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characterized by population growth, over-exploitation, drought and desertification
that can reduce crop productivity. Microorganisms are essential members of these
arid soil environments. They help in rock weathering and soil crust formation. Soil
microbial communities are largely responsible for key ecosystem services including
soil fertility and climate regulation (Maestre et al. 2015 ). Also, microorganisms
have enormous metabolic potential and can adapt quickly to the rapidly-changing
environmental conditions of moisture availability, temperature fluctuations, UV
radiation, etc., making them suitable candidates for biotechnological applications.
Thus, the selection and enrichment of beneficial microorganisms in the vicinity
of plants should help to enhance the fitness and productivity of crop species. The
use of beneficial microorganisms as a low-cost and environmentally-friendly strat-
egy for productivity enhancement is highly relevant in dryland agriculture. In this
chapter, the interaction of soil microbes with key components of dryland agriculture
and their potential benefits with functional aspects are discussed.
2 Soil Microbial Communities
According to William Whitman, there are 5 × 10^30 prokaryotes on earth with 26 ×
1028 in the top 8 m of soil. One gram of healthy soil may contain as much as 108–9
bacteria, 105–8 actinomycetes, 105–6 fungi and 103–6 microalgae indicating the abun-
dance of microbial life. Further, 1 m^3 of soil may house hundreds of species of
bacteria, actinomycetes, fungi and algae, indicating diversity within the same group
of microorganisms. However, the estimated number of species is much higher than
that described, with even fewer cultured in the laboratory. There may be 1.5 million
species of fungi but only 5 % are described, and as many as one million species of
bacteria but only about 5000 have been described (Tilak 2000 ). The two parameters,
i.e. abundance and diversity, become important when evaluating the significance of
microorganisms in the soil. While abundance may increase or decrease over short
periods in response to weather, natural resources, ecology, cropping systems and
crop husbandry, diversity is a more complex and stable attribute which reflects a
state of near equilibrium. Diversity is more important in the understanding of the
functional significance of microorganisms at a given site. Abundance can vary
between soil types, seasons and land uses. Given the large fluctuations and unstable
numbers, microbial biomass is often used as a more reliable parameter to assess
abundance. In terms of biomass, fungi dominate in the soil followed by bacteria and
actinomycetes. Total population and live biomass only reflect the status of the soil
at a given point in time, without giving a clear picture of the living diversity as influ-
enced by different land use practices over time. The greatest uncertainty in popula-
tion counts reflects our inability to recover all of the organisms in the culture.
Generally, about 5–10 % of the organisms in the soil can be recovered through
normal viable counts. Even direct counting methods do not reflect the true
M. Grover et al.