Potentially any carbon containing compound which can be labelled with 13C can be added to soil directly and the 13C values of PLFAs followed to assess utilisation of the labelled carbon component of the compound by various soil mmicrobe groups. The added 13C substrates include plant litter, root residue, root exudates and environmental pollutants. The use of 13C labeled CH4 or CO2 coupled with compound specific analysis of PLFAs has been also used to study specific microbial communities in soil-plant system