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Vol. 18 No. 3 - July 2012

Microbes for Soil Sustainability and Crop Productivity

By: Chandra Shekhar Nautiyal*

Feeding our ever-expanding population is one of the greatest challenges facing mankind. Increasing population of the world is raising alarm that the requirement of food will be 28.8 million ton, while their availability will be only 21.6 million ton by 2020. Soil being a critical component on earth not only for sufficient food production but also for maintaining the sustainable global environmental conditions, attract many researchers to evaluate its role in various direct and indirect physico-chemical and biological processes. The soil quality has been defined by many researchers as ‘the capacity of a specific kind of soil to function, within natural or managed ecosystem boundaries, to sustain biological productivity, promote environmental quality, and maintain plant and animal health. Soil erosion, atmospheric pollution, extensive soil cultivation and grazing, salinization and desertification decrease the productive agricultural land.

A unique balance of chemical, physical and biological components contribute towards maintaining soil quality. Agri-ecosystem functioning is governed largely by soil microbial dynamics. Sustainable productive agriculture depends on a healthy community of soil microbes that decompose organic matter and contribute to the biological recycling of chemical nutrients that affect soil fertility. Thus interactions between the diversity of primary producers (plants) and decomposers (microbes) are the two key functional groups that form the basis of all ecosystems and have major consequences on the functioning of agricultural ecosystems.

Soil microorganisms are highly diverse and abundant organisms on earth; one gram of soil may contain billions of microbes with thousand of different species. Several biotic or abiotic factors lead to the alteration of microbial community structure and composition which may directly or indirectly influence the soil ecosystem, nutrient cycle activity and crop production. In addition, anthropogenic intervention for the management and treatment of soil involving pesticide, chitin, compost, manure or genetically modified microorganisms and plants also influence microbial diversity. The excessive use of chemical fertilizers and pesticides to attain the green revolution has made the food and environment contaminated with chemicals and thereby putting ill effects on our health. Though the use of chemical fertilizers (nitrogen, phosphorus and potassium: NPK) enhance crop yield, but brings alteration in soil properties, functional diversity in microbial population and their enzymatic activities. Further, there is more demand for agrochemicals with degrading soil quality, and the enhanced cost is prohibitive and even unaffordable by small and marginal farmers. Contrary to this, several researchers have demonstrated that organic farming (compost and green manure) leads to improved soil quality with higher microbiological activity involving crop rotations, reduced application of synthetic nutrients and no pesticides. Along with the cost of chemical fertilizers now people are getting aware about the negative effects of chemical fertilizers on the soil health and crop productivity. In addition, we can now see a renewed recognition of the ill effects of agrochemicals and the central role of soil resources for assuring food security and the increased awareness that soil microorganisms play a fundamental role in sustainable agriculture that has triggered numerous studies and projects taking initiatives of organic farming. Several studies have been made to evaluate the application of fresh and composted organic wastes modifying the structure, size and activity of soil microbial community.

Although long-term impact of chemical fertilizers on soil microbial biomass and diversity is not well documented, however, it has been shown that chemical fertilizers could increase the soil microbial biomass, carbon and nitrogen, and exerts no significant change in the microbial characteristics of the soil. Changes in microbial parameter are correlated with the soil organic carbon content and not to the application of P and N. Evidences linking direct impact of chemical fertilizers on microbial diversity function and phylogeny are not so evident. Crop productivity greatly depends upon the amount of available nutrients in the soil, which is governed by transformations of soil microbial biomass. Thus the growth and activity of microorganisms are functions of soil properties, such as nutrition, texture, pH, temperature, moisture content, as they are sensitive indicators of changes in soil properties. Supporting the choice of organic farming, with credible science can be vital for improving the overall productivity, food security, food sovereignty and environmental impact on agriculture in the country. For conversion of a conventional field to organic, first step is to build up the lost fertility of the soil. This can be achieved by complete restriction on the use of synthetic agrochemicals and increased use of organic and biological inputs. For nutrient management and soil fertility, built up crop residue, animal dung, forest leaf litter, bone meal, blood meal, slaughter house waste and green manures are important organic sources. All such organic material needs to be composted properly for appropriate impact. Nutrient value of the raw material and composting methodology determines the quality of produce. Biological resources such as biofertilizers, biopesticides and other microbiological inputs to improve the soil fertility and reducing the use of chemical fertilizers and pesticides have also attracted lot of attention. Plant growth-promoting rhizobacteria (PGPR) are the rhizosphere bacteria that can enhance plant growth by a wide variety of mechanisms. Recent progress in our understanding on the diversity of PGPR in the rhizosphere along with their colonization ability and mechanism of action should facilitate their application as a reliable component in the management of sustainable agricultural system. In accordance with their mode of action, PGPRs can be classified as biofertilizers, phytostimulators and biopesticides with certain bacteria having overlapping applications. However, screening strategies for selecting the best rhizobacterial strain for rhizosphere competence and studies on the ecology of introduced PGPR with the resident PGPR and other microbial species in the plant rhizosphere will require more comprehensive knowledge, although the involvement of ACC deaminase gene, siderophore, phosphate, phytohormones like IAA, cytokinin, gibberellins etc., noduation, disease suppression and their coordinated expression seemed to be responsible in enhancing the plant growth, yield and nutrient uptake of various crop plants in different agro ecosystems. Short-term interventions to provide food, water and basic needs such as seeds and fertilizer to kick-start agriculture in response to food crisis and extreme weather is not the solution. Longer-term and large-scale measures are needed in order to build greater resilience to soil degradation, drought and climate change and reduce human vulnerability to disasters.

 Change in land use / agro-ecosystems can change the microbial community structure of the soil and vice-versa. Consequently, the impact of different crop species that are cultivated in various combinations is likely to be an important factor in determining the structure of microbial community that can beneficially function in nutrient cycling, the production of plant growth hormones and suppression of root disease. Improvement in agricultural sustainability requires optimal use and management of soil fertility and soil physical properties.

 There is a lot to discover and understand the complex yet beneficial aspect of plant and microbial interaction in light of the present challenge like drought, soil fertility loss, climate change and soil contamination. In order to secure the food security through sustainable agricultural practices role of soil microbes is certainly a high priority component for productivity of agro-ecosystem.

Carefully controlled field trials of crop plants inoculated along with rhizobacteria are necessary for maximum commercial exploitation of PGPR strains. An increased knowledge of microbe based symbioses in plants could provide potential ways of developing sustainable agriculture in order to ensure human and animal food production with minimal risk to the environmental health. These measures need to be promoted for popularization among farmers on large scale in order to achieve sustainable agriculture and food security.

*Director, CSIR-National Botanical Research Institute, Rana Pratap Marg, Lucknow – 226001, India; E-mail: csn@nbri.res.in

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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