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The application of plant growth promoting rhizobacteria (PGPR) in low input and organic cultivation of graminaceous crops; potential and problems

Cummings S. P.

Environmental Biotechnology

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2009

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Vol. 5, No. 2

43-50

EN

Although the uses of plant growth stimulating bacteria (PGPR) to improve the yield of graminaceous crops have been studied for over seventy years the utility of the technology remains uncertain. Increases in crop yield have often been inconsistent, reflecting a lack of understanding of the mechanisms by which PGPR exert their effects. Because PGPR are able to fix N2, this was initially assumed to boost crops by supplementing soil N. However, it is now clear, that for most free living PGPR, other mechanisms affecting root development, and nutrient uptake can account for the observed increase in crop yields. Here we review the current state of our understanding of PGPR in graminaceous crop cultivation, identifying their potential contribution to more sustainable agricultural practices but also highlighting issues that need to be addressed before this technology can be appropriately assessed as a replacement for inorganic N addition.

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The potential and pitfalls of exploiting nitrogen fixing bacteria in agricultural soils as a substitute for inorganic fertiliser

Cummings S. P., Humphry D. R., Santos S. R., Andrews M., James E. K.

Environmental Biotechnology

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2006

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Vol. 2, No. 1

1-10

EN

Nitrogen fixing bacteria have been used for centuries to improve the fertility of agricultural soils. Since the introduction of inorganic nitrogen (N) fertiliser that provides a reliable boost to crop yields whilst reducing land and labour requirements, the use of biological nitrogen fixation has been in decline. Recently, concerns have been expressed about the sustainability of inorganic N fertiliser application, however, there remain doubts about whether N2 fixing bacteria alone can provide agriculture with sufficient fixed N to feed a burgeoning global population. In this paper we review the current state of our knowledge regarding those diazotrophic bacteria that have a role to play in agriculture. We focus on our current areas of research, particularly, the importance of understanding the classification and mechanism of action of N2 fixing bacteria that are used in agricultural soils. We discuss the applications of N2 fixing bacteria that illustrate their potential to provide sustainable N, particularly focussing on Australian and South American agricultural systems where these bacteria are widely exploited to maintain soil fertility. We also identify problems with the use of bacteria as inoculants, including ineffective inoculation due to poor quality preparation, the use of appropriate isolates and issues with sustainability. We review the outlook for biological N fixation highlighting how molecular biology may enable the expression of N fixation in non-leguminous crops.

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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

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2005

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Vol. 1, No. 1

1--10

EN

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.