Biochar surface properties and chemical composition determine the rhizobial survival rate.

Biochar may be potentially used as a rhizobial carrier due to its specific chemical compositions and surface properties, but the relationship between these properties and rhizobial survival rate is largely unknown. Here, we analysed the physicochemical characteristics and carrier potential of six types of biochars made from various feedstocks at 600 °C using slow pyrolysis method, and results were compared with conventional carrier material peat. Liquid suspension of Bradyrhziobium japonicum CB1809 was used to inoculate all the carrier materials. Shelf life and survival rate was determined via colony forming unit (CFU) method for up to 90 days under two storage temperature conditions (28 °C and 38 °C). The determined physicochemical characteristics of biochars were categorized into major elements, trace elements, relative ratios, surface morphology, functional groups, and key basic properties; and their interaction to shelf life was analysed using hypothesis-oriented structure equation modelling (path analysis). Results revealed that different types of biochars had different capacity to impact on shelf life due to their different physicochemical properties. Among all biochars pine wood BC was the most suitable carrier with the highest counts of 10.11 Log 10 CFU g-1 and 9.76 Log 10 CFU g-1 at the end of 90 days at 28 °C and 38 °C storage, respectively. Path analysis revealed that rhizobial shelf life was largely explained by total carbon (TC), manganese (Mn), specific surface area (SSA), pore size, CO (ketonic carbon), and O-CO (carboxyl carbon) functional groups, and all these indicators exhibited positive direct impact on shelf life. Pinewood BC showed the highest values of Mn, SSA, pore size and functional groups (CO and O-CO), contributing to its highest rhizobial shelf life and survival rate among other biochars and peat tested.