which is followed by S5 (10.26 cm) and S2 (9.24 cm) (Fig. 4b). The stem and root lengths of wheat seedlings are remarkably increased by 37.4–57.2% and 8.52–73.5%, respectively. In addition, wheat treated with VRBF also significantly increases in fresh and dry weights compared to the control, with the highest fresh weight of wheat seedlings in the S5 treatment (0.4239 g/plant), followed by the S1 treatment (0.4022 g/plant), with the treatments increasing by 47.1–71.4%, respectively (Fig. 4c). The highest dry weight of wheat seedlings is in the S5 treatment at 0.0593 g/plant, which increases significantly by 25.6% (S1), 41.3% (S2), 54.3% (S3), 50.5% (S4), 57.9% (S5), and 49.8% (S6) in S1-6 treatments, respectively, compared to CK (Fig. 4d).
Different VRBFs have a significant effect on chlorophyll content in wheat seedlings (Fig. 5). Compared with the control (0.827 mg/g), S5 treatment has the maximum chlorophyll a content (1.431 mg/g), followed by S3 (1.270 mg/g), S4 (1.090 mg/g), and the lowest performer is S1 (0.954 mg/g). Finally, the chlorophyll a content of the treatments is significantly increased by 15.4–73.0%, respectively (Fig. 5a). The highest chlorophyll b contents are found in S2 (0.316 mg/g) and S6 (0.288 mg/g), which increase their chlorophyll b contents in wheat seedlings by 65.8% and 51.3%, respectively, compared with the control group (0.191 mg/g). The lowest one is S3, which is only 0.125 mg/g (Fig. 5b), even though the chlorophyll b content of S3 treatments is slightly lower than that of the CK group. Its total chloroplast content is greater than that of the CK group. Figure 5c reflects the total chlorophyll content of wheat seedlings in each treatment. Compared with the CK group (1.017 mg/g), the average total chlorophyll contents under the S1-6 treatments are 1.185, 1.31, 1.395, 1.359, 1.649, and 1.258 mg/g, respectively, which are increased by 16.6–62.2%. The best performance of total chlorophyll content is under the S5 treatment. Soluble sugars play a key role in dehydration tolerance during plant seedling development. The highest soluble sugar content in the leaves of wheat seedlings treated with VRBF is found in the S4 treatment (3.517 mg/g), followed by S5 (3.479 mg/g) and S6 (3.420 mg/g). Compared to the control, the soluble sugar content of the S1-6 treatments significantly increased by 14.9%, 37.5%, 49.6%, 64.5%, 62.7%, and 59.9% (Fig. 5d). Similarly, a significant increase in proline content is observed under different VRBF treatments, ranging from 26.0 to 94.0% in the S1-6 treatments, respectively, compared to the control. The highest proline contents are found in S5 (11.982 µg/g) and S4 (11.794 µg/g), and the lowest is in S1 treatment with 7.780 µg/g. In the present study, there is a reduction in MDA content in wheat seedlings under different VRBF treatments (Fig. 5f). The MDA content in the CK group is 17.350 µg/g. S5 treatment has the lowest MDA content (13.176 µg/g), followed by S6 (14.961 µg/g), S1 (14.970 µg/g). S5, S6, and S1 treatments have the MDA content decreases by 24.05%, 13.83%, and 13.77%, respectively.
Figure 6 shows the variation of nutrient content in wheat seedlings after the application of different VRBFs. The nitrogen (N) content of wheat seedlings in the CK group and VRBF treatments is 4.32%, 5.37%, 5.02%, 6.18%, 5.95%, 6.88%, and 6.18%, respectively (Fig. 6a). The S5 treatment has the highest N content, while there is no significant difference (p < 0.05) in the S3 and S6 treatments compared to the CK group. The S5, S3, and S6 treatments improve wheat N content by 59.2%, 43.1%, and 43.1% respectively. When VRBF is added to the soil, N-containing compounds decompose slowly and provide a stable supply of N to wheat seedlings. Additionally, nitrogen-fixing bacteria increase N utilization in the soil and improve uptake by the wheat root system. This leads to the accumulation of more N in wheat seedlings. The highest levels of phosphorus (P) in wheat seedlings are found in S5 (0.585%), S2 (0.545%), and S6 (0.532%), while the CK group only has 0.36569% of P (Fig. 6b). Compared with the CK group, the P content of wheat in the S2 and S5 treatments is 48.9% and 59.8% higher than that of CK, respectively (p < 0.05). From Fig. 6c, the highest potassium concentration in wheat seedlings is found in S5 treatment (1.883%), followed by S4 (1.715%). Wheat K concentration is significantly (P < 0.05) higher in S5 and S4 treatments by 59.7%, 45.5% compared to CK group (1.179%).
