14C -: a tool for separation of autotrophic and heterotrophic soil respiration

We assessed the potential of using C-14 contents of soil respired CO2 to calculate the contributions of heterotrophic and autotrophic respiration to total soil respiration. The partitioning of these fluxes is of utmost importance to evaluate implications of environmental change on soil carbon cycling and sequestration. At three girdled forest stands in Sweden and Germany, where the tree root (autotrophic) respiration had been eliminated, we measured both flux rates and C-14 contents of soil respired CO2 in girdled and control plots in the summers of 2001 or 2002. At all stands, CO2 flux rates were slightly higher in the control plots, whereas the C-14 contents of respired CO2 tended to be higher in the girdled plots. This was expected and confirmed that heterotrophically respired CO2 cycles more slowly through the forest ecosystem than autotrophically respired CO2. On the basis of these data, the contributions of hetero- and autotrophic respiration to total soil respiration were calculated using two separate approaches (i.e. based on flux rates or C-14). Fractions of heterotrophic respiration ranged from 53% to 87%. Values calculated by both approaches did not differ significantly from each other. Finally, we compared the C-14 contents of soil respired CO2 in the girdled plots with the C-14 contents of heterotrophically respired CO2 calculated by three different C-14 models. None of the models matched the measured data sufficiently. In addition, we suspect that inherent effects of girdling may cause the C-14 content of CO2 respired in the girdled plots to be lower than 'true' heterotrophically respired CO2 in an undisturbed plot. Nevertheless, we argue that measurements and modeling of C-14 can be developed into a valuable tool for separating heterotrophic and autotrophic soil respiration (e.g. when girdling cannot be performed).