Methanogenic Archea are small microorganisms involved in the decomposition of organic matter in soil, by using H2 or some times CO2 as a substrate and eventually produce methane which is the 2nd highest green house gase after after CO2.
Archea identification, activity, diversity and community structure can determine the production and oxidation rate of methane.
Lipids will only provide a semiquantitative measure for archaeal abundance and the lipid pool will comprise compounds from alive and dead organisms (also looking at intact polar lipids will not necessarily give you a measure for alive biomass). I can give you a protocol for GC-detectable lipids (isoprenoidal di-ethers are in the "alcohol fraction"), which includes isoprenoidal glycerol di-ethers but I'd suggest to use a different approach such as fluorescence in situ hybridisation with which you can count eg archaeal cells by fluorescence microscopy.
Helge Niemann! Thanks for your answer, and also for the valuable suggestion, i myself is planning to use FISH technology.