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Keywords:

  • boreal soils;
  • carbon stabilization;
  • CPMAS 13C NMR;
  • jack pine;
  • soil fractionation;
  • soil organic matter

Abstract

Boreal forests, containing >20% of the total organic carbon (OC) present at the surface of the Earth, are expected to be highly vulnerable to global warming. The objective of this study was to compare soil OC stocks and chemistry in jack pine stands located along a latitudinal climatic transect in central Canada. Total OC stocks (0–1 m) increased with decreasing mean annual temperature (MAT). We used a combination of physical fractionation of soil OC pools, 13C isotopic determination and cross-polarization, magic-angle spinning 13C nuclear magnetic resonance (NMR) spectroscopy to further characterize OC composition at all sites. Soil OC was dominated by labile pools. As illustrated by the C/N ratios, δ13C data and results from the 13C NMR analysis, the light fraction showed little alteration within the soil profiles. Instead, this fraction reflected the importance of fresh litter inputs and showed an increase in root contribution with depth. As opposed to the light fraction, the clay- and silt-stabilized OC exhibited an increase in δ13C and a decrease in C/N with depth, indicating an increase in its degree of decomposition. These changes with depth were more marked at the southern than the northern sites. Results hence suggest that if the MAT were to increase in the northern boreal forest the overall jack pine soil OC stocks would decrease but the remaining OC would become more decomposed, and likely more stabilized than what is currently present within the soils.