Soil Burial Contributes to Deep Soil Organic Carbon Storage

Soil Biology & Biochemistry 69 (2014) 251e264

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Soil Biology & Biochemistry

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Review Soil burial contributes to deep soil organic carbon storage

Nina T. Chaopricha a,*, Erika Marín-Spiotta a,b a Nelson Institute for Environmental Studies, University of WisconsineMadison, 550 North Park Street, Madison, WI 53706, USA b Department of Geography, University of WisconsineMadison, 550 North Park Street, Madison, WI 53706, USA article info abstract

Article history: Understanding the source of soil organic carbon (SOC) in deep soil horizons and the processes influ- Received 14 August 2013 encing its turnover is critical for predicting the response of this large reservoir of terrestrial C to envi- Received in revised form ronmental change and potential feedbacks to climate. Here, we propose that soil burial is a globally 16 October 2013 important but greatly underestimated process contributing to the delivery and long-term persistence of Accepted 13 November 2013 substantial SOC stocks to depths beyond those considered in most soil C inventories. We draw from Available online 2 December 2013 examples in the paleosol and geomorphology literature to identify the effects of soil burial by volcanic, aeolian, alluvial, colluvial, glacial, and anthropogenic depositional processes on soil C storage. We Keywords: Soil organic carbon describe how the state factors affecting soil formation affect the persistence and decomposition of SOC in Paleosol buried soils. Organic horizons and surface mineral soils that become buried under layers of sediment can Subsoil store C several meters below the earth’s surface for millennia or longer. Buried SOC concentrations can Deep soil rival those of surface soils, and soils buried under volcanic deposits generally contain higher concen- Soil burial trations of SOC than those under alluvial or non-permafrost loess deposits. The dearth of quantitative Deposition research on buried SOC specifically, and on deep C pools in general, makes it difficult to estimate the Erosion global importance of burial as a terrestrial C storage mechanism on contemporary time scales. The handful of studies that provide data on soil C stocks in buried horizons and estimate their spatial extent suggest that buried soils can contain significant regional OC reservoirs that are currently ignored in inventories and biogeochemical models. Recent research suggests that these buried SOC stocks may cycle biologically on annual-to-decadal time scales if the processes contributing to their protection from decomposition are altered. We discuss the vulnerability of buried SOC pools to disturbance from climate change and human activities that may reconnect these deep SOC pools with the atmosphere. We also provide recommendations on how burial processes can be incorporated into soil biogeochemical models to more accurately predict dynamics of deep SOC pools under different landscapes and environmental conditions. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction (Richter and Mobley, 2009; Zabowski et al., 2011). The long-held assumption that SOC with old radiocarbon (14C) ages is not Soils hold one of the largest terrestrial reservoirs of organic responsive to changes in environmental conditions has also carbon (OC), and while most of this pool cycles on very slow time contributed to underestimates of the amount and dynamics of C scales (centuries to millennia), climate change and landscape stored in deep soils (cf. Fontaine et al., 2007). Evidence that modern disturbance can affect the proportion of soil organic carbon (SOC) 14C is incorporated into subsurface soils (Koarashi et al., 2012) and with the potential for more rapid exchange with the atmosphere that ancient SOC can respond to disturbances on annual to decadal (Houghton et al., 2001; Trumbore, 2009). Most of our under- time scales (Paul et al., 2006), in conjunction with reinterpretations standing of the processes affecting C storage and release from soils of relationships between age and reactivity (Torn et al., 2009), comes from studies of the top 30 cm of soil, which has been the provide new insights into the role of deep SOC in the contemporary focus of agricultural studies concerned with nutrient cycling global C cycle. A growing number of studies report a dynamic response of SOC pools that are located at and below 1 m depth to changes in land use and land cover (Devine et al., 2011; Harrison et al., 2011; Veldkamp et al., 2003). * Corresponding author. Present address: Cornell International Institute for Food, Recent investigations of deep SOC have focused on belowground Agriculture and Development, B75 Mann Library, Ithaca, NY 14853, USA. Tel.: þ1 607 342 4793. root inputs and the vertical transport of particulate and dissolved E-mail address: [email protected] (N.T. Chaopricha). organic matter through bioturbation, physical mixing, gravity, and

