The δ15N signature of the detrital food web tracks a landscape-scale soil phosphorus gradient in a Costa Rican lowland tropical rain forest

In this study, we investigated whether landscape-scale variation of soil P accounts for C-13 and N-15 composition of detrital invertebrates in a lowland tropical rain forest in Costa Rica. The top 10-cm soil, leaf-litter samples and plant foliage were collected among 18 plots representing a three-fold soil P gradient during 2007-2009. Body tissue of litter invertebrates (extracted from leaf-litter samples) along with soil, leaf litter and green foliage were analysed for total C, total N, delta C-13 and delta N-15 values. Differences in delta C-13 and delta N-15 signatures across plots and relative trophic distances of detrital food webs (Delta delta N-15), and their variation with soil P gradient were evaluated. We found soil P gradient had a significantly positive correlation with delta N-15 of Asterogyne martiana foliage, leaf litter, collembolans and oribatid mites. The delta N-15 of the collembolans and pseudoscorpions positively correlated to leaf-litter delta N-15. Delta delta N-15 between the trophic levels remained consistent across the soil P gradient. Higher delta N-15 in the collembolans and oribatid mites might be derived from their consumption on N-15-enriched decayed debris or fungal hyphae growing on it. It suggests that fine-scale soil P variation can affect trophic dynamics of detrital arthropods via regulation of microbial community and nutrient dynamics.