Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | SOIL Discuss., 1, 709–756, 2014 www.soil-discuss.net/1/709/2014/ doi:10.5194/soild-1-709-2014 SOILD © Author(s) 2014. CC Attribution 3.0 License. 1, 709–756, 2014 This discussion paper is/has been under review for the journal SOIL. Please refer to the Permafrost soils and corresponding final paper in SOIL if available. carbon cycling Permafrost soils and carbon cycling C. L. Ping et al. 1 2 3 1 4 C. L. Ping , J. D. Jastrow , M. T. Jorgenson , G. J. Michaelson , and Y. L. Shur Title Page 1Agricultural and Forestry Station, Palmer Research Center, University of Alaska Fairbanks, Abstract Introduction 1509 Geogeson Drive, Palmer, AK 99645, USA Conclusions References 2Biosciences Division, Argonne National Laboratory, Argonne, IL 60439, USA 3 Alaska Ecoscience, Fairbanks, AK 99775, USA Tables Figures 4Department of Civil and Environmental Engineering, University of Alaska Fairbanks, Fairbanks, AK 99775, USA J I Received: 4 October 2014 – Accepted: 15 October 2014 – Published: 30 October 2014 J I Correspondence to: C. L. Ping ([email protected]) Back Close Published by Copernicus Publications on behalf of the European Geosciences Union. Full Screen / Esc Printer-friendly Version Interactive Discussion 709 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract SOILD Knowledge of soils in the permafrost region has advanced immensely in recent decades, despite the remoteness and inaccessibility of most of the region and the 1, 709–756, 2014 sampling limitations posed by the severe environment. These efforts significantly in- 5 creased estimates of the amount of organic carbon (OC) stored in permafrost-region Permafrost soils and soils and improved understanding of how pedogenic processes unique to permafrost carbon cycling environments built enormous OC stocks during the Quaternary. This knowledge has also called attention to the importance of permafrost-affected soils to the global C cycle C. L. Ping et al. and the potential vulnerability of the region’s soil OC stocks to changing climatic condi- 10 tions. In this review, we briefly introduce the permafrost characteristics, ice structures, and cryopedogenic processes that shape the development of permafrost-affected soils Title Page and discuss their effects on soil structures and on organic matter distributions within Abstract Introduction the soil profile. We then examine the quantity of OC stored in permafrost-region soils, as well as the characteristics, intrinsic decomposability, and potential vulnerability of Conclusions References 15 this OC to permafrost thaw under a warming climate. Tables Figures 1 Introduction J I J I Permafrost is defined as ground (soil or rock and included ice) that remains at or below 0 ◦C for at least two consecutive years (Washburn, 1973). In this review the permafrost Back Close material that receives the most attention is ice-cemented permafrost because of the Full Screen / Esc 20 phenomena associated with ice formation, freeze-thaw cycles, and frost heave, which affect soil forming processes and resulting soil properties. Permafrost underlies nearly Printer-friendly Version 24 % of the exposed land surface on Earth, mostly in the lowlands of the circumpo- lar north regions, the boreal regions, high alpine and plateau regions in the Northern Interactive Discussion Hemisphere, and limited areas in the high alpine regions of the Southern Hemisphere 25 and Antarctica (Bockheim, 1995; Brown et al., 1998). The total ice-free land areas of the northern circumpolar region encompass 17.8 × 106 km2 and are estimated to store 710 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1034 Pg organic carbon (OC) in the surface 0–3 m. The permafrost-affected soils ac- count for only 58 % of this land area yet they store 70 % of the soil OC (Hugelius et SOILD al., 2014). Recently, permafrost and permafrost-affected soils have received greater 1, 709–756, 2014 attention because of their large OC stocks and the potential, with warming, for releas- 5 ing significant amounts of this C as the greenhouse gases carbon dioxide (CO2) and methane (CH4) (McGuire et al., 2009; Schuur et al., 2011). The release of C is exacer- Permafrost soils and bated by rising soil temperatures and accelerated rates of permafrost thawing, creating carbon cycling a strong positive feedback loop, especially for CH with its greater warming potential 4 C. L. Ping et al. compared to CO2 (Oechel et al., 1993; Koven et al., 2011). But, recent reports (Walter 10 et al., 2006, 2007; Zimov et al., 2006) suggest that while a large amount of CH4 is pro- duced in thermokarst features created by permafrost thawing, this could be mitigated Title Page over the long term by increased primary production (Walter Anthony et al., 2014). How- ever, what controls the rate of gas release is not necessarily total OC stocks but rather, Abstract Introduction environmental factors such as changes in soil hydrology, temperature, and vegetation Conclusions References 15 communities (Olefeldt et al., 2013). In addition, differences in