3.3 Effect of vinegar residue biofungal fertiliser on the physico-chemical properties of potting saline soils
Soil pH and EC are direct indicators of soil salinity and can affect soil acidity. The application of VRBDF group treatments (S1-6) significantly reduced soil pH and EC compared to the CK group (Fig. 7a,b). In the CK group, soil pH was 8.52 and EC was 0.861 dS/m. The treatments that showed the largest decreases in soil pH and EC were S5 (7.93) and S4 (0.320 dS/m), respectively, which were 6.9% and 62.8% lower. The treatment with the highest soil organic matter content is S5 (22.1 g/kg), followed by S4 (21.88 g/kg), S6 (20.6 g/kg), and S2 (20.5 g/kg). The lowest organic matter content is found in S1 (17.92 g/kg) compared to the control group (13.24 g/kg). The S4, S6, and S2 treatments are 66.9%, 65.3%, and 54.8% higher, respectively, than the CK group (Fig. 7c). The nutrient (N, P, and K) levels of the potting soil are displayed in Fig. 7d. In the CK group, the effective phosphorus content is 12.5 mg/kg. The S5 group of treatments has the highest soil AP content, with values of 26.77 mg/kg for S5, 23.81 mg/kg for S2, and 22.90 mg/kg for S6. These values are 114.1%, 90.4%, and 83.2% higher, respectively, compared to the CK group (Fig. 7d). In the CK group, the soil AK content is 146.6 mg/kg. The S5 group has the highest soil AK content at 175.6 mg/kg, followed by S4 with 174.53 mg/kg. These values are 19.8% and 19.1% higher, respectively, than the CK group (Fig. 7e). The soil AN content of the treatment’s ranges from 41.4 to 67.67 mg/kg. The highest AN content is observed in the soil of the S4 treatment, which is 63.5% higher compared to the CK group. However, there is no significant difference in AN content between the S2 and S6 treatments (P < 0.05). The total soil phosphorus content in the S1 (0.57 g/kg), S2 (0.39 g/kg), S3 (0.55 g/kg), S4 (0.54 g/kg), S5 (0.58 g/kg), and S6 (0.53 g/kg) groups increased by a total of 25.8–87.1% compared to the CK group (0.31 g/kg). In terms of soil TK content, the treatment of group S4 has the highest level at 15.54 g/kg. This represents a significant increase of 102.8% compared to the control group (7.66 g/kg). On the other hand, group S6 has the lowest TK content at 10.98 g/kg, possibly due to the salt content inhibiting the activity of potassium-solubilizing bacteria to some extent (Fig. 7h). Figure 7i shows the TN content of soil in potted plants. The soil TN content for each treatment group (CK, S1, S2, S3, S4, S5, and S6) significantly increases by 40–112% compared to the control group of CK. Notably, the TN content is higher in the S4, S2, and S6 treatments, indicating that the application of VRBF effectively increases the nitrogen content in the soil.
Figure 7 Effect of different fertiliser treatments on soil physico-chemical properties in potting trials. (a) pH;(b) EC༛(c) OM༛(d) AP༛(e) AK༛(f) AN;(g) TP༛(h) TK༛(i) TN. Vertical lines indicate standard errors and different lower case letters indicate significant differences at p < 0.05
3.4 Effect of vinegar residue biofungal fertilizer on enzyme activity in potted saline soil
The changes in enzyme activities in the potting soil are depicted in Fig. 8. The S1-6 group exhibits significantly higher soil enzyme activities compared to the CK group (1.96 mL/g-h). However, no significant differences in catalase activities are observed between the treatment groups (p < 0.05). The soil catalase activity reaches its peak in the S5 and S3 treatments, both recording 2.27 mL/g·h, while the lowest activity is seen in S2 with 2.13 mL/g-h. In comparison to the CK group, the catalase activity in the S1-6 treatments shows an increase of 8.67–18.88%. Additionally, the highest sucrase activity is recorded in S4 (18.84 mg/g/d), closely followed by S3 (18.61 mg/g/d). The sucrase activity in the S1-6 group exhibits a significant increase of 24.26–41.9% compared to the CK group (13.27 mg/g·d) (Fig. 8b). The CK group has an alkaline phosphatase activity of 0.74 mg/g-24h. In the S1-6 group, the ALPase activity is 0.89, 1.31, 1.20, 1.08, 0.95, and 1.02 mg/g·24h, which represents increases of 20.3%, 77.02%, 62.2%, 45.9%, 28.4%, and 37.8% respectively. Figure 8d demonstrates that all the S1-6 groups have higher urease activities compared to the CK group (0.94 mg/g·24h), with S3 exhibiting the highest urease activity (1.27 mg/g·24h), followed by S6 (1.26 mg/g·24h). The groups S1-6 show an increase in soil urease enzymes ranging from 18.08–35.1% when compared to the control soil.
