Estimation of the longevity of C in terrestrial detrital food webs using radiocarbon (14C):: how old are diets in termites?

We propose that diets of consumers in a food web have various ages, where age is defined as the time elapsed since carbon (C) in the diet was fixed from atmospheric CO2 by primary producers. To examine the diet ages for primary consumers in a detrital food web, we measured the radiocarbon (C-14) content of termites collected in Thailand in 1998 and 2004. Diet ages were estimated by comparing the C-14 content of samples with records of atmospheric (CO2)-C-14, which doubled in the early 1960s as a result of nuclear weapons tests and decreased after the nuclear test ban treaty. For comparison, we measured the C-14 content of bees as primary consumers in a grazing web at the same study site. Stable carbon and nitrogen (N) isotope ratios were also analysed. The C-14 contents of the same species of termites decreased during the sampling interval, indicating that they used organic matter produced after the peak in atmospheric (CO2)-C-14. The diet ages were estimated to be 12-18, 7-13 and 5-9 years for the wood-feeder (Microcerotermes crassus), the soil-feeders (Dicuspiditermes makhamensis and Termes comis) and the fungus-grower (Macrotermes carbonarius), respectively. One colony of soil-feeder (T. comis), which nested in a fallen tree trunk, had exceptionally low C-14 content, and its diet age was estimated to be around 50 years. The two bee species had lower C-14 contents compared with the termites, and their diet ages were estimated to be 0 (Apis florea) and 2-4 years (Trigona sp.). Stable C and N isotope ratios of termites showed similar patterns as previously reported, and no clear difference was observed between 1998 and 2004. Although the bees and the fungus-growing termite had similar stable C and N isotope ratios, their diet ages differed. Our study suggests that radiocarbon can be used to estimate the diet ages of consumers in terrestrial food webs. Diet age should provide new insight into the trophic positions of organisms in grazing and detrital food webs and the interactions between these two webs.