0038-0717/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.soilbio.2013.11.011 252 N.T. Chaopricha, E. Marín-Spiotta / Soil Biology & Biochemistry 69 (2014) 251e264 preferential flowpaths as the main modes of delivery of OC to the the buried soil” (NRCS, 2013). Soil layers underlying a plaggen subsoil (Chabbi et al., 2009; Lorenz and Lal, 2005; Marín-Spiotta epipedon, an anthropogenic surface layer with thickness of 50 cm et al., 2011; Rumpel and Kögel-Knabner, 2011; Tonneijck and or greater resulting from long-continued manuring, also are Jongmans, 2008; Wilkinson et al., 2009). Depositional processes considered to be a buried soil. Much of what is known about have received considerable attention in the context of long-range buried soils comes from the study of paleosols in the geo- soil erosion and sedimentation on land (Berhe and Kleber, 2013; morphology and Quaternary paleopedology literature (Jacobs and Berhe et al., 2012, 2007; Quinton et al., 2010; Van Oost et al., Sedov, 2012). While the term paleosol is sometimes used to 2007). The role of soil burial in the sequestration of C photo- describe any soil that developed under different climatic or synthesized in situ at depositional sites has been largely ignored in environmental conditions than today and was once buried, even discussions of deep SOC dynamics. Burial can disconnect a soil from when overlying material has eroded and it is now exposed at the atmospheric conditions and slow or inhibit the microbial decom- surface, we use the term here to refer only to currently buried position of native or transported SOC. Buried soil horizons, or sur- soils (Johnson, 1998). Depositional processes leading to soil burial face soils that have been buried through various depositional are certainly not limited to the distant past. Current environ- processes, can store more SOC than would exist at such depths from mental changes driven or influenced by human activities also root inputs and leaching from upper horizons (Gurwick et al., lead to landscape disturbances that can result in the rapid burial 2008a; Hoffmann et al., 2013; VandenBygaart et al., 2012). of recently fixed organic C on land. Here, we propose that soil burial processes are important con- Buried SOC can be highly variable spatially and thus challenging tributors to the accumulation and long-term persistence of to quantify due to the strong dependence of buried soils on land- considerable quantities of SOC at depths beyond those considered scape geomorphology and climate history (Johnson et al., 2011; by global soil C inventories. While the USDA Soil Taxonomy places Rosenbloom et al., 2006). Furthermore, most studies of buried the lower limit of soil at 2 m below the surface (Soil Survey Staff, soils report point locations and do not quantify SOC stocks within 2010), we expand our consideration of soil to include material the soil profile or across the landscape. Here, we report on a handful above weathered rock that is biologically active and plays an of studies that provided either SOC stocks or soil bulk density and important role in biogeochemical cycling, sensu Richter and OC concentration data, which allowed us to calculate stocks given Markewitz (1995). Organic matter makes up the bulk of the soil the thickness of the horizons. Only two studies we found reported many meters in depth in peatlands (Tarnocai et al., 2009; Ellery et estimates of spatial coverage for buried soils, which allowed for the al., 2012) and in permafrost soils (Bockheim and Hinkel, 2007; quantification of reservoir size. Johnson et al., 2011; Schuur et al., 2008). Most recent reviews on Buried soil horizons can contain large SOC content, comparable deep SOC have focused on the top 1 m of soil, whereas here, we to surface soils. One of the few studies on SOC dynamics in deep focus on SOC in deeper buried soils to highlight the large stores of buried soil horizons, which also reported soil bulk density values, is potentially biologically active C beyond depths commonly that of Basile-Doelsch et al. (2005) about a volcanic paleosol measured. Our review excludes very deep paleosols that may have sequence on the island of La Réunion. Extrapolating between depth been subjected to metamorphism and behave more like sedimen- intervals for which specific data were reported, we estimated that tary rocks. the two deepest buried soil horizons sampled (90e276 cm) con- We review the contributions of geomorphic, pedogenic, and tained a total of 512 Mg C/ha, or 72% of the whole soil profile SOC to anthropogenic depositional processes to deep SOC storage and that depth, including surface horizons. Volcanic paleosol sequences describe how environmental conditions or state factors (sensu below 1 m and up to 3 m depths on substrates of varying age in Jenny, 1941)influence SOC persistence in buried soils, both during Mexico contained between 216 and 565 Mg C/ha, 23e50% of whole and since burial. Understanding the mechanisms contributing to soil profile SOC (Peña-Ramírez et al., 2009). These soils are located the stabilization or mobilization of SOC from deep buried soils is in the Sierra del Chichinautzin Volcanic Field, an area of ca. important for predicting the response of soil C pools to changes in 2400 km2 covered by volcanic deposits with ages between 50,000 environmental conditions. Buried soils have been studied histori- and 1670 years B.P. The high SOC densities of these buried soil cally for information about past environments (Valentine and horizons (with average values of 2.1 0.4 kg/m2) suggest that Dalrymple, 1976; Zech et al. 2013) and can also serve as case burial in volcanic deposits contributes to storage of significant SOC studies for understanding both the sensitivity of landscape pro- stocks deep below the surface. Organic-rich volcanic paleosols cesses to future environmental change and the mechanisms buried 3 m below the surface on the slopes of Mt. Kilimanjaro were contributing to soil organic matter stabilization (Chaopricha, 2013; estimated to contribute a regionally important C “hotspot” of Marín-Spiotta et al., in review) Where specific information on 82 Mg C (Zech et al., 2014). factors affecting soils buried below 1 m is scarce, we cite studies of Buried soils can amount to significant C reservoirs even with low shallower buried soils. Building upon what is known about the SOC concentrations due to the vast volumes of soil resulting from processes contributing to the stabilization of deep SOC in buried deep profiles with compacted bulk densities, as well as extensive soils, we discuss vulnerability of these underestimated C pools to geographic distributions, especially where buried soils are repre- landscape and climatic disturbances that may reconnect potentially sentative of regional-scale geomorphic processes, as is common ancient OC with the atmosphere. Finally, we propose that burial with loess deposition (Miao et al., 2007). In the U.S. central Great processes and landscape disturbance be incorporated into soil Plains, a 1-m thick paleosol buried in loess deposits 6 m below the biogeochemical models to better predict the response of below- modern surface could contain a maximum of 2.7 Pg C, assuming ground OC pools to global change. homogeneous coverage over the large area across multiple states where the soil has been identified (Marín-Spiotta et al., in review). 2. Buried soils as deep SOC reservoirs Even if the loess paleosol covered only one tenth of that area, it would still amount to a considerable stock of 0.27 Pg C. These large Aburiedsoilisdefined by the National Resources Conserva- SOC stocks are located at depths below those accounted for in tion Service as a soil that is “covered with a surface mantle of global C inventories and models and can become exposed to the new soil material that either is 50 cm or more thick or is 30e modern surface through geologic or human disturbances including 50 cm thick and has a thickness that equals at least half the total landslides, road construction, and mining (Chaopricha, 2013; thickness of the named diagnostic horizons that are preserved in Karlstrom et al., 2008). N.T. Chaopricha, E. Marín-Spiotta / Soil Biology & Biochemistry 69 (2014) 251e264 253

In addition to their role in sequestering C from the atmosphere, deposits (Basile-Doelsch et al., 2005; Zech, 2006), aeolian loess buried soils can provide important ecosystem services. Deep buried (Antoine et al., 2013; Mason et al., 2008), flood and alluvial sedi- soil horizons with higher SOC concentrations than overlying loess ments (Blazejewski et al., 2009; Carter et al., 2009), colluvial ma- may reduce groundwater contamination by retaining contaminants terial landslides and other mass wasting processes (Mayer et al., and inhibiting their downward flow (Duffy et al., 1997). In riparian 2008; VandenBygaart et al., 2012), glacial sediments (Harris et al., wetlands, deep buried SOC can influence denitrification potential 1987; Mahaney et al., 2001), or multiple types of depositional and thus regulate nitrogen losses (Hill and Cardaci, 2004). materials (Rawling et al., 2003; Schirrmeister et al., 2013). Human activities can influence the rate and geographic extent of most of 3. Soil burial processes contributing to deep SOC these depositional processes as well as contribute to the burial of SOC under impervious surfaces in urbanized or constructed Storage of SOC at depth results from multiple pedogenic, biotic, landscapes. and geomorphic factors that contribute to the delivery, accumula- Environmental conditions pre- and post-burial and specificto tion, and persistence of organic matter in the subsurface of soils. different depositional processes can influence the types of organic Soil burial processes occur in different types of environments and matter that persist in deep buried soils. Not all SOC currently in can occur repeatedly over centuries to millenia, resulting in vertical buried horizons was there at the time of burial. Roots and bur- sequences of multiple buried soil horizons (Fig. 1). Soil burial is rowing animals can extend down into deep horizons and penetrate recognized to be important in the buildup of permafrost through through multiple paleosols in a soil profile (Gocke et al., 2010)so successive cryoturbation events (Becher et al., 2013; Bockheim, that it may be difficult to isolate residual SOC from new inputs after 2007). SOC in surface organic and mineral soil horizons also can the burial process. Disturbance events subsequent to burial can also become buried, for example, under lava flows and other volcanic result in the delivery of more recent C into buried soil horizons. For

