HR: 15:10h
AN: A23E-07 INVITED     [Abstracts]
TI: Legumes, N$_{2}$ fixation and the H$_{2}$ cycle
AU: * Layzell, D B
EM: layzelld@biology.queensu.ca
AF: Queen's University, Biology Department, Kingston, ONT K7L 3N6 Canada
AB: Legume plants such as soybean or pea can form symbiotic, N2 fixing associations with bacteria that exist in root nodules. For every N$_{2}$ fixed, 1 to 3 H$_{2}$ are produced as a by-product of the nitrogenase reaction. Therefore, a typical N$_{2}$ fixing legume crop produces about 200,000 L H$_{2}$ gas (at STP) per hectare per crop season. This paper will summarize our current understanding of the processes leading to H$_{2}$ production in legumes, the magnitude of H$_{2}$ 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: $\sim$ In symbioses lacking uptake hydrogenase (HUP) activity (thought to be the majority of crop legumes), the H$_{2}$ 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 H$_{2}$); $\sim$ Laboratory studies indicate that 60% of the reducing power from H$_{2}$ is coupled to O$_{2}$ uptake, whereas 40% is coupled to autotrophic CO$_{2}$ fixation. The latter process should increase soil carbon stocks by about 25 kg C/ha/yr; $\sim$ At the site of the nitrogenase enzyme, H$_{2}$ production is autocatalytic such that the higher the H$_{2}$ concentration, the more H$_{2}$ is produced and the less N$_{2}$ fixed. The variable O$_{2}$ diffusion barrier in legumes can act to restrict H$_{2}$ diffusion from the nodule, thereby increasing the relative magnitude of H$_{2}$ production versus N$_{2}$ fixation; $\sim$ Studies to understand why legume symbioses make such an energy investment in H$_{2}$ production have led to the discovery that H$_{2}$ 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 H$_{2}$ economy will have on the H$_{2}$ cycle.
DE: 1615 Biogeochemical processes (4805)
DE: 0315 Biosphere/atmosphere interactions
DE: 0394 Instruments and techniques
DE: 0400 Biogeosciences
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting