The continued use of chemical fertilizers and manures for enhanced soil fertility and crop productivity often results in unexpected harmful environmental effects, including leaching of nitrate into ground water, surface run-off of phosphorus and nitrogen run-off, and eutrophication of aquatic ecosystems. Integrated nutrient management systems are needed to maintain agricultural productivity and protect the environment. Microbial inoculants are promising components of such management systems. This review is a critical summary of the efforts in using microbial inoculants, including plant growth-promoting rhizobacteria and arbuscular mycorrhizal fungi for increasing the use efficiency of fertilizers. Studies with microbial inoculants and nutrients have demonstrated that some inoculants can improve plant uptake of nutrients and thereby increase the use efficiency of applied chemical fertilizers and manures. These proofs of concept studies will serve as the basis for vigorous future research into integrated nutrient management in agriculture.

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Plant-microbes interactions in enhanced fertilizer-use efficiency.

Biological N2 fixation (BNF) by associative diazotrophic bacteria is a spontaneous process where soil N is limited and adequate C sources are available. Yet the ability of these bacteria to contribute to yields in crops is only partly a result of BNF. A range of diazotrophic plant growth-promoting rhizobacteria participate in interactions with C3 and C4 crop plants (e.g. rice, wheat, maize, sugarcane and cotton), significantly increasing their vegetative growth and grain yield. We review the potential of these bacteria to contribute to yield increases in a range of field crops and outline possible strategies to obtain such yield increases more reliably. The mechanisms involved have a significant plant growth-promoting potential, retaining more soil organic-N and other nutrients in the plant– soil system, thus reducing the need for fertiliser N and P. Economic and environmental benefits can include increased income from high yields, reduced fertiliser costs and reduced emission of the greenhouse gas, N2O (with more than 300 times the global warming effect of CO2), as well as reduced leaching of NO3 –N to ground water. Obtaining maximum benefits on farms from diazotrophic, plant growth promoting biofertilisers will require a systematic strategy designed to fully utilise all these beneficial factors, allowing crop yields to be maintained or even increased while fertiliser applications are reduced. q 2004 Elsevier Ltd. All rights reserved.

Non-symbiotic bacterial diazotrophs in crop-farming systems : can their potential for plant growth promotion be better exploited?

Nitrogen fixation in endophytic and associative symbiosis

The genus Azospirillum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Azospirillum inoculation on plants . . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . Inoculation effects on root development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Root colonization by Azospirillum . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Proposed mode of action of Azospirillum on plant growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nitrogen fixation by Azospirillum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hormonal effects of Azospirillum on plants . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Improvement of root development, mineral uptake, and plant-water relationships by Azospirillum.. . . . . . . . . . . . Azospirillum nitrate reductase in plants . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Specificity and variability in Azospirillum. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . Interaction of Azospirillum with other soil-rhizosphere microflora . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . Azospirillum as a competitor in the rhizosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Interaction of Azospirillum with soil particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Genetics and immunology of Azospirillum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Agrotechnical aspects: inoculants and interaction with pesticides. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks and future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Current status of Azospirillum inoculation technology: Azospirillum as a challenge for agriculture

A β-glucoronidase (GUS)-marked strain of Herbaspirillum seropedicae Z67 was inoculated onto rice seedling cvs. IR42 and IR72. Internal populations peaked at over 106 log CFU per gram of fresh weight by 5 to 7 days after inoculation (DAI) but declined to 103 to 104 log CFU per gram of fresh weight by 28 DAI. GUS staining was most intense on coleoptiles, lateral roots, and at the junctions of some of the main and lateral roots. Bacteria entered the roots via cracks at the points of lateral root emergence, with cv. IR72 appearing to be more aggressively infected than cv. IR42. H. seropedicae subsequently colonized the root intercellular spaces, aerenchyma, and cortical cells, with a few penetrating the stele to enter the vascular tissue. Xylem vessels in leaves and stems were extensively colonized at 2 DAI but, in later harvests (7 and 13 DAI), a host defense reaction was often observed. Dense colonies of H. seropedicae with some bacteria expressing nitrogenase Fe-protein were seen within leaf and stem epide...

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Infection and Colonization of Rice Seedlings by the Plant Growth-Promoting Bacterium Herbaspirillum seropedicae Z67

A spontaneous mutant ofAzospirillum lipoferum, resistant to streptomycin and rifampicin, was inoculated into the soil immediately before and 10 days after transplanting of rice (Oryza sativa L.). Two rice varieties with high and low nitrogen-fixing supporting traits, Hua-chou-chi-mo-mor (Hua) and OS4, were used for the plant bacterial interaction study. The effect of inoculation on growth and grain and dry matter yields was evaluated in relation to nitrogen fixation, by in situ acetylene reduction assay,15N2 feeding and15N dilution techniques. A survey of the population of marker bacteria at maximum tillering, booting and heading revealed poor effectivety. The population of nativeAzospirillum followed no definite pattern. Acetylene-reducing activity (ARA) did not differ due to inoculation at two early stages but decreased in the inoculated plants at heading. In contrast, inoculation increased tiller number, plant height of Hua and early reproductive growth of both varieties. Grain yield of both varieties significantly increased along with the dry matter. Total N also increased in inoculated plants, which was less compared with dry matter increase.15N2 feeding of OS4 at heading showed more15N2 incorporation in the control than in the inoculated plants. The ARA,15N and N balance studies did not provide clear evidence that the promotion of growth and nitrogen uptake was due to higher N2 fixation.

The fate of markerAzospirillum lipoferum inoculated into rice and its effect on growth, yield and N2 fixation of plants studied by acetylene reduction,15N2 feeding and15N dilution techniques

D. N. Nayak

Papers

The fate of markerAzospirillum lipoferum inoculated into rice and its effect on growth, yield and N2 fixation of plants studied by acetylene reduction,15N2 feeding and15N dilution techniques