Dynamics of pyrogenic carbon in permafrost-affected soils on short- and long-timescales

Wildfires occur regularly in boreal forests of Northern Canada and are increasing in frequency and intensity due to the impacts of projected global climate change. A by-product of these forest fires is pyrogenic carbon (PyC) as a residue of incomplete combustion. The short-and long-term dynamics of this important soil organic carbon (SOC) pool in permafrost-affected mineral soils, however, is largely unknown. Here we studied eleven boreal forest soils at distinct landscape positions under continuous (northern sites) and discontinuous (southern sites) permafrost. In these we assessed the short-term fate of ¹³C-labeled PyC and its precursor grass organic matter over two year in-situ soil core incubations. Further, we isolated PyC by hydrogen pyrolysis (PyC_HyPy) for quantification and radiocarbon measurements to investigate long-term pools across the landscape.

Losses of PyC after two years were dominated by decomposition with up to three times more PyC losses at northern sites (36%) compared to the southern sites (11%). The losses of the grass organic matter were substantial (69-84%) but losses were larger in southern soils. The PyC persistence depended on site and soil specific properties and not solely on its chemical resistance. Fresh PyC was increasingly decomposition in nutrient limited mineral soils under continuous permafrost, indicating that polyaromatic compounds can act as a nutrient source. Mineral interactions were important and contributed to the stabilization of ~40% of recovered PyC. Mineral-associated PyC mainly remained in particulate forms as identified on the microscale using NanoSIMS. Beside large grass organic matter losses, remaining fractions were recovered predominantly as particles in northern soils but highly dispersed on mineral surfaces in the southern soils on the microscale. Our results highlight that permafrost-affected boreal forest soils are sensitive to fresh PyC and organic matter inputs with substantial losses by decomposition even under continuous permafrost conditions with unique stabilisation mechanisms at the organo-mineral interface.

On the long-term, we identified that the native PyC_HyPy represents a millennial age carbon pool with significantly higher ages in continuous (5.5-7.8 cal. ka BP; F¹⁴C=0.44-0.54) than in discontinuous (1.2-2.2 cal. ka BP; F¹⁴C=0.76-0.88) permafrost soils in 0-15 cm soil depth. The PyC_HyPy was markedly older than the bulk SOC (modern with F¹⁴C=0.65-1.11). With soil depths, PyC_HyPy ages increased to >18 cal. ka BP (F¹⁴C<0.10) in cryoturbated soils. In accordance with the age, the PyC_HyPy stocks were larger at northern (3.4 ± 0.3 Mg PyC_HyPy ha^-1) compared to southern (1.4 ± 0.1 Mg PyC_HyPy ha^-1) sites. The PyC_HyPy stocks were found to be independent of permafrost intensity and landscape position within the regions and did not reflect observed SOC variability. By considering the results of the two-year incubation with the long-term observations, we identified that the processes on different temporal scales are not directly linked. A better understanding of PyC dynamics along temporal and spatial scales is required to evaluate soil carbon feedbacks of high-latitude soils with global warming and associated permafrost thaw and shifts in vegetation and wildfire regimes.