The Fate of Pyrogenic Carbon in Boreal Forest Soils

Wildfires are common in the circumpolar region and are expected to become more frequent with climate change. These fires lead to the formation of pyrogenic carbon (PyC) as a by-product of incomplete combustion of vegetation biomass. After PyC enters the soil, its more labile fractions get decomposed and mineralized on a relatively short timescale, but a relevant fraction of PyC is highly aromatic and condensed, thus more resistant to biotic and abiotic decomposition. These components can remain in the soil for millennia, finally compensating the wildfire carbon emissions and acting as an atmospheric carbon sink. Further, translocation, physical-chemical altering, and content of PyC in soils can be driven by soil texture, permafrost, and translocation due to steepness. The aim of this master’s thesis is to provide a detailed investigation of the main drivers controlling the translocation and physicalchemical altering of PyC stored in soil organic carbon fractions along two Canadian landscape gradients, one in the South Slave Lake Region, (AB and NWT) and one in the Inuvik Region (NWT). The applied methods comprehend soil organic carbon fractionation by size and density, diffuse reflectance infra-red Fourier transformation spectroscopy (DRIFT), chemothermal oxidation at 375 °C (CTO-375) and benzene polycarboxylic acids (BPCA) analysis. Results highlighted that soils from the Inuvik Region, which are affected by continuous permafrost conditions, lower pH, and greater clay content, store more SOC and PyC than soils from the South Slave Lake Region, which are affected by sporadic permafrost conditions and a pH > 6 due to carbonates. The higher SOC and PyC stocks and concentrations in the soils of the Inuvik Region were attributable to the continuous permafrost conditions and higher soil moisture content, which likely reduce the decomposition rate and bury soil organic matter at greatest depths through cryoturbation, protecting trapped soil organic carbon from physical and chemical alteration. Further, also DRIFT analysis suggests a higher potential SOC cycling in the South Slave Lake Region, for both the bulk samples and SOC fractions, because of the lower aliphatic/aromatic and cellulose/lignin ratios, despite no difference for the organic layer and the particulate organic matter fraction were found. Differences in SOC and PyC quantities along the landscape gradients only partially correlate with differences in steepness. Indeed, the homogeneous distribution of SOC and PyC in the South Slave Lake Region correlates with smooth differences in height between top and bottom of the catena and with the sporadic permafrost conditions, while the 50 m difference in height between top and bottom of the catena in the Inuvik Region, together with continuous permafrost conditions, likely cause a transport of SOC and PyC along the catena and in greater soil depths. The analysis of PyC quality at each landscape position and depth revealed that more physical-chemical altered PyC is not necessarily found at the bottom of the catenae, which is attributable to the protection of also more labile structures by permafrost. The proportion of SOC and PyC in soil fractions revealed a higher stability and resistance against decomposition in case of association with silt and clay, and sand and aggregates, whereas protection against decomposition was lower if SOC and PyC were found in association with particulate organic matter. Further, the association of soil carbon with specific fractions is attributable also to the soil texture, and thus to the extent to which each fraction contributes to the bulk soil. Thus, permafrost can be considered as the most relevant driver of both translocation and physical-chemical altering of PyC in boreal forest soils, while soil texture drives its association with specific soil fractions (all of which exhibit a different ability to stabilize carbon in soils), and differences in steepness only partially explain the translocation of PyC along landscape gradients.

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