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The role and future potential of nitrogen fixing bacteria to boost productivity in organic and low-input sustainable farming systems

Cummings S. P.

Environmental Biotechnology

2005

Vol. 1, No. 1

1--10

Biological nitrogen fixation (BNF) results from the interaction between a plant and diazotrophic bacteria. The bacteria are either free living in the soil or live in symbiosis with the plant. Despite biological nitrogen fixation offering a sustainable solution to nitrogen limitation in agricultural soils its use is in decline. Problems with this technology can arise for two major reasons. Firstly, the inappropriate use of diazotrophs with the expectation of achieving N2 fixation. Free-living diazotrophs have been used as inoculants of non-legume crops for many years, however, their mechanism of action remains to be thoroughly characterised. While some may interact with crops to increase available N in soil, many achieve increases in crop yield through the production of plant hormones. This adds nothing to the soil N budget and increases in yields observed are often variable. The second problem occurs when legumes are used to increase soil N in combination with rhizobial symbionts. Frequently poor nodulation of the legumes is observed in the field even when inoculated with .elite. strains of rhizobia. These observations are a consequence of one or more factors, including the use of low quality inoculants, the inability of the rhizobial inoculant to tolerate soil conditions, or their lack of competitiveness for nodule occupancy with indigenous soil rhizobia. These issues can be overcome by the use of more rigorous criteria in inoculant selection and production. The use of inoculants developed from indigenous soil rhizobia offers a tailor made solution to obtaining inoculant strains that are competitive in a particular soil with a specific crop. Here, examples of where this approach has been successful and the potential of this technology to increase the use of BNF in more marginal soils are discussed.