The stock and age of pyrogenic carbon in boreal forest soils of the Mackenzie Basin, Northern Canada

Wildfires occur regularly in the boreal forests of Northern Canada and an increasing frequency and intensity due to the global climate change is projected. A by-product of these forest fires is pyrogenic carbon (PyC) as a residue of incomplete combustion. The quantity and age of PyC in boreal forest soils, however, are largely unknown although boreal soils contribute to a large extent to the global soil organic carbon (SOC) stocks. The Mackenzie River is a major export pathway for PyC between terrestrial and marine environments, with exported PyC ages on geological timescales. This indicates that soil may play an important role as an intermediate pool prior to the PyC export. We sampled eleven forest soils (with nine replicates) in the Canadian Taiga Plains and Shield within the Mackenzie River basin. Our sample sites were located in regions with soils under continuous permafrost in the Inuvik region (northern sites) and under sporadic and discontinuous permafrost in the South Slave Lake regions (southern sites). All sites were unaffected by fire for at least four decades. We used the hydrogen pyrolysis (HyPy) method to separate the PyCHyPy from the non-fire-derived SOC in the upper 0-15 cm to determine PyCHyPy stocks and performed radiocarbon dating upon both bulk soil and isolated PyCHyPy. The total SOC stocks were lower in the soil from the southern sites with on average 26 ± 20 Mg ha-1 (10-153 Mg ha-1) compared to 57 ± 29 Mg ha-1 (16-188 Mg ha-1) in the northern sites. The radiocarbon dating revealed much older PyCHyPy compared to the bulk soil SOC radiocarbon age, supporting the persistent nature of PyC and stabilization in soils. The PyCHyPy found in the soil of the southern sites, however, was much younger with ages in the range of 495-3 275 radiocarbon years BP than in the northern sites with ages on the range of 2 083-10 407 radiocarbon years BP. The larger SOC stocks and higher ages of PyCHyPy in the soils of the northern sites indicate the importance of permafrost conditions for the whole carbon cycle of boreal forests soils.

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