Isotopic detection of recent photosynthate carbon flow into grassland rhizosphere fauna

In this study, we measured the incorporation of recent photosynthate-C inputs into active rhizosphere fauna (earthworms, enchytraeids, mites and collembolans) in an upland grassland soil under natural environmental conditions. This was achieved by means of a 13CO2 pulse-chase experiment made during the growing season, followed by a 20-day dynamic sampling of soil fauna for 12C/13C analysis by IRMS. The effect of post-13C labelling defoliation (cutting) on fauna 12C/13C ratios was also examined. Results showed that earthworms made up over 93% of the extracted fauna biomass, while mites, collembolans and enchytraeids together accounted for less than 7%. All fauna groups showed evidence of tracer 13C in their tissues within a week of 13CO2 pulse labelling in both control and cut treatments. Cutting significantly increased the amount of tracer 13C entering the organisms (P=0.0002). Similarly, the fauna group also had a significant effect (P=0.0001). Time did not have any effect on fauna 13C content between groups as differences were only significant at the last sampling occasion. The interaction time×animal group, however, had a significant effect (P=0.0054). Collembolans accounted for most of the tracer 13C measured within the fauna biomass, i.e. mean±standard deviation of 44.78±12.75% and 44.29±14.69% of fauna 13C in control and cut treatments, respectively. Mites and earthworms contained between 22.13% and 28.45%, and enchytraeids less than 6% of the tracer 13C. We conclude that, during the growing season, there was a rapid incorporation of recent photosynthate-C into rhizosphere mesofauna. This carbon transfer was most significantly increased by defoliation in mites and collembolans (P<0.01). These results provide evidence that soil foodweb carbon dynamics are not solely underpinned by detrital decomposition but are also affected by short-term plant rhizodeposition patterns.