Cereal Nitrogen Fixation: Problems and Potentialities

Estimates of nonsymbiotic and associative nitrogen fixation indicate that nitrogen fixation occurs at magnitudes that may be of agronomic significance. Long-term N balances for crop production, although difficult to measure, are necessary for estimating the amounts of N2 fixed. Techniques using N directly to measure nitrogen fixation and problems involved in employing these techniques are discussed. Acetylene-reduction assays (ARA) are very sensitive but there are limitations to their use in quantification of nitrogen fixation as well as infield studies. Current understanding about the source of energy for cereal nitrogen fixation and the effect of light, temperature, soil moisture, plant genotype, plant age, and combined nitrogen on nitrogen fixation is illustrated with examples. Possibilities of improving the ability of cereals to support nitrogen fixation through plant breeding are discussed. Types of bacteria involved and methods used to isolate, count, and test their nitrogenase activity influence the results of such studies. Problems associated with selecting bacteria for field studies, their performance, and mode of benefiting crops from inoculations are discussed. Future areas of work are highlighted.