495Interestingly, the mechanisms described above are interlinked and contribute to
the biogeochemical cycle. In dryland systems, the process of carbon recycling is
influenced by natural processes as well as human interventions. Disturbing the natu-
ral resource base often leads to the loss of carbon from cultivable soils thus render-
ing them uncultivable. For instance, when land is not cultivated, the soil surface is
exposed to extreme weather events such as intense rainfall events, extreme tempera-
tures and high wind velocities, thereby reducing microbial diversity and increasing
carbon loss.
The association of mycorrhizal fungi with plant roots improves the plant’s
absorption of soil phosphates and other nutrients. The mycorrhizae also increase the
capacity of biological nitrogen fixers to fix more nitrogen. Soil microorganisms
multiply and die at enormous rates and, in doing so, release a constant stream of
nutrients available to plants. Plants then grow better, assimilate more energy and
provide more food in the form of root exudates which attract more microbes. In this
mutually-beneficial two-way process, the size of the microbial population is gov-
erned by the plant inputs. Soils with high carbon matter have high populations of
soil microorganisms compared with soils with low carbon content (drylands). Low
populations of microorganisms mean low nutrient turnover and reduced plant
growth. In such situations, strategies to increase the carbon content of soils through
organic inputs and management practices will be beneficial. In tests conducted by
the Texas Plant and Soil Laboratory at Edinburgh, Texas, the mineral uptake of
crops grown in soils with very high amounts of minerals such as phosphorus, cal-
cium, magnesium, potassium, and sulfur was deficient; in a plant tissue and petiole
test, plants had only 20 % of the necessary mineral uptake for good plant growth and
health (http://www.geotexsupply.com/geotexsupply6). The addition of microbes
increased plant mineral uptake by 60 % to 80 % compared with the pre-treatment
test. The microbes were able to digest (solubilize) the minerals in the soil into the
soil solution making them available for uptake by the plant.
Alfisols and Vertisols, two predominant soil types in dryland areas, are generally
deficient in essential nutrients. However, the application of manures and mineral
fertilizers to improve soil fertility significantly impacts microbial diversity. Many
studies have shown the positive effect of organic manures on soil biological health.
The use of chemical fertilizers (nitrogen, phosphorus and potassium: NPK) enhances
crop yield but can alter soil properties and the functional diversity and enzymatic
activities of microbial populations. Sharma et al. ( 1983 ) reported that the applica-
tion of nitrogen fertilizers such as ammonium sulfate only increased the fungal
population while FYM and NPK applications increased the populations of fungi,
bacteria and actinomycetes. Long-term fertilizer trial plots in alfisols had more
microbial species diversity in the FYM plots than the chemical fertilizer and control
plots. While organic-manured plots had more population and species richness, the
continuous application of NPK did not significantly change the diversity of fungi or
bacteria compared with the control plots. Certain species such as Chaetomium,
Monilia, Trichoderma and Spicaria were more frequently isolated in FYM plots
than control or chemical fertilizer plots (Venkateswarlu and Srinivasarao 2005 ). A
positive effect of organic inputs alone or in conjunction with chemical fertilizers has
Application of Microbiology in Dryland Agriculture