Nitrogen fixation associated with grasses and cereals: Recent progress and perspectives for the future

Over the last 20 years many new species of N₂-fixing bacteria have been discovered in association with grasses, cereals and other non-nodulating crops. Virtually all of these bacteria are microaerophylic, fixing N₂ only in the presence of low partial pressures of oxygen. Until a few years ago much attention was focussed on members the genusAzospirillum and it was assumed that N₂ fixation was restricted to the rhizosphere or rhizoplane of the host plants. Through the use of N balance and¹⁵N techniques it has been shown that in the case of lowland rice, several tropical pasture grasses and especially sugar cane, the contributions of biological N₂ fixation (BNF) are of agronomic significance.More detailed study of the N₂-fixing bacteria associated with sugar cane (Acetobacter diazotrophicus andHerbaspirillum spp.) has shown that they occur in high numbers not only in roots of this crop but also in the stems, leaves and trash but are rarely found in the soil. Some of these endophytic diazotrophs have now also been found in forage grasses, cereals, sweet potato and cassava, although evidence of significant BNF contributions is still lacking.The identification of these endophytic diazotrophs as the organisms probably responsible for the high contributions of N₂ fixation observed in sugar cane suggests that it may be possible to attain significant BNF contributions in some other gramineae and perhaps root crops.