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

We assessed the potential of using ¹⁴C 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 ¹⁴C 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 ¹⁴C 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 ¹⁴C). 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 ¹⁴C contents of soil respired CO₂ in the girdled plots with the ¹⁴C contents of heterotrophically respired CO₂ calculated by three different ¹⁴C models. None of the models matched the measured data sufficiently. In addition, we suspect that inherent effects of girdling may cause the ¹⁴C 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 ¹⁴C can be developed into a valuable tool for separating heterotrophic and autotrophic soil respiration (e.g. when girdling cannot be performed).