3.1. Characteristics of the Substrates for Amendment
The characteristics of the substrates amended with the digestate are shown in Table 2. Essentially, the pH of the ash and vegetable changed from 10.5 and 8.89 respectively to 9.76, which suggests that blending can stabilize soil pH in line with Franke-Whittle et al., (2014a). Similarly, changes in the values of N and P were observed when ash and vegetable were combined and clearly shows the complementary synergy between individual elements (Tambone et al., 2007). Conversely, an increase was noticed for K (vegetable only) and a decrease when combined with ash and vegetable (Cortés et al., 2020; Tambone et al., 2007).
Table 2
Substrate Amendment Characteristics
Bulking agent | pH | N (mg/kg) | P (mg/kg) | K (mg/kg) |
Ash | 10.50 | 2432.43 | 2698.11 | 28540.00 |
Vegetable | 8.89 | 32675.68 | 5343.82 | 31708.00 |
Wood Ash + Vegetable | 9.76 | 19554.06 | 3920.97 | 29124.00 |
In addition, the characteristics of the substrate for AD and the digestate obtained from the different bio-wastes are as shown in Tables 3 and 4 respectively. Parameters examined were total solid (TS) (%), volatile solid (VS) (%), total organic carbon (TOC) (%) and total nitrogen (TN).
Table 3
Characteristics of AD Substrates
Digestates | TS (%) | VS (%TS) | TOC (%) | TN (%) |
Cattle Rumen (CR) | 10.85 ± 0.150 | 89.00 ± 0.500 | 5.57 ± 0.115 | 0.30 ± 0.107 |
Food Waste (FW) | 26.53 ± 0.950 | 86.83 ± 0.764 | 7.23 ± 0.379 | 0.46 ± 0.117 |
Fruit waste (FRW) | 9.13 ± 0.814 | 94.50 ± 0.500 | 4.23 ± 0.252 | 0.27 ± 0.086 |
CR + FW | 19.63 ± 0.950 | 89.67 ± 0.289 | 9.63 ± 0.115 | 0.51 ± 0.163 |
CR + FW + FRW | 13.75 ± 0.289 | 92.67 ± 0.289 | 8.50 ± 0.173 | 0.46 ± 0.164 |
Table 4
Characteristics of Digestates
Digestates | TS (%) | VS (%TS) | TOC (%) | TN (%) |
Cattle Rumen (CR) | 1.21 ± 0.050 | 55.69 ± 1.140 | 0.22 ± 0.056 | 0.01 ± 0.002 |
Food Waste (FW) | 14.37 ± 2.674 | 61.75 ± 0.702 | 1.90 ± 0.669 | 0.08 ± 0.029 |
Fruit waste (FRW) | 3.07 ± 0.494 | 73.04 ± 1.212 | 0.75 ± 0.167 | 0.03 ± 0.008 |
CR + FW | 7.00 ± 2.570 | 64.44 ± 5.354 | 1.77 ± 0.357 | 0.08 ± 0.015 |
CR + FW + FRW | 3.48 ± 1.067 | 75.63 ± 2.006 | 1.08 ± 0.177 | 0.05 ± 0.007 |
Table 4
Bacterial isolates occurrence in the various digestate amended with ash and vegetable
Digestate | Bacterial Isolates | Amendment | Percentage (%) |
Ash | Vegetable | Ash + Vegetable |
CR | Bacillus sp. | + | + | + | 22.73 |
| Enterobacter | + | - | - | 7.58 |
| Salmonella | + | + | + | 13.64 |
| E. coli | + | + | + | 19.70 |
| Proteus | + | - | + | 12.12 |
| Bacillus subtilis | - | + | + | 12.12 |
| Serratia | - | + | - | 9.09 |
| Chromobacterium sp | - | + | - | 3.03 |
FW | Bacillus sp. | + | + | + | 22.73 |
| Pseudomonas | + | + | + | 18.18 |
| Salmonella | + | + | + | 15.15 |
| E. coli | + | + | + | 19.70 |
| Klebsiella | + | + | + | 16.67 |
| Bacillus spp. | - | + | - | 7.58 |
FRW | Bacillus spp. | - | + | + | 24.19 |
| Bacillus subtilis | + | + | + | 16.13 |
| Pseudomonas | + | + | + | 17.74 |
| E. coli | + | + | + | 24.19 |
| Salmonella | + | + | + | 17.74 |
CR + FW | Bacillus spp. | + | + | + | 22.06 |
| Bacillus subtilis | + | + | + | 22.06 |
| Salmonella | + | + | + | 14.71 |
| Klebsiella | - | + | + | 8.82 |
| E. coli | + | + | + | 19.12 |
| Serratia | - | + | + | 13.24 |
CR + FW + FRW | E. coli | + | + | + | 18.75 |
| Bacillus spp. | + | + | + | 18.75 |
| Bacillus subtilis | + | + | + | 18.75 |
| Salmonella | + | + | + | 13.75 |
| Pseudomonas | + | + | + | 16.25 |
| Proteus | + | + | + | 13.75 |
In Table 3, values of TS, VS, TOC and TN showed considerable reduction as shown in Table 4. The difference suggests the influence of microbial activities before and after the AD process
3.2. Microbial profile of the Amended Digestate
The bacterial loading of the composting materials throughout the process are shown in Fig. 2 (A – E) for the digestate amended with ash, vegetable or a combination of both. During composting process, the bacterial load for the various conditions increased from weeks 1 to 4 and decreased at week 5 (Fig. 1). At week 5, the highest bacterial load was 5.78 log10 CFUs/g and the lowest was 3.46 log10 CFUs/g for digestate from fruit waste amended with ash/vegetable and fruit waste amended with ash, respectively. During amendment, the addition of ash and vegetable induced changes in temperature, aeration, pH, moisture content and nutrient availability; thus, increasing the numbers of microorganisms (Shafi et al., 2017). The increase in bacterial counts observed during weeks 1–4 suggested the ability of bacteria to break down (via hydrolytic enzymes) and utilize the complex polymers present in the organic matter of composting mixtures (Lim et al., 2020; Ren et al., 2020).
