Dual-chamber measurements of δ13C of soil-respired CO2 1 partitioned using a field-based three end-member model

The contribution of ‘historical’ soil C (SOM) to total soil respiration (RS) in forest has been a crucial topic in global change research, but yet remains still uncertain. One of the contributing factors of this uncertainty is the difficulty to in reliably measuringe and partitioning key carbon-cycle processes. Isotopic methods, such as natural variations in carbon isotope composition (δ13C) of soil respiration, are more frequently being applied, and show promise in separating heterotrophic and autotrophic contributions to RS. However, natural and artificial modification of δ13CRs can cause isotopic disequilibria in the soil-atmosphere system generating a mismatch between what is usually measured and what process-based models can predict. Here we report the partitioning of the soil-surface CO2 flux in a warm Mediterranean forest into components derived from root, litter/humus and SOM sources (using a new, three end-member mixing model,) and compare this with the conventional partitioning into autotrophic and heterotrophic components. The three end-member mixing model takes into account both the discriminations during CO2 respiration/decomposition of the three components, as well as the fractions of their CO2 fluxes integrated over their total soil profile mass. In addition, we used a novel dual-chamber technique to ensure that δ13CRs was subjected to minimal artefacts during measurement. We observed that by using measured soil surface CO2 concentrations as a baseline 30 level for the dual-chamber operation, it was possible to achieve and monitor the 31 necessary conservation of the soil CO2 steady-state diffusion conditions during the 32 measurements, without using permanent collars inserted deeply into the soil. When 33 RS (8.64 gCO2·m2·d-1) was partitioned into two in two ways, the mean autotrophic 34 and heterotrophic respiration was 56 and 44%, respectively. Whereas, wWhen RS 35 was partitioned using the three- way model, however, roots, litter/humus, and SOM 36 contributed 30, 33, and 37% of the total flux. Our results clearly confirm that the use 37 of natural variations in carbon isotope composition of RS, and its components, can 38 represent an important toolbe used for to separateing autotrophic and heterotrophic 39 components. However, in order to improve the estimateions of the partitioning 40 method, it is also important to distinguish the fractional contribution of the long-term 41 SOM-derived flux from younger and more labile sources.JRC.H.7-Climate Risk Managemen