Legumes, N2 fixation and the H2 cycle

Legume plants such as soybean or pea can form symbiotic, N2 fixing associations with bacteria that exist in root nodules. For every N₂ fixed, 1 to 3 H₂ are produced as a by-product of the nitrogenase reaction. Therefore, a typical N₂ fixing legume crop produces about 200,000 L H₂ gas (at STP) per hectare per crop season. This paper will summarize our current understanding of the processes leading to H₂ production in legumes, the magnitude of H₂ production associated with global cropping systems, and the implications for its production and oxidation on both the legumes and the soils in which they grow. Specific points may include: ∼ In symbioses lacking uptake hydrogenase (HUP) activity (thought to be the majority of crop legumes), the H₂ diffuses into the soil where it is oxidized by soil microbes that grow up around the legume nodules. The kinetic properties of these microbes are very different (higher Km and Vmax) from that of microbes in soils exposed to normal air (ca. 0.5 ppm H₂); ∼ Laboratory studies indicate that 60% of the reducing power from H₂ is coupled to O₂ uptake, whereas 40% is coupled to autotrophic CO₂ fixation. The latter process should increase soil carbon stocks by about 25 kg C/ha/yr; ∼ At the site of the nitrogenase enzyme, H₂ production is autocatalytic such that the higher the H₂ concentration, the more H₂ is produced and the less N₂ fixed. The variable O₂ diffusion barrier in legumes can act to restrict H₂ diffusion from the nodule, thereby increasing the relative magnitude of H₂ production versus N₂ fixation; ∼ Studies to understand why legume symbioses make such an energy investment in H₂ production have led to the discovery that H₂ treated soils have improved fertility, supporting the growth and yield of legume and non-legume crops. This observation may account for the benefits of legumes when used in rotation with cereal crops, a phenomenon that has been used by farmers for over 2000 years, but which has remained unexplained. An attempt will be made to position these results and insights in the context of the impact that a future H₂ economy will have on the H₂ cycle.