Separating soil respiration components with stable isotopes: natural abundance and labelling approaches

Due to the potential of forest ecosystems contributing to CO2 increase as well as to climate change mitigation, forest-atmosphere CO2 exchange has been intensively studied over last decades. However, the contribution of individual components of belowground carbon pools is still poorly known. In particular, there is no unequivocal means to separate root respiration (autotrophic) from heterotrophic respiration by soil microflora and fauna. Most studies investigating soil respiration disturbed the soil and tried to exclude autrophic respiration. Here we review alternative non invasive methods to separate the two components. Those methods share the application of the stable carbon isotope C-13, using either local changes in natural abundance of (CO2)-C-13 or artificial labelling of trees with CO2 enriched or depleted in C-13. We conclude that the applicability of natural stable isotope methods is still limited in forest ecosystems because only in a few cases there are large enough differences in C-13 among soil carbon pools (usually due to the earlier presence of a C-4 canopy, which is seldom in forests). On the other hand, artificial labelling with CO2 either enriched or depleted in C-13 is now in a widely used for partitioning soil respiration components. However, recent findings gave clear evidence that measurements of soil CO2 efflux can be substantially influenced by the return efflux of the abiotic label. Still, especially the combination of Free Air Carbon Enrichment (FACE) with dual delta C-13 and delta O-18 stable isotope approach has the potential to provide new answers on the response sensitivity of turnover dynamics of the largest belowground soil carbon storage to elevated temperature and CO2.