In soil-plant ecosystems, a common form of 13C labelling used in conjunction with compound specific isotope analysis of soil PLFAs is that where atmospheric CO2 is labelled. The 13CO2 is first fixed by plants in the process of photosynthesis; labelled plant inputs then enter the soil to be potentially taken up and assimilated by soil microbe, subsequently labelling their PLFAs. Two main approaches are used to deliver 13CO2 to the plants: either pulse or continuous steady state labelling. In pulse labelling, as its name suggests, the 13CO2 is delivered to plants over discrete period of time