Cow manure compost-based products as alternative rhizobial carrier materials

Cow manure compost (CMC) has been used widely as soil amendment to improve soil fertility and health, and to minimize environmental impacts. However, direct use of commercial CMC-based products as alternative rhizobial carrier materials is rare, it could promote inoculant commercialization. This research aimed to explore the potential of four CMC-based products (CMC powder, CMC granular, CMC + 50% gypsum, and CMC + 30% biochar) as a rhizobial carrier. Peat was used as a standard (control) material. All the carriers were directly mixed with Bradyrhizobium japonicum CB 1809 strain to prepare four CMC-based inoculants. Shelf life of inoculants was evaluated up to 120 days of storage and survival rate in soil was determined after 30 days under both optimum moisture (55% WHC-water-holding capacity) and drought stress (15% WHC) conditions. Results revealed that overall, CMC + 50% gypsum product showed better physicochemical characteristics such as WHC (126.9 & PLUSMN; 6.87%), Ca (134.15 & PLUSMN; 5.78 mg kg(-1)), and S (104.9 & PLUSMN; 6.52 mg kg(-1)) with the best shelf life during storage at 28 & DEG;C and 35 & DEG;C temperatures. The difference in shelf life between the CMC + 50% gypsum and other CMC-based products became more apparent at higher temperature (35 & DEG;C) and longer-term storage (120 days). Additionally, when introduced into the soil, the CMC + 50% gypsum carrier outperformed the peat (control), showing the best strain survival rate of 95% (under optimum moisture) and 87% (under drought) after 30 days of storage. The strain population of CMC + 30% biochar as carrier was the lowest with values 8.66 Log 10 CFU g(-1) and 7.52 Log 10 CFU g(-1) in the shelf-life test at both storage temperatures 28 & DEG;C and 35 & DEG;C, respectively, likely due to its alkaline pH. Multivariate analysis revealed that the carriers' WHC, Ca (involved in nodule formation and stability), and S (essential for nodule metabolism and nitrogenase activity) contents had a strong positive correlation with the long-term shelf life and the survival rate in soil, confirming the importance of carrier materials' moisture retention and availability of specific elements (Ca, S). Hence, CMC + 50% gypsum showed the highest values of WHC, Ca and S, and optimum pH, reflecting its highest rhizobial population density and survival rate as compared to other carriers. This work has demonstrated the great potential of CMC-based commercial products as alternative carrier materials, opening an alternative avenue for rapid commercial development of inoculants.