Pyrogenic carbon stocks and storage mechanisms in podzolic soils of fire-affected Quebec black spruce forests Laure N. Soucémarianadin ⁎ ,1 , Sylvie A. Quideau, M. Derek MacKenzie Department of Renewable Resources, University of Alberta, Edmonton, Alberta, Canada abstract article info Article history: Received 30 July 2013 Received in revised form 20 November 2013 Accepted 22 November 2013 Available online 16 December 2013 Keywords: Pyrogenic carbon Podzols Black spruce forests Quebec, Canada Boreal Carbon storage Wildfire, a recurrent disturbance in the boreal, converts part of the forest floor into pyrogenic carbon (PyC). The latter is an important component of the global soil carbon pool, yet knowledge of its stocks and storage mecha- nisms in these boreal ecosystems is scarce. Podzolization processes, which are frequent under boreal vegetation, result in distinctive patterns of soil organic carbon (SOC) accumulation in the mineral subsoil; how this may affect PyC storage remains largely unknown. The objectives of this study were to estimate SOC and PyC stocks in podzolic soils from fire-affected black spruce forests, and to explore the storage mechanisms taking place in their mineral horizons. We also compared PyC stocks in mineral soils to forest floor stocks. Samples were collect- ed from 23 soil profiles under black spruce forests located throughout the province of Quebec. To further explore the relationship between podzolization and PyC storage mechanisms, we measured SOC and PyC contents in size and density fractions of a subset of 11 podzolic B horizons. Total SOC stocks in the mineral horizons and forest floors were comparable. Pyrogenic carbon stocks in the mineral soils, estimated by a H 2 O 2 /dilute HNO 3 digestion, averaged 0.2 (±0.1) kg C m -2 . This was significantly lower than forest floor stocks, which ranged from 0.2 to 1.2 kg C m -2 . Consequently, PyC constituted a smaller fraction of total SOC (2–15%) in mineral soils than in for- est floors, where it was as high as 68% (±5) in some horizons. In the mineral soils, SOC and PyC concentrations were strongly correlated. While some PyC was found in unprotected particulate organic matter (POM), the rest was associated with organo-mineral and organo-metallic complexes in the micro-aggregate protected POM and fine fraction. Patterns of PyC accumulation in mineral soils were similar to SOC, and the greater PyC stocks were found in podzolic B horizons. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Boreal ecosystems cover 24 × 10 6 km 2 , which corresponds to 17% of the Earth's land surface (Kasischke, 2000). Boreal forest soils cover a smaller area of the biome than permafrost soils, but represent an almost equally large carbon pool, estimated at 227 × 10 12 kg (Kasischke, 2000). Boreal forests are exposed to recurring wildfires, which act as a major driver of the carbon cycle (Bond-Lamberty et al., 2007; Hatten and Zabowski, 2009). Fires result in the production of significant amounts of CO 2 , and temporarily turn these forests into carbon sources (e.g., Harden et al., 2000; Neff et al., 2005). The often large boreal fires occurring in North America are thought to be relatively homogeneous with stand-replacing, high intensity crown fires. However, in Quebec boreal black spruce forests, combustion of the thick forest floor layers by wildfire is often incomplete and pyrogenic carbon (PyC), also called black carbon, residues are a major by-product. Forest floor PyC stocks may increase over successive fires, as long as fire severity is not high enough to consume all organic layers. Some PyC may also be translocated and accumulate in mineral soils, although the extent of this process is currently poorly understood and likely varies from one ecosystem to another (Czimczik and Masiello, 2007). According to some studies, PyC is an important component of the stable soil carbon pool (González-Pérez et al., 2004; Kasischke, 2000; Preston and Schmidt, 2006), although several studies caution against assuming that all PyC is recalcitrant (Czimczik and Masiello, 2007; Hammes et al., 2008; Harden et al., 2000; Hockaday et al., 2006; Preston, 2009). Significant PyC accumulation has been observed in some Canadian boreal forest soils (e.g., Bélanger and Pinno, 2008; Harden et al., 2000), but the amount stored appears to vary greatly among ecosystems (Czimczik et al., 2005; Kane et al., 2007, 2010; Preston and Schmidt, 2006). Little is known on how much PyC accumu- lation may arise from physical interactions with the mineral soil matrix (Schmidt et al., 2011), and if this can explain, at least partially, PyC per- sistence in pyrogenic ecosystems. To introduce PyC into carbon models and to test its potential as a passive carbon pool under future climate change conditions, there is urgent need for a better understanding of its storage mechanisms, as well as better estimates of overall soil stocks. Geoderma 217–218 (2014) 118–128 ⁎ Corresponding author at: Department of Renewable Resources, University of Alberta 442 Earth Sciences Building, Edmonton, AB T6G 2E3, Canada. Tel.: +1 780 492 5397; fax: +1 780 492 1767. E-mail address: soucmari@ualberta.ca (L.N. Soucémarianadin). 1 Present address: Department of Medical Biochemistry and Biophysics, Umeå University, SE-901 87 Umeå, Sweden. Tel.: +46 90 7866 723. 0016-7061/$ – see front matter © 2013 Elsevier B.V. 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