Effect of the Relative Abundance of Conifers Versus Hardwoods on Soil δ13C Enrichment with Soil Depth in Eastern Canadian forests

Soils are a major component of the global C cycle, and considerable effort has been dedicated to improve our understanding of factors controlling soil organic C (SOC) turnover and stabilization in the last decades. Carbon stable isotopes are useful in this respect as they represent an integrative indicator of SOC biogeochemical processing. In the present study, C concentration and delta C-13 were measured in soil horizons of 21 forest sites located at the transition zone between the hardwood and the conifer forest in Qu,bec, Canada, and related to 13 biophysical variables to identify the main drivers of SOC storage and turnover. Carbon concentrations in the forest floor (FF) and the B- and C-horizons were, respectively, strongly correlated with percentage of clay (Pclay), the mean annual precipitation: potential evapotranspiration ratio (MAP:PET), and percentage of hardwoods (Phwd). In FF, delta C-13 was poorly correlated with the studied variables, whereas in mineral horizons, it was significantly correlated with mean annual air temperature (MAAT) and the percentage of conifers (Pc) and Pclay. Across the studied area, delta C-13 increased on average by 2.0aEuro degrees from the FF to the C-horizon. The isotopic enrichment with soil depth (beta) was strongly negatively correlated with Pc, which explained 55% of its variability among sites. This suggests that the vegetation type is an important driver of soil C long-term turnover rate in forest ecosystems. Overall, our data suggest that hardwood forest expansion in response to climate change might reduce the stability and the storage of SOC in the future.