Microbiological mechanism of hydrogen fertilizer effect in soil

For a long time, intercropping and rotation of leguminous with non-leguminous crops is widely used to reduce the application of nitrogen fertilizer and increase yield in agroecosystems. At present, most researchers considered that this management measure is helpful for reducing fertilizer consumption and increasing its efficiency, as it can improve nutrient supply of legumestonon-legumes, the spatial nutrient utilization efficiency by enhancing soil spatial heterogeneity, and improve soil structure and disease resistance. However, current theories cannot fully explain the positive effect of crop rotation and inter-cropping systems involving legumes. A large amount of hydrogen (H₂) can be produced as an obligatory by-product of nitrogenase responsible for nitrogen (N₂) fixation in the root nodules of leguminous plants. Despite of substantial amounts of H₂ enriched in the rhizosphere of legumes, only a minor proportion of H₂ is found to leak to soil surface. Increasing evidence showed that most H₂ released in soil is immediately depleted in the surrounding of N₂-fixing nodules by H₂-oxidizing bacteria (HOB) thriving in soil. HOB can use H₂ as an electron donor to assimilate and fix CO₂ through redox reactions to synthesize cellular substances and consequently promote plant growth. To date, however, little is known about the biological mechanism and ecological process behind the "hydrogen fertilizer effect". Therefore, we review the H₂-induced plant growth-promoting effects and its microbiological mechanisms. Our aims were to explore a new way for enhancing agroecosystem production, and to provide scientific basis for future utilization of H₂ in agricultural production practices.