Use of radiocarbon to estimate diet ages of earthworms across different climate regions

Natural abundance of radiocarbon (¹⁴C) has been applied to estimate the turnover time of soil carbon (C) across different climate regions. However, despite the important functional role played by soil animals in decomposition processes, little is known about variation in their ¹⁴C concentrations across different climate regions. In this study, we measured ¹⁴C concentrations of earthworms collected in three forests in Japan. In addition, we also reviewed ¹⁴C data on earthworms that were previously reported. We used these data to test whether the diet ages (defined as time elapsed since C in the diet of earthworms was fixed from atmospheric CO₂ by photosynthesis) differed according to feeding habits and across study sites in various climate regions ranging from cool temperate forest to tropical savanna. Multiple regression analysis showed that the diet ages of earthworms were significantly affected by both feeding habits and study sites. The diet ages of endogeic (soil-feeding) earthworms (8.3±0.4 years, mean±SE) were significantly older than those of epigeic (litter-feeding) earthworms (2.6±0.5 years), with anecic (litter-/soil-feeding) earthworms (5.7±0.9 years) having intermediate diet ages. When mean diet age was compared for each feeding habit, only that of endogeic earthworms differed significantly across the sites. However, it did not necessarily become younger in warmer climate regions. These results either suggest that the degree of decomposition of soil organic matter used by earthworms differs among the study sites, or that the difference in the turnover time of soil organic C used by earthworms across the sites is relatively small and variable due to factors other than temperature, such as soil texture and vegetation.