The fungal loading of the samples during composting is shown in Fig. 3 (A – E) for the various digestate amended with ash, vegetable and a combination of both. During composting, the fungal load for the various conditions increased from week 1 to 4 and decreased in week 5 (Fig. 3). At week 5, the highest fungal count was 5.58 log10 CFUs/g and the lowest was 3.82 log10 CFUs/g for digestate from cattle rumen + food waste + fruit waste amended with vegetable and cattle rumen amended with vegetable respectively. This observation is in agreement with (Ezemagu et al., 2021) and indicates the positive impacts of fungal in maintaining nutrient availability (Franke-Whittle et al., 2014a; Ren et al., 2018; Zeng et al., 2016a)
Furthermore, saprophytic fungi contribute to the composting process by degrading recalcitrant biomass such as lignin and cellulose (Langarica-Fuentes et al., 2014; Escobar et al. (2020).
The comparison of mean variation for bacteria load and fungal load in the digestates amended with ash and vegetable are shown in Fig. 4.
In comparison, the microbial mean of CR digestate amended with vegetable was significantly higher than FW and FRW amended with vegetable with values 0.80, 0.48 and .044 respectively. For fungal load, the highest significant value of 0.65 was for FW digestate amended with vegetable. Following Azeem et al (2020), digestate amendment with an organic substrate such as vegetable is a potential key factor in increasing plant nutrient availability. By implication, there is increased biological activity of the microbes in the rhizosphere and enhanced metabolic activities thereby increasing nutrients in the soil (Ezemagu et al., 2021; Franke-Whittle et al., 2014a; Tambone et al., 2015b). In other words, CR + FW and CR + FW + FRW as shown is a mean effect of the individual digestate resulting from the combination of components. Similarly, and as reflected in the fungal count, FW was influenced significantly by vegetable amendment for CR (Zeng et al., 2016b). The overall synergy of the combined effects was noticeable in CR + FW as well as CR + FW + FRW which again, is corroborated by the findings of Jin et al. (2020) and Lin et al. (2018) on the importance of digestate amendment to increase carbon storage, soil nutrient and subsequent impacts in the soil physicochemical properties (Akyol et al., 2019)
The percentile of identified bacterial isolates was computed in terms of the number of species treated to the total number of species in the entire experimental treatment as shown in Table 4. The bacterial isolates identified from the various digestate amended with ash and vegetable were found to be predominantly Bacillus subtilis, E. coli, Pseudomonas aeruginosa and Serratia marcescens as shown in Table 3 The highest frequency of occurrence among the isolates was Bacillus spp (24.19%) while Chromobacterium had the lowest percentage occurrence of 3.03%. These identified bacterial isolates found in the amended digestate when applied to soil can improve soil microbiomes (Escobar et al., 2020) thereby acting as growth-promoting rhizobacteria (Aloo et al., 2019). Also, the species B. subtilis and P. aeruginosa are important organic matter degrading bacteria known to dissolved minerals and make nutrients available in the soil (Li et al., 2019).
The identified fungal isolates were Mucor piriformis, Rhizopus stolonifer, Aspergillus niger, Penicillium expansum and Saccharomyces cerevisiae (Table 5). Of the fungal isolates, Aspergillus niger was the most prevalent isolate occurring in the digestate amended with ash and vegetable with a percentage frequency of 29.0% while Saccharomyces cerevisiae was the least isolated fungi with a percentage frequency of 7.3%. From the results of this study, fungal isolates Aspergillus and Trichoderma were dominant in terms of frequency of occurrence. The presence of these fungal isolates plays an important role in lignocellulosic biomass degradation thus increasing the C/N ratio resulting in high microbial activities (Escobar et al., 2020; Fernandez-Bayo et al., 2020).
