Abstract
Abstract. In the boreal region, soil organic carbon (OC) dynamics are strongly governed by the interaction between wildfire and permafrost. Using a combination of field measurements, numerical modeling of soil thermal dynamics, and mass-balance modeling of OC dynamics, we tested the sensitivity of soil OC storage to a suite of individual climate factors (air temperature, soil moisture, and snow depth) and fire severity. We also conducted sensitivity analyses to explore the combined effects of fire-soil moisture interactions and snow seasonality on OC storage. OC losses were calculated as the difference in OC stocks after three fire cycles (~500 yr) following a prescribed step-change in climate and/or fire. Across single-factor scenarios, our findings indicate that warmer air temperatures resulted in the largest relative soil OC losses (~5.3 kg C m−2), whereas dry soil conditions alone (in the absence of wildfire) resulted in the smallest carbon losses (~0.1 kg C m−2). Increased fire severity resulted in carbon loss of ~3.3 kg C m−2, whereas changes in snow depth resulted in smaller OC losses (2.1–2.2 kg C m−2). Across multiple climate factors, we observed larger OC losses than for single-factor scenarios. For instance, high fire severity regime associated with warmer and drier conditions resulted in OC losses of ~6.1 kg C m−2, whereas a low fire severity regime associated with warmer and wetter conditions resulted in OC losses of ~5.6 kg C m−2. A longer snow-free season associated with future warming resulted in OC losses of ~5.4 kg C m−2. Soil climate was the dominant control on soil OC loss, governing the sensitivity of microbial decomposers to fluctuations in temperature and soil moisture; this control, in turn, is governed by interannual changes in active layer depth. Transitional responses of the active layer depth to fire regimes also contributed to OC losses, primarily by determining the proportion of OC into frozen and unfrozen soil layers.
Highlights
High-latitude soils store large quantities of organic carbon (OC), accounting for nearly 50% of the global belowground OC pool (Tarnocai et al, 2009)
(1) What is the sensitivity of active layer depth (ALD) and soil climate to air temperature, soil moisture, snow depth, and fire severity? (2) What is the sensitivity of soil OC storage to air temperature, soil moisture, snow depth, and fire severity? (3) What are the relative effects of ALD, soil climate, and ALD-soil climate interactions on soil OC losses across climate and fire scenarios? By addressing these questions, we aim to quantify the importance of individual climatic and disturbance factors governing soil OC dynamics in the boreal region
Recent field studies and modeling efforts have helped shape our understanding of soil OC dynamics in relation to wildfire and permafrost dynamics in the boreal region (Carrasco et al, 2006; Harden et al, 2006; Fan et al, 2008; Yi et al, 2009a, 2011; O’Donnell et al, 2011)
Summary
High-latitude soils store large quantities of organic carbon (OC), accounting for nearly 50% of the global belowground OC pool (Tarnocai et al, 2009). Recent warming at high latitudes has caused localized thawing of permafrost (Osterkamp and Romanovsky, 1999; Romanovsky et al, 2010), resulting in the release of old OC from terrestrial ecosystems (Schuur et al, 2009). Permafrost thaw can alter local soil thermal and hydrologic conditions (Jorgenson et al, 2001; Yi et al, 2009b), which can influence rates of microbial activity and OC loss from decomposition. Wildfire has the potential to exacerbate rates of permafrost thaw (Yoshikawa et al, 2003) and OC losses (Harden et al, 2000) from soils of the boreal region. Through the combustion of surface organic horizons, wildfire reduces thermal insulation and increases active layer depth (ALD; Burn, 1998; Yoshikawa et al, 2003; Johnstone et al, 2010)
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