A Simulation Study of Gaseous Diffusion Resistance, Nodule Pressure Gradients and Biological NitrogenFixation in Soyabean Nodules

A simulation of a normally functioning soyabean nodule, witha variable gaseous diffusion barrier in the inner cortex, hasbeen used to calculate rates of nitrogen fixation and the concentrationsof O₂, CO₂, H₂ and N₂ at various tissue locations, in responseto variations in diffusion-resistance and external O₂ concentration. A small diffusion-resistance allowed increased nitrogen fixationin air, but lead to diminished rates at increased external O₂concentrations. Large diffusion-resistances provide increasedprotection against the effects of high O₂ concentrations butdiminish nitrogen fixation in air. These effects depend on therespiratory activity (V_max) of the bacteroids. In general, efficiency(moles of N₃ fixed/moles of O₂ used) is affected more than N₂fixation rates at increased external O₂ concentrations. As a result of differential fluxes and solubilities of the gasesinvolved in nitrogen fixation, significant negative pressuredifferences (about 24 kPa in air) would be generated betweenthe outer cortex and the nodule central tissues, provided thatthe structure is sufficiently inflexible, and the central tissueis isolated from undue influences of water and gas. The calculations also show that the concentrations of H₂ nearthe bacteroids remain low (2–3 per cent of concentrationsof dissolved N₂) and are thus unlikely to inhibit N₂ fixationexcept at high values of the diffusion resistance. Nitrogen fixation, diffusion, pressure