Role of microbial biomass in biogeochemical processes in paddy soil environments

Soil microbial biomass (hereafter referred to as microbial biomass), defined holistically as the living component of soil organic matter (i.e., all organisms with a volume less than 5,000 micro cubic m, 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-flxation, 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 biomass.