Interactions of Biochar with Establishment of Legume-Rhizobia Symbiosis.

Rhizobial infection of legumes facilitates a significant part of the nitrogen (N) cycle, converting N₂ gas to ammonium through biological N fixation (BNF). BNF fixes approximately 150 billion kg N yr^-1, matching the fuel-intensive Haber-Bosch process. With a global need for agricultural intensification and the decline of non-renewable energy resources, soil microbial symbionts are increasingly important in making nutrients available to plants. Agricultural management, however, can hinder establishment of these symbioses by disturbing the soil habitat. Our research seeks to understand whether biochar, the latest trend in agricultural management, inhibits establishment of legume-rhizobia mutualism by interfering with plant-microbe signaling. Due to high aromaticity and surface area, many biochars have a great capacity to adsorb aromatic, organic compounds present in the soil. This interaction could have implications for plant-microbe symbioses that depend on highly aromatic flavonoid signaling compounds for initiation. In legume-rhizobia symbiosis, flavonoid exudation is the first step in a complex signaling process, functioning to chemoattract host-specific rhizobia and induce rhizobial nod genes. Any interference with this process could inhibit or reduce rhizobial infection. In a greenhouse study, we assessed nodulation in cowpea plants grown in soils amended with biochar at rates of 0, 10, and 25 g kg^-1 (equivalent to 0, 20, and 50 t ha^-1). As biochar amendment increases soil pH, limed and un-limed controls were used to account for any pH effects on nodulation. For limed controls, CaCO₃ rates were determined based on soil buffer capacity and biochar liming potential. CaCO₃ was added to raise the soil pH to equal that of biochar-amended pots. Plants were grown in an agricultural silt loam soil as well as in sand culture watered with a modified Hoagland solution.