Examining biochar as a carrier for Rhizobium spp. on pea crop

Abstract

The symbiotic relationship formed between legumes and rhizobia plays an integral role in the agriculture industry as the bacteria fix atmospheric dinitrogen (N2) to plant available nitrogen (N). Inoculating legume crops with rhizobia is a common agricultural practice with peat and clay being the preferred inoculant carriers. Both peat and clay are slowly renewable, natural resources with limited availability. This leaves room to explore alternative, more sustainable, carriers that can compete biologically and economically with current carriers. A potential alternative carrier is biochar which is the product of thermal degradation of organic materials in the absence of air (pyrolysis). Feedstock, pyrolysis temperature, and degree of oxidation during the production of biochar affects the resulting biochar characteristics. The aim of this research was to (i) characterize the physical and chemical properties of a variety of biochars and examine their abilities to support rhizobia, (ii) manipulate a subset of biochars to achieve increased surface area, (iii) assess the potential phytoxicity of each biochar, and (iv) evaluate the ability of each biochar to deliver nodulating rhizobia to pea seed. Nine biochars produced from different feedstocks and sources were examined. The biochars displayed a wide range of physical and chemical properties resultant from the varying feedstocks and production conditions. Six of the nine biochars were able to support rhizobia while three showed an inability to support rhizobia. The % carbon (C) and C:N ratio of the biochars was found to positively correlate with Rhizobium survival. The manipulated biochars had exponentially larger surface areas than the original biochars but failed to support rhizobia immediately following inoculation. It was observed that the manipulated biochars were very alkaline most likely causing rhizobia to become stressed upon inoculation and subsequently unable to survive in the high pH conditions. There were some phytotoxic effects on garden cress seed with undiluted biochar extracts where, conversely, the diluted biochar treatments resulted in the biostimulation of garden cress. There were no conclusive results assessing the biochars ability to deliver rhizobia to pea seed as the uninoculated and sterile treatments were successfully nodulated via native rhizobia. These findings suggest that biochar has the ability to support rhizobia but due to biochar’s complex nature, further research is needed in developing it as an inoculant carrier.