3.5 Relevance analysis
Table 1
Principal Component Analysis (PCA) of soil physicochemical properties and enzyme activities of potting soils.
Parameters | PC1 | PC2 | PC3 |
pH | -0.26536 | 0.17861 | 0.50104 |
EC | -0.31364 | 0.06497 | -0.09138 |
SOM | 0.31006 | -0.05933 | -0.16478 |
TN | 0.24408 | 0.36597 | 0.04623 |
AN | 0.28539 | -0.13073 | 0.24882 |
TK | 0.28668 | -0.13334 | 0.08186 |
AK | 0.30356 | -0.09424 | -0.14792 |
TP | 0.23832 | 0.51921 | -0.15616 |
AP | 0.26855 | -0.19358 | -0.45792 |
S-CAT | 0.27796 | 0.38919 | 0.05409 |
S-SC | 0.2705 | 0.26335 | 0.43225 |
S-ALP | 0.22376 | -0.48548 | 0.43581 |
S-UE | 0.3005 | -0.13824 | 0.084 |
Eigenvalue | 9.771 | 1.294 | 0.888 |
Percentage of total variance | 75.161 | 9.955 | 6.832 |
Cumulative percentage | 75.160 | 85.116 | 91.948 |
Principal Coordinate Analysis (PCA) showed the effect of VRBF on the relationship between physicochemical properties and enzyme activities of saline soils. Table 1 shows the results of analysis of variance (ANOVA). The cumulative variance explained by the first (75.161%), second (9.955%) and third (6.832%) axes accounted for 91.948% of the total variance. The significant differences in nutrient content and enzyme activities in the soil after application of VRBF further confirmed the great changes in soil fertility. Except for pH and EC, the loadings of the first principal component were all positive, indicating that the other soil nutrient indicators had strong positive correlations with PC1. However, pH and EC values were negatively correlated with PC1. Soil chemical variables such as P, K, N and OM contributed the most to the first principal component, which was mainly attributed to the increase in soil organic matter (SOM) content and nutrient levels of N, P and K by VRBF. This indicates that PC1 represents soil nutrient content. The second component mainly reflects the changes in soil enzyme activities after VRBF application. The third component, PC3, was mainly influenced by soil pH. PCA analysis (Fig. 9a) showed that soil nutrient content was the first driver of soil quality in saline soils, followed by soil enzyme activity, and pH and salinity were the third drivers. The correlation study (Fig. 9b) examined the relationship between wheat seedling growth and soil health under different VRBF treatments in a pot experiment. The results showed that there were significant positive correlations between soil pH and electrical conductivity and MAD. In addition, negative correlations (p < 0.05) were found with OM, AN, TK, AK, AP, S-CAT, S-UE and other wheat parameters except Chlb and MAD. This suggests that lowering soil pH in saline soils can effectively improve soil quality and promote wheat growth. In addition, soil electrical conductivity was significantly (p < 0.05) negatively correlated with other soil physicochemical properties, enzyme activities and wheat growth parameters excluding Chlb and MAD. Wheat root length (RL) was negatively correlated with soil pH and EC, but significantly correlated with soil OM, AN, AK, AP and S-UE. This highlights the close relationship between wheat root-soil environment and soluble N, P, K and urease contents. Except for wheat MAD, all other growth parameters were significantly and positively correlated with soil nutrient content and enzyme activities. This suggests that promotion of wheat growth depends on the increase in soil nutrient content and enzyme activity.