Associative nitrogen fixation and root exudation - What is theoretically possible in the rhizosphere?

Root exudation is a key driver of many rhizosphere processes including nitrogen fixation by diazotrophic bacteria residing in the soil. We critically review our knowledge of rhizosphere carbon flow and determine the extent to which rhizodeposition could fuel associative N₂ fixation by soil microorganisms. We conclude that most estimates of rhizosphere C flow are fundamentally flawed due to the use of inappropriate methodology combined with a poor mechanistic understanding of root C flow. Using a mathematical model, we predicted that rhizodeposition could under optimal conditions support the fixation of between 0.2 to 4 kg N ha^-1 year^-1 which is in good agreement with experimentally derived values for natural ecosystems (0.05 to 5 kg N ha^-1 y^-1). Our model indicated that fixation was highly dependent upon the number of potential N₂ fixers in the rhizosphere relative to the total microbial population. If N₂ fixer populations could be enhanced, we predict that fixation rates may reach up to 20 kg N ha^-1 y^-1 given highly optimal conditions which again agree with experimentally derived results. We conclude that whilst the potential for rhizodeposition-driven N₂ fixation in the soil is small in comparison to inorganic and symbiotic-N₂ fixation inputs, it may be of importance in N-limited ecosystems.