493composition of the soil population. Therefore, most researchers attempt to deter-
mine the culturable biodiversity (Venkateswarlu and Srinivasarao 2005 ). In recent
years, with the development of novel molecular biology techniques, some reports
on molecular biodiversity have used metagenomic approaches.
Microorganisms play an essential role in plant establishment through favorable
activities that contribute to plant growth. These include (1) transformation of soil
organic matter and hence nutrient cycling; (2) formation of stable soil structure; (3)
plant growth regulation (through the production of phytohormones, enzymes and
other growth regulators); (4) antibiosis; (5) induced resistance (against pests); and
(6) induced tolerance (to various abiotic stresses) (Lynch and Bragg 1985 ; Glick
1995 ; Grover et al. 2011 , 2015 ; Miransari 2013 ;). Several groups of organisms act
both competitively and synergistically to mediate the above processes.
The functional diversity of microorganisms in stressed ecosystems has been
extensively studied (Graham 1992 ; Zahran 1999 ; Venkateswarlu et al. 2008 ). The
occurrence of Rhizobium, Bradyrhizobium, Azotobacter, Azospirillum, Pseudomonas
and Bacillus has been reported in desert ecosystems, acid soils, saline and alkaline
areas, and highly-eroded hill slopes in India (Grover et al. 2011 ). The diversity of
bacterial communities associated with different plants grown under various arid
regions has been reviewed recently (Soussi et al. 2015 ). Selection and deployment
of these plant-associated bacterial assemblages are mediated by a combination of
bio-pedo-agroclimatic conditions and the plant species/varieties. Diversity and
functional redundancy of these microorganisms make them active in supporting
plant growth under stressed environments (Soussi et al. 2015 ). However, proper
management of soil microflora for agricultural sustainability is the major
challenge.
3 Interactions Between Key Components of Dryland
Agriculture and Soil Microbes
3.1 Nutrient Management
To date, 17 macro- and micro-nutrients are essential for optimal plant growth and
development. Nutrient availability is an important factor affecting plant growth.
Any imbalance in the soil due to the absence or deficiency of any of these nutrients
can adversely affect plant growth and productivity. Soil biological communities
play a key role in nutrient management by transforming nutrients in organic and
inorganic forms. In this transformation process, soil microorganisms make usable
(inorganic) forms of different nutrients available to plants. Soil organisms also store
nutrients in the soil as organic compounds. The different kinds of transformations
performed by soil microorganisms are discussed below.
Application of Microbiology in Dryland Agriculture