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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: [email protected] |