Fig. 1. Example of different soil burial processes (top panel) and the paleosol sequences they form (bottom panel): a) volcanic burial at Mt. Kilimanjaro, Tanzania (photo from Zech, 2006); b) aeolian burial at the Old Wauneeta Roadcut in southwestern Nebraska, USA (photo modified from Feggestad et al., 2004); and c) alluvial burial at Pressey Park, South Loup River in central Nebraska, USA (photo by Joseph Mason). 254

Table 1 Examples of soil organic carbon (SOC) stocks in volcanically buried soil horizons below 1 m depth. Numbers (1, 2) indicate buried soils from different profiles or sites reported in the same study. We report mean and standard error (SE) SOC stocks for buried soils in cases when vertical sequences of multiple buried soils were reported by the authors. Total SOC stocks were calculated by summing SOC stocks for all buried soil layers within a soil profile. Burial times/soil ages are based on dates of the burial deposits or buried horizons as reported by the authors. YBP ¼ years before present; m.a.s.l. ¼ meters above sea level.

Buried soil Depth Mean SOC SE SOC Total SOC Burial time or soil age (y) Location Current vegetation MAT ( C) MAP (mm) Altitude Reference range (cm) stocks stocks stocks (m.a.s.l.) 2 2 (kg C/m ) (kg C/m ) (Mg C/ha) ..Capih,E aí-pot olBooy&Bohmsr 9(04 251 (2014) 69 Biochemistry & Biology Soil / Marín-Spiotta E. Chaopricha, N.T. 1) Tlaloc P 95e142 1.6 0.5 62.0 6200 85 Mexico Pine forest 10e14 1000e1200 3100 Peña-Ramírez et al., 2009 2) Chautzin P 79e270 3.7 1.3 256.6 1835 55 Mexico Pine forest 10e14 1000e1200 3100 3) Pelado P1 82e200 1.9 1.1 77.8 9620 160 to 10,900 280 Mexico Pine forest 10e14 1000e1200 3100 4) Pelado P2 88e180 3.0 1.8 89.7 9620 160 to 10,900 280 Mexico Pine forest 10e14 1000e1200 3100 5) Malacatepetl P 95e290 0.8 0.5 41.5 30,500 1160 Mexico Pine forest 10e14 1000e1200 3100 6) Malacatepetl A 75e293 2.4 0.3 143.8 30,500 1160 Mexico Fir forest 10e14 1000e1200 3200

