Assimilation of plant-derived freshly fixed carbon by soil collembolans: Not only via roots?

• We traced C isotopic label from the spruce trees to Collembola in the field study.
• The frequency of label was high and increased with time in euedaphic springtails.
• The label was also recovered in epigeic species, but was rare in hemiedaphic ones.
• Soil-dwelling and epigeic species likely assimilated plant C via different pathways.

Large amounts of freshly fixed carbon in forest ecosystems are allocated belowground and fuel soil food webs. This supply is usually generalized as the ‘root carbon’, but particular pathways by which freshly fixed carbon is assimilated by soil animals remain poorly understood. We followed the appearance of the isotopic label in springtails inhabiting different layers of soil and litter (euedaphic, hemiedaphic and epigeic life forms) after in situ pulse-labelling of spruce trees with 13CCO2. Isotopic label was most frequently observed in euedaphic Collembola (26% of all samples), mainly in Protaphorura armata. The label was very rare in litter-dwelling (hemiedaphic) species (4%). Surprisingly, the label was often observed in epigeic Collembola (11%). During the period of 44 days after labelling, the proportion of labelled samples increased with time in euedaphic, but not in epigeic species, suggesting that freshly fixed carbon is assimilated by these collembolan groups via different pathways. Most likely, euedaphic species are trophically linked to plant roots, whereas epigeic species receive freshly fixed carbon from aboveground. The exact mechanisms and potential importance of the latter pathway should be evaluated in further research.