landscape position and the ice content of permafrost create widely varying conditions that affect soil drying or Tables Figures wetting after permafrost thaw, and ultimately affect the accumulation or loss of soil OC (Jorgenson et al., 2013). J I The terms “permafrost soils” and “permafrost-affected soils” are commonly used syn- J I 20 onymously. Permafrost-affected soils are referred to as Cryosols in both the World Ref- erence Base (WRB) (IUSS Working Group WRB, 2006) and the Canadian soil classi- Back Close fication system (Soil Classification Working Group, 1998), Gelisols in the US system Full Screen / Esc (Soil Survey Staff, 1999), Cryozem in the Russian system (Shishov et al., 2004), and permagelic suborders in the Chinese system (Gong et al., 1999). Despite differences Printer-friendly Version 25 in nomenclature and the properties used for classification, these systems share the basic requirement of the presence of permafrost at a certain depth, generally within Interactive Discussion 2 m of the surface. However, these systems differ substantially in their classification hierarchy. In the Russian and WRB systems the genetic lineage or soil material type, such as organic vs. mineral, supersedes soil thermal regimes. For example, organic 711 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | soils (Histosols) key out first and those with permafrost key out in the subclass as Cryic Histosols. But, in the Canadian and US systems, Cryosols and Gelisols, respectively, SOILD key out first because permafrost is recognized as the controlling factor in land use 1, 709–756, 2014 interpretation and ecosystem functions. 5 Gelisols of the US classification system have three suborders: Histels, Turbels, and Orthels. Histels are organic soils, Turbels are mineral soils affected by cryoturbation, Permafrost soils and and Orthels are mineral soils lacking cryoturbation (Soil Survey Staff, 1999). Gelisols carbon cycling are ubiquitous throughout the continuous permafrost zone of the northern circumpo- lar Arctic region. In this zone, Gelisols also form in lowlands with restricted drainage C. L. Ping et al. 10 (Rieger et al., 1979; Ping et al., 2004, 2005a). Within the discontinuous and sporadic permafrost zones of the boreal regions, slope and aspect of uplands and mountains Title Page play a controlling role in distributions of permafrost and Gelisols (Péwé, 1975; van Cleve et al., 1983; Ping et al., 2005b). In the Southern Hemisphere, permafrost and Abstract Introduction Gelisols occur on the Antarctic continent, in the sub-Antarctic islands, and in mountain Conclusions References 15 areas (Beyer et al., 1999, 2000; Blume et al., 1997; Bockheim, 1995). However, the OC content of permafrost-affected soils in the southern circumpolar region is extremely low Tables Figures when compared to the northern permafrost region (Bockheim, 1990). Hence, the focus of this review is on permafrost soils of the northern permafrost region. J I J I 2 Permafrost characteristics and transformations Back Close 20 2.1 Permafrost formation and distribution Full Screen / Esc There are two general types of permafrost. The most common one is a paleoclimate Printer-friendly Version remnant of the Quaternary age. This old permafrost is widespread throughout the low- lands and hills of the circumpolar region (Gubin, 1993; Péwé, 1975; Kanevskiy et al., Interactive Discussion 2011; Winterfeld et al., 2011). Most notable for this kind are the Yedoma formations 25 in northeast Russia, Arctic and boreal Alaska (Reyes et al., 2010; Schirrmeister et al., 2011; Kanevskiy et al., 2011, 2014), and the Yukon regions in Canada (Froese et al., 712 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2008). Yedoma is also called “Ice-Complex” because of the huge ice wedges formed in thick deposits of syngenetic origin (i.e., concurrent upward growth of deposits and the SOILD permafrost surface). Old permafrost that consists of stratified sediments is also found in 1, 709–756, 2014 deep loess deposits, such as those in central Alaska and the Yukon Territory of Canada 5 (Péwé, 1975). The second, more modern kind of permafrost was formed during the Holocene as a result of deglaciation followed by vegetation succession under cold cli- Permafrost soils and matic conditions and is mostly located in the sub-Arctic and boreal regions (Zoltai and carbon cycling Tarnocai, 1975). This more recently formed type has been termed “ecosystem-driven permafrost” (Shur and Jorgenson, 2007). In general, most permafrost in the Arctic is C. L. Ping et al. 10 of late-Pleistocene age (Brown, 1965; Gubin, 1993; Kanevskiy et al., 2011; Ping et al., 1997, 2008a). In the Dry Valley region of Antarctica, the permafrost is even older as Title Page it formed during the pre-late Quaternary (Bockheim, 1990). However the upper per- mafrost of the old formations is subject to contemporary climate fluctuations. Abstract Introduction 2.2 Cryogenesis and cryostructure Conclusions References Tables Figures 15 Permafrost affects soil formation through physical and biological processes.