Microbial biomass in upland soil and rhizosphere.




Soil microbial biomass (hereafter referred to as microbial biomass), defined holistically as
the living component of soil organic matter (Le., all organisms with a volume less than
5,000 fJ-m3 , e.g., bacteria, fungi, protozoa), is actively involved in biogeochemical processes
that occur in soil microniches of paddy soils. These processes include organic matter
decomposition, microbial oxidoreduction, and cycling of N, C, and plant nutrients. The
nature and extent of these biogeochemical processes cannot be approximated without
understanding the involvement of microorganisms in these processes. Microbially-mediated
biogeochemical processes, such as photosynthesis, N2-fixation, organic matter decomposition,
subsoil oxidoreduction, and nutrient immobilization, lead to increases in the content
of microbial biomass, while processes, such as biomass turnover and mineralization, lead to
its decrease. In addition, the level of microbial biomass in the paddy soil ecosystem is
affected by many biotic and abiotic factors, such as N fertilization, organic matter applications,
soil type, flooded-upland soil rotation, and soil depth. Methods to measure soil microbial
biomass as a single pool of organic matter include substrate-induced respiration,
chloroform fumigation-incubation, chloroform fumigation-extraction, and adenosine triphosphate.
However, these holistic methods provide little information about the community
composition and physiological state of the soil microbial biomass and reasons why the soil
microbial biomass changes over time and under different conditions. However, these methods
are useful in understanding of cycling and dynamics of soil organic matter, especially
where whole suites of organisms are involved. Culturing and isolation of microorganism
provide answers to the shortcomings of the single pool biomass methods. Newer methods
that can provide valuable information about the physiological state of soil microbial communities
and provide more sensitivity to detect changes in these communities include:
gene-based analytical methods, microbial activity, tracer isotopes, and analysis of biomarkers.
Depending upon the objectives of a study or the problem to be addressed, any of the
analytical methods described offer a best and efficient approach to analyze soil microbial

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