Table 5
Fungal isolates occurrence in the various digestate amended with ash and vegetable
Digestate | Fungal Isolates | Amendment | Percentage (%) |
Ash | Vegetable | Ash + Vegetable |
CR | Aspergillus niger | + | + | + | 22.73 |
| Penicillium spp | + | + | + | 18.18 |
| Trichoderma viridae | + | + | + | 22.73 |
| Mucor spp. | + | + | + | 16.67 |
| Yeast cells | + | + | + | 19.70 |
FW | Aspergillus niger | + | + | + | 16.85 |
| Penicillium spp | + | + | + | 16.85 |
| Trichoderma viridae | + | + | + | 16.85 |
| Mucor | + | + | + | 12.36 |
| Aspergillus tamarii | + | + | + | 12.36 |
| Yeast cells | + | + | + | 15.73 |
| Bjerkandera adusta | - | + | + | 8.99 |
FRW | Mucor spp. | + | + | + | 12.86 |
| Neurospora spp. | + | + | + | 12.86 |
| Aspergillus niger | + | + | + | 21.43 |
| Aspergillus tamarii | + | + | + | 17.14 |
| Bjerkandera adusta | + | + | + | 20.00 |
| Yeast cells | + | + | + | 15.71 |
CR + FW | Trichoderma viridae | + | + | + | 17.65 |
| Penicillium spp | + | + | + | 17.65 |
| Mucor spp. | + | + | + | 7.06 |
| Neurospora spp. | + | + | + | 10.59 |
| Aspergillus niger | + | + | + | 17.65 |
| Aspergillus tamarii | + | + | + | 14.12 |
| Yeast Cells | + | + | + | 15.29 |
CR + FW + FRW | Bjerkandera adusta | + | + | + | 11.49 |
| Mucor spp. | + | + | + | 13.79 |
| Neurospora spp. | + | + | + | 17.24 |
| Aspergillus niger | + | + | + | 17.24 |
| Aspergillus tamarii | + | + | + | 17.24 |
| Yeast cells | + | + | + | 13.79 |
| Penicillium spp | + | + | + | 9.20 |
All the identified bacterial isolates were screened for rhizobacterial potentials. The rhizobacterial potential screened were phosphate solubilization, Indole acetic acid, nitrogen and ammonia production. The percentage results of plant growth-promoting potential of the various isolates are shown in Table 6. All the isolates showed a high level of rhizobacterial potential with B. subtilis and Bacillus spp having the highest potential of 100% while Pseudomonas had the least of 50%.
Table 6
Plant growth promoting potential of bacterial isolates
Digestate | Bacterial Isolates | N2 | NH3 | IAA | PSB | Percentage (%) |
CR | Bacillus sp. | + | + | + | + | 100 |
| Enterobacter | + | - | - | + | 50 |
| Salmonella | + | + | - | + | 75 |
| E. coli | + | + | - | + | 75 |
| Proteus | + | + | - | - | 50 |
| Bacillus subtilis | + | + | + | + | 100 |
| Serratia | + | + | - | + | 75 |
| Chromobacterium | + | + | - | + | 75 |
FW | Bacillus sp. | + | + | + | + | 100 |
| Pseudomonas | + | - | - | + | 50 |
| Salmonella | + | + | - | + | 75 |
| E. coli | + | + | - | + | 75 |
| Klebsiella | + | + | - | - | 50 |
| Bacillus spp. | + | + | + | + | 100 |
FRW | Bacillus spp. | + | + | + | + | 100 |
| Bacillus subtilis | + | + | + | + | 100 |
| Pseudomonas | + | - | - | + | 50 |
| E. coli | + | + | - | + | 75 |
| Salmonella | + | + | - | + | 75 |
CR + FW | Bacillus spp. | + | + | + | + | 100 |
| Bacillus subtilis | + | + | + | + | 100 |
| Salmonella | + | - | - | + | 75 |
| Klebsiella | + | + | - | + | 75 |
| E. coli | + | + | - | + | 75 |
| Serratia | + | + | - | + | 75 |
CR + FW + FRW | E. coli | + | + | - | + | 75 |
| Bacillus spp. | + | + | + | + | 100 |
| Bacillus sp. | + | - | - | + | 100 |
| Salmonella | + | + | - | + | 75 |
| Pseudomonas | + | - | - | + | 50 |
| Proteus | + | + | - | - | 50 |
Bacteria with rhizobacterial potentials can enhance plant growth directly or indirectly in multifunctional ways (Aloo et al., 2019). The bacterial isolates tested for rhizobacterial potential showed positive results to phosphate solubilization, IAA, nitrogen and NO3 production. The level of rhizobacterial potential depends on the isolate. In the study, it was observed that Bacillus spp displayed greater potential. This aligns with the work of Hayat et al., (2013) that Bacillus spp and Enterobacter aerogenes have rhizobacterial potential. Bacillus spp have been confirmed to have many special functions and properties in plant rhizosphere such as phytostimulation, bio-fertilization and bio-protection (Aloo et al., 2019). Evidence had shown that Bacillus spp produce a wide range of rhizobacteria properties (Jiang et al., 2015; Shafi et al., 2017). This suggests that the isolates’ rhizobacterial potential could help in increasing soil nutrients for crop improvement (Ren et al., 2020). reported that rhizobacterial Bacillus spp. secrete metabolites that can enhance nutrient availability to plants. The identified bacteria showed different genera which are similar to those identified by Chandna et al. (2013).