Short-term incorporation of freshly fixed plant carbon into the soil animal food web: field study in a spruce forest

Abstract

We analyzed the dynamics of the short‐term incorporation of recently fixed carbon into the below‐ground food web in a boreal forest. Five young spruce trees (Picea abies) were pulse‐labeled with 13CO2 and the isotopic label was traced in soil invertebrates during 5 weeks. The freshly fixed plant carbon quickly entered both litter‐located and soil‐located compartments of the detrital food web. Among invertebrates inhabiting the mineral soil layers, a trophic link to the root‐derived C was most pronounced in species with higher δ15N values, suggesting this energy source to be more important in deeper mineral soil horizons. The label appeared faster in saprophagous animals than in predators (the median time lag after labeling was 6 and 12 days, respectively), but the difference was not significant. The label was recovered in 15 of 38 species of saprophagous animals and in 20 of 63 species of predators. Among saprophages, the frequency and intensity of the label was relatively high in endogeic collembolans and in bibionid larvae, but earthworms and enchytraeids were not labeled. Several groups of predators, lithobiid centipedes in particular, quickly acquired the root‐derived carbon, possibly indicating the feeding on live roots or mycorrhizal mycelium. In total, only 35 % of species or genera examined acquired the label. This suggests that majority of invertebrate taxa in the decomposer food web are unlikely to depend heavily on freshly fixed plant carbon provided by roots and root‐associated microorganisms.