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 (CO2)-C-13 pulse-chase experiment made during the growing season, followed by a 20-day dynamic sampling of soil fauna for analysis by IRMS. The effect of post-C-13 labelling defoliation (cutting) on fauna C-12/C-13 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 C-13 in their tissues within a week of, (CO2)-C-3 Put Se labelling in both control and cut treatments. Cutting significantly increased the amount of tracer (13) C 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 C-13 content between groups as differences were only significant at the last sampling occasion. The interaction time x animal group, however, had a significant effect (P = 0.0054). Collembolans accounted for most of the tracer C-13 measured within the fauna biomass, i.e. mean +/- standard deviation of 44.78 +/- 12.75% and 44.29 +/- 14.69% of fauna C-13 in control and cut treatments, respectively. Mites and earthworms contained between 22.13% and 28.45%, and enchytraeids less than 6% of the tracer C-13. We conclude that, during the growing season, there was a rapid incorporation of recent photosynthate-C into rhizosphere mesofatma. 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. (c) 2006 Elsevier Ltd. All rights reserved.