1) 1750 N 100e210 0.4 0.0 48.7 55,000 YBP to early Holocene Tanzania Up tow1800 m.a.s.l.: Max 3000 1750 Zech et al., 2014 2) 1850 N 100e250 0.5 0.0 53.7 savannah/cultivated mm/year 1850 3) 1400 W 100e130 0.6 0.0 18.2 land. Abovew1800 m: between 2000 1400 4) 1700 N 100e200 0.8 0.1 40.0 sub-montane and and 2400 m.a.s.l., 1700 5) 2800 N 100e300 2.3 0.5 411.1 montane forests much less above 2800 6) 2200 N 100e160 3.9 0.9 117.9 and below 2200 e 7) 2600 Ne160 100 210 1.8 5.7 0.2 0.8 105.7 627.4 those elevations1700 2600 9) 23508) 1700 100e200 100 3.6 0.6 285.4 2350 10) 1750 100e215 3.2 0.7 319.0 1750 11) 2140 100e215 10.3 4.5 413.4 2140 12) 2450 100e230 4.7 1.6 282.1 2450 13) 2250 100e235 6.6 0.7 463.9 2250 14) 750 100e300 0.4 0.1 28.5 750 15) 1430 100e300 2.1 0.1 213.8 1430 16) 2750 115e160 8.1 2.1 242.8 2750 17) 2550 115e213 4.1 0.8 122.5 2550 e 264 1) Tephra ash 90e276 3.2 1.0 511.6 31,700 YBP and earlier La Réunion Forest 13 1700 1720 Basile-Doelsch et al., 2005 a 0.1 36.1 79 AD Pompeii, Italy Urban 5.7 Vogel and 1) Sandy loam 560e590 1.2 Märker, 2011 2) Sandy loam 640e670 1.0a 0.3 30.2 Urban 6.1 3) Sandy loam 550e600 0.5a 0.1 27.3 Urban 7.6 4) Sandy loam 610e670 1.2a 0.2 70.7 Urban 7.7 5) Silty loam 610e670 2.2a 0.1 130.8 Urban 8.9 a Estimated using mean soil bulk density (0.6 g/cm 3) for volcanic paleosols from Zech et al. (2014) and Basile-Doelsch et al. (2005). N.T. Chaopricha, E. Marín-Spiotta / Soil Biology & Biochemistry 69 (2014) 251e264 255 example, in the Rocky Mountains, the accumulation of black C in persistence may differ for modern surface soils and paleosols deep buried soil horizons was attributed to the incorporation of (Peña-Ramírez et al., 2009). new inputs from recent surface burning that were transported downwards during clay translocation (Leopold et al., 2011). 3.2. Loess deposits In the following sections, we examine different burial processes and discuss how differences in the time scales, geographic extent, Aeolian deposits formed from the accumulation of windblown and properties of the sediment and depositional environment dust tend to exist in currently or formerly arid areas with a dust affect the amount and persistence of SOC. We provide generaliza- source, wind, and minimal vegetative cover. Such conditions have tions on the relative amounts of SOC buried under different developed through the wind erosion of deserts and during degla- depositional processes and the frequency and spatial occurrence of ciation as glacial meltwater decreased and floodplains dried up, specific burial processes with the awareness that geographic biases exposing glacial silts and clays to erosional winds. Loess deposition by researchers investigating buried soils could very well contribute can cover very large geographic areas such as the U.S. Great Plains to some of these observed differences. We report values for SOC (Bettis et al., 2003) and plateaus in western China (Lu and Sun, concentrations (mg C/g soil) for different depositional processes in 2000). The resulting soil building process tends to occur gradu- Tables 1e4 as provided by the authors. For volcanic and loess soils ally, sometimes over thousands of years, as dust accumulates and (Tables 1 and 2), we also estimated SOC content (kg/m2) using bulk soil thickness increases over time. In the U.S. Great Plains, grassland density values for similar buried soils. Mollisols aggraded throughout the Holocene as loess accrued in multiple events due to changes in climate and vegetative cover, 3.1. Volcanic deposits resulting in sequences of A horizons that developed at the end of the Pleistocene and today are buried up to several meters below the Tephra and lava flows have buried soils around the world in modern soil surface (Jacobs and Mason, 2005). regions with currently or formerly active volcanoes under widely Our review of the literature revealed that many paleosols in varying climates and vegetation types. Volcanic events often loess deposits tend to have low SOC concentrations (Table 2), which occur repeatedly over geologic and historic periods of time, in part may be related to the formation of these soils during rela- resulting in multiple layers of vertical paleosol sequences such as tively brief periods of slower loess deposition in environments with those found on the southern slopes of Mt. Kilimanjaro in low primary production. Episodes of dune mobilization and wind Tanzania (Zech et al., 2014) and near the Nevado de Toluca vol- erosion can be associated with periods of aridity and increasing fire cano in Central Mexico (Sedov et al., 2003, 2001). The global frequency, leading to the accumulation of black C in buried soils extent of soil buried through volcanic deposition tends to be overlain by loess sediments (Marín-Spiotta et al., in review; Wang concentrated in specific regions under the influence of paleo or et al., 2005). The highest SOC stocks found in paleosols were active volcanism, unlike other depositional events with much those in Siberian yedoma loess permafrost (Table 2), which forms wider geographic footprints. when wind-blown sediments bury roots and other organic matter Volcanic deposition typically occurs very rapidly, burying live that then become incorporated into deeper permafrost. Loess vegetation and organic residues in soils under flows of lava, ash, permafrost can contain consistently large SOC content down 20 or pumice, or debris avalanches. The properties of the volcanic de- 30 m (Zimov et al., 2006a). Freezing temperatures protect yedoma posits are likely to affect the rate of burial, the depth of the deposits, SOC from decomposition, making these buried soils highly and the potential transformations of organic matter. During certain vulnerable to changes in climate. combustion conditions, plant and soil organic matter can be con- verted into condensed aromatic structures known as black C, which can persist in some soil environments for thousands of years 3.3. Alluvial and colluvial deposits (Spokas, 2010). Burial by volcanically-derived dust during cold and arid glacial periods is thought to have contributed to the accumu- Sediment deposited in stream terrace fill, alluvial fans, and lation of SOC-rich (100 mg/g soil) paleosol sequences up to 3 m overbank floodplain deposits can also bury and stabilize SOC. Thick, deep on the slopes of Mt. Kilimanjaro in Tanzania (Zech et al., 2014). dark A horizons buried in alluvial deposits were found in U.S. Soils developing on weathered lava tend to accumulate high Central Plains stream valleys as deep as 11 m below ground, where amounts of organic C due to the abundance of noncrystalline or they have been buried for approximately 10,000 years (Mandel, amorphous minerals (Basile-Doelsch et al., 2005; Torn et al., 1997). 2008). Floodplain sediments can bury peat, as they have in the The high sorptive capacity of most volcanic soils and the presence Mkuze coastal floodplain of South Africa, protecting organic matter of black C can lead to the long-term persistence of high amounts of from threats to currently exposed peatlands for approximately SOC. In a tephra-soil sequence in central Japan, the oldest buried 6000 years (Ellery et al., 2012). In Venezuela, a sequence of several soil layer at a depth of 4 m from a ca. 29 cal ka BP eruption was peat paleosols was found buried under alluvium, and one buried found to contain the highest C concentrations (>60 mg/g soil) peat layer at a depth of 13 m from an interglacial period 60e70,000 (Inoue et al., 2011). years ago contained approximately 150e750 mg SOC/g soil In the literature surveyed, volcanic paleosols showed a wide (Mahaney et al., 2001). range of mean SOC contents, from a low of 0.4 0.1 kg C/m2 to a Riparian zone soils store considerable SOC in buried organic- high of 10.3 4.5 kg C/m2 (Table 1), which could reflect the di- rich soil horizons. In the Pawcatuck River watershed of Rhode Is- versity of current and past climates where volcanic activity was land, 17 of 22 sites studied contained OC-rich material buried in common. There are no consistent trends between depth of buried former surface horizons or lenses beