Minor stable carbon isotope fractionation between respired carbon dioxide and bulk soil organic matter during laboratory incubation of topsoil

A common assumption in paleoenvironmental reconstructions using soils is that the carbon isotope composition of soil-respired CO₂is equivalent to the carbon isotope composition of bulk soil organic matter (SOM). However, the occurrence of a non-zero per mil carbon isotope enrichment factor between CO₂and SOM ([Formula: see text]) during soil respiration is the most widely accepted explanation for the down-profile increase in SOM δ¹³C values commonly observed in well-drained soils. In order to shed light on this apparent discrepancy, we incubated soil samples collected from the top 2 cm of soils with pure C₃vegetation and compared the δ¹³C values of soil-respired CO₂to the δ¹³C values of bulk SOM. Our results show near-zero [Formula: see text] values (−0.3 to 0.4 ‰), supporting the use of paleosol organic matter as a proxy for paleo soil-respired CO₂. Significantly more negative [Formula: see text] values are required to explain the typical δ¹³C profiles of SOM in well-drained soils. Therefore our results also suggest that typical SOM δ¹³C profiles result from either (1) a process other than carbon isotope fractionation between CO₂and SOM during soil respiration or (2) [Formula: see text] values that become increasingly negative as SOM matures.