The loss of organic and inorganic carbon from roots into soil underpins nearly all the major changes that occur in the rhizosphere. In this review we explore the mechanistic basis of organic carbon and nitrogen flow in the rhizosphere. It is clear that C and N flow in the rhizosphere is extremely complex, being highly plant and environment dependent and varying both spatially and temporally along the root. Consequently, the amount and type of rhizodeposits (e.g. exudates, border cells, mucilage) remains highly context specific. This has severely limited our capacity to quantify and model the amount of rhizodeposition in ecosystem processes such as C sequestration and nutrient acquisition. It is now evident that C and N flow at the soil–root interface is bidirectional with C and N being lost from roots and taken up from the soil simultaneously. Here we present four alternative hypotheses to explain why high and low molecular weight organic compounds are actively cycled in the rhizosphere. These include: (1) indirect, fortuitous root exudate recapture as part of the root’s C and N distribution network, (2) direct re-uptake to enhance the plant’s C efficiency and to reduce rhizosphere microbial growth and pathogen attack, (3) direct uptake to recapture organic nutrients released from soil organic matter, and (4) for inter-root and root–microbial signal exchange. Due to severe flaws in the interpretation of commonly used isotopic labelling techniques, there is still great uncertainty surrounding the importance of these individual fluxes in the rhizosphere. Due to the importance of rhizodeposition in regulating ecosystem functioning, it is critical that future research focuses on resolving the quantitative importance of the different C and N fluxes operating in the rhizosphere and the ways in which these vary spatially and temporally.

Carbon flow in the rhizosphere: carbon trading at the soil–root interface

Production of the phytohormone indole-3-acetic acid (IAA) is widespread among bacteria that inhabit the rhizosphere of plants. Several different IAA biosynthesis pathways are used by these bacteria, with a single bacterial strain sometimes containing more than one pathway. The level of expression of IAA depends on the biosynthesis pathway; the location of the genes involved, either on chromosomal or plasmid DNA, and their regulatory sequences; and the presence of enzymes that can convert active, free IAA into an inactive, conjugated form. The role of bacterial IAA in the stimulation of plant growth and phytopathogenesis is considered.

Bacterial biosynthesis of indole-3-acetic acid

Because of their ability to transform atmospheric N2 into ammonia that can be used by the plant, researchers were originally very optimistic about the potential of associative diazotrophic bacteria to promote the growth of many cereals and grasses. However, multiple inoculation experiments during recent decades failed to show a substantial contribution of Biological Nitrogen Fixation (BNF) to plant growth in most cases. It is now clear that associative diazotrophs exert their positive effects on plant growth directly or indirectly through (a combination of) different mechanisms. Apart from fixing N2, diazotrophs can affect plant growth directly by the synthesis of phytohormones and vitamins, inhibition of plant ethylene synthesis, improved nutrient uptake, enhanced stress resistance, solubilization of inorganic phosphate and mineralization of organic phosphate. Indirectly, diazotrophs are able to decrease or prevent the deleterious effects of pathogenic microorganisms, mostly through the synthesis of anti...

Plant Growth-Promoting Effects of Diazotrophs in the Rhizosphere

Phosphorus (P) is one of the major plant growth-limiting nutrients although it is abundant in soils in both inorganic and organic forms. Phosphate solubilizing micro-organisms (PSMs) are ubiquitous in soils and could play an important role in supplying P to plants in a more environmentally friendly and sustainable manner. Although solubilization of P compounds by microbes is very common under laboratory conditions, results in the field have been highly variable. This variability has hampered the large-scale use of PSMs in agriculture. Many reasons have been suggested for this variability, but none of them have been extensively investigated. In spite of the importance of PSMs in agriculture, the detailed biochemical and molecular mechanisms of P solubilization are not known. Recent work in our laboratory has shown that the conditions employed to isolate PSMs do not reflect soil conditions and that PSMs capable of effectively releasing P from soil are not so highly abundant as was suggested in earlier studies. These studies have also indicated that the mineral phosphate solubilizing (mps) ability of microbes could be linked to specific genes, and that these genes are present even in non P solubilizing bacteria. Understanding the genetic basis of P solubilization could help in transforming more rhizosphere-competent bacteria into PSMs. Further research should also focus on the microbial solubilization of iron (Fe) and aluminum (Al) phosphates, as well as mobilization of the organic phosphate reserves present in the soils.

Role of soil microorganisms in improving P nutrition of plants

Enhancement of drought stress tolerance in crops by plant growth promoting rhizobacteria.

Summary. Treating seedling roots of several plant species with cultures of Azotobacter paspali changed plant growth and development and significantly increased weight of leaves and roots; effects were probably caused by plant growth regulators. Culture supernatant fluids contained indolyl-3-acetic acid, at least 3 gibberellins and 2 cytokinins. The added inoculum of A. paspali survived in plant rhizospheres for only a few weeks and no nitrogen was fixed in the root zone of young Paspalum notatum, the grass with which A. paspali is associated.

Effects on plant growth produced by Azotobacter paspali related to synthesis of plant growth regulating substances.

Azotobacter paspali grows in the rhizospheres of Paspalum notatum where it is thought to fix nitrogen actively and hence improve pasture growth. In the experiments described here A. paspali improved growth primarily by producing growth regulating substances. Nitrogenase activity was not detected in rhizospheres of young plants. Its occurrence in older plants was not necessarily associated with the presence of A. paspali and was greater in the rhizopheres of control than of inoculated plants.

Role of Azotobacter paspali in Association with Paspalum notatum

Similar Effects on Tomato Plants of Azotobacter Inoculation and Application of Gibberellins

Brown, M. E. & Garr, G. R. 1984 Interactions between Azotobacter chroo-coccum and vesicular-arbuscular mycorrhiza and their effects on plant growth. Journal of Applied Bacteriology56, 429–437.



The effects of simultaneous inoculation of roots of lettuce seedlings with vesicular-arbuscular endophytes and Azotobacter chroococcum are described. Endophytes alone increased yields of lettuce grown in partially sterilized P-deficient soil, but not in the same unsterile soil. Dual inoculation with endophytes and Azotobacter in both soils produced larger plants than either inoculum alone. In another poorly structured unsterile soil with adequate P for plant growth, endophytes depressed yields of lettuce. There was no association between endophytes and Azotobacter. In none of the experiments did Azotobacter influence the level of endophyte infection in the roots; but numbers of Azotobacter on the root systems were decreased in the presence of the endophytes. Azotobacter alone always increased growth of young plants before the effects of the endophytes were observed.

Interactions between Azotobacter chroococcum and vesicular‐arbuscular mycorrhiza and their effects on plant growth

Summary: Micro-organisms isolated from rhizospheres and rhizoplanes of wheat plants, and from root-free soil, produced growth regulating substances with the properties of gibberellins and indolyl-3-acetic acid (IAA). Substances inhibiting extensions of pea plant internodes and lettuce hypocotyls were also produced, especially by bacteria from the root region of seedlings 6 days old. Bacteria producing growth promoting substances were most abundant on roots of older plants. Seedlings grown aseptically with added gibberellic acid (GA3) and IAA, or grown with a soil inoculum, developed similarly and differed in their morphology from those grown aseptically without additives.

Plant growth substances produced by micro‐organisms of soil and rhizosphere

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