The isotopic composition of soil organic carbon on a north-south transect in western Canada

The minor isotopes of carbon (C-13 and C-14) are widely used as tracers in studies of the global carbon cycle. We present carbon-isotope data for the 0-5 cm layer of soil on a transect from 49.6degreesN to 68degreesN, from mature forest and tundra ecosystems in the boreal-arctic zone of interior western Canada. Soil organic carbon in the <2000 mu m fraction of the soil decreases from 3.14 kg m(-2) in the south to 1.31 kg m(-2) in the north. The C-14 activity of the organic carbon decreases as latitude increases from 118.9 to 100.7 per cent modern carbon (pMC). In addition, the C-14 activities of organic carbon in the particle-size fractions of each sample decrease as particle size decreases. These results suggest that organic carbon in the 0-5 cm layer of these soils transfers from standing biomass into the coarsest size fractions of the soil and is then degraded over time, with the residue progressively transferred into the more resistant finer particle sizes. We calculate residence times for the coarsest size fractions of 21 years in the south to 71 years in the north. Residence times for the fine size fractions (< 63 mum) are considerably longer, ranging from 90 years in the south to 960 years in the north. The delta(13)C of the organic carbon decreases from -26.8 +/- 0.3parts per thousand. in soil under forest in the south to -26.2 +/- 0.1parts per thousand. for tundra sites in the north. At all sites there is an increase in delta(13)C with decreasing particle size of 0.7-1.6parts per thousand. These changes in 613 C are due to the presence of, old' carbon in equilibrium with an atmosphere richer in C-13, and to the effects of microbial degradation.