03163naa a2200181 a 450000100080000000500110000800800410001910000140006024501440007426000090021852023550022770000150258270000160259770000180261370000160263170000130264777303210266013150532008-10-06       2008    bl uuuu     u00u1 u    #d1 aHYODO, F.  aGradual enrichment of 15N with humification of diets in a belowground food webbrelationship between 15N and diet age determined using 14C.  c2008  aStable nitrogen (N) isotope has been widely used to disentangle food web and to infer trophic positions of organisms based on an assumption that the stepwise enrichment occurs along trophic levels. The enrichment of 15N in soil organisms with diet humificaition has also been reported , but the underlying mechanism has not been fully examined. To examine the effect of diet humification on 15N, we estimated the stable N isotope ratios and diet ages of earthworms and termites. These organisms feed on organic matter with various degrees of humification, ranging from undecomposed plant materials to humified organic matter (soil organic matter), in a gallery forest and a savannah in the Ivory Coast. We defined diet age as the time elapsed since carbon (C) in the diet of earthworms and termites was fixed from atmospheric CO2 by photosynthesis; it was estimated by comparing the radiocarbon (14C) content of these organisms to atmospheric 14CO2 records. Stable N isotope ratios increased along the humification gradient of diets, and values for earthworms and termites varied from 1.8 to 9.9? and from ? 1.5 to 15.9?, respectively. Epigeic (litter-feeding) earthworms had younger diet ages (2?4 y), whereas endogeic (soil-feeding) earthworms generally exhibited older diet ages (5?9 y). Grassfeeding termites had young diet ages (2 y), and wood/soil-feeding termites had the oldest diet ages (~50 y). Soil-feeding termites were similar in diet age (7?12 y) to wood feeders (8?11 y), with the exception of one species (18?21 y) that consumes large-diameter wood. A significant positive relationship was found between diet ages and stable N isotope ratios of the two groups in the savannah. This relationship held in the gallery forest when termites feeding on woody tissues were not considered. These results show that the stable N isotope ratios of organisms can increase with diet age, unless C in the diet has been stored as organic matter, such as woody tissue, that is able to age without being subject to humification processes. Given that aboveground food webs are often sustained directly by material and energy flow from belowground food webs, in addition to trophic interactions, gradual enrichment of 15N with the humification of belowground diets should be considered when interpreting stable N isotope ratios of terrestrial food webs.1 aTAYASU, I.1 aKONATÉ, S.1 aTONDOH, J. E.1 aLAVELLE, P.1 aWADA, E.  tIn: INTERNATIONAL COLLOQUIUM ON SOIL ZOOLOGY, 15; INTERNATIONAL COLLOQUIUM ON APTERYGOTA, 12., 2008, Curitiba. Biodiversity, conservation and sustainabele management of soil animal: abstracts. Colombo: Embrapa Florestas. Editors: George Gardner Brown; Klaus Dieter Sautter; Renato Marques; Amarildo Pasini. 1 CD-ROM.