Tree species' influences on soil carbon dynamics revealed with natural abundance 13C techniques

The carbon (C) sequestration potential of land-use practices is increasingly important. Trees sequester atmospheric C into biomass and above and belowground litter but may also prime the decomposition of soil organic matter (SOM). We compared the influence of Acer pseudoplatanus (Sycamore) and Larix x. europlepsis (Hybrid Larch) on soil C decomposition. We used natural abundance C-13 to partition soil-surface CO2 efflux into root and SOM sources. CO2 was sampled from incubated root-free soil and from live tree roots using in-situ chambers. Combined surface efflux delta(CO2)-C-13 was measured using dynamic chambers and cavity-ringdown spectroscopy. Under Sycamore, CO2 emissions were dominated (80-90 %) by root respiration. SOM contributed 10-20 % with a mean residence time of centuries. Under Larch, 24-33 % of total CO2 efflux was root respiration, the remainder originating from an SOM pool with a turnover time of decades. Total soil C stocks were similar between the two plot types. Root-respired delta(CO2)-C-13 was consistently different by c. 2 aEuro degrees between the species. The decomposition rate of soil C and its mean residence time are markedly different under the two tree species. Species differences in root-respired delta(CO2)-C-13 may reflect plant C allocation or respiratory fractionation.