Soils are extremely complex ecosystems, containing an enormous diversity of microorganisms and soil fauna, which are difficult to study without disturbing the natural soil system. Various techniques are used to study these complicated systems. In particular, ratios of the carbon isotopes ¹²C, ¹³C, and ¹⁴C, which differ in natural materials, can be used to trace carbon in the environment using either natural abundances or labeling methodologies.

Carbon-14 natural abundances have been used to estimate turnover time of soil organic matter (SOM). Toyota et al. (submitted) compared carbon-14 signals in the forest soils, which have different vegetation history, and showed different accumulation patterns in relation to the vegetation change. They also estimated the diet age of earthworms, a dominant macro-invertebrate, using carbon-14 and showed the difference in age-dependence of soil animals corresponding to the feeding habits. I show some other on-going studies using carbon-14 signatures and emphasize a potential utility of carbon-14 natural abundances on the study of carbon assimilation and carbon sequestration. Finally, I conclude the utility of carbon and nitrogen stable isotope ratios together with carbon-14 analysis in the study of soil food webs.