Climate controls on forest soil C isotope ratios in the Southern Appalachian Mountains

A large portion of terrestrial carbon (C) resides in soil organic carbon (SOC). The dynamics of this large reservoir depend on many factors, including climate. Measurements of ¹³C:¹²C ratios, C concentrations, and C:N ratios at six forest sites in the Southern Appalachian Mountains (USA) were used to explore several hypotheses concerning the relative importance of factors that control soil organic matter (SOM) decomposition and SOC turnover. Mean δ¹³C values increased with soil depth and decreasing C concentrations along a continuum from fresh litter inputs to more decomposed soil constituents. Data from the six forest sites, in combination with data from a literature review, indicate that the extent of change in δ¹³C values from forest litter inputs to mineral soil (~20 cm deep) is significantly associated with mean annual temperature. The findings support a conceptual model of vertical changes in forest soil δ¹³C values, C concentrations, and C:N ratios that are interrelated through climate controls on decomposition. We hypothesize that, if other environmental factors (like soil moisture) are not limiting, then temperature and litter quality indirectly control the extent of isotopic fractionation during SOM decomposition in temperate forest ecosystems.