Isotopic Composition Of Microbial Biomass

Determining the isotopic composition of microbial biomass C is a further important tool for studying soil microbial ecology and the decomposition and immobilization of soil organic C. The use of the CFI method is restricted to the isotopes 14C, 13C, and 15N, whereas the CFE method can be used with a larger range of isotopes (i.e., 14C, 13C, 15N, 32P, and 35S). Recently, 13C studies have gained increasing interest due to the improved sensitivity of 13C measurements and due to its nonradioactive nature (in contrast to 14C studies). Since the tissues of C3 plants (e.g., wheat) and C4 plants (e.g., maize) differ in their natural abundance of 13C, these plant materials have a natural label that can be used for decomposition studies in both microcosm and field studies. 13C/12C determinations are performed with an offline sample preparation technique combined with isotope analysis by a dual-inlet IRMS (isotope ratio mass spectrometer) or an online analysis using an element analyzer connected to an isotope ratio mass spectrometer. A new method is based on the UV-catalyzed liquid oxidation of fumigated and nonfumigated soil extracts combined with trapping of the released CO2 in liquid N2; 613CO2-C is subsequently determined with a gas chromatograph connected to an IRMS (Potthoff et al., 2003). The 13C analysis can also be done using an automated continuous-flow IRMS.

signature molecules as a measure of microbial biomass and microbial community structure

Various cellular constituents can be used to estimate microbial biomass and subdivide community members into broad groups. These signature molecules include adenosine triphosphate (ATP), microbial membrane components, and respiratory quinones. Each type of molecule differs in its level of resolution. The amount of ATP extracted from soil gives a measure of the energy charge of all soil biota, whereas ergosterol, a component of fungal cell walls, is attributed only to the fungal biomass. Signature molecules such as phospholipid fatty acids or respiratory quinones are used as indicators of the microbial community's structural diversity. A prerequisite for the use of biochemical compounds as signature molecules is that they are unstable outside the cell, because the compound extracted from soil should represent living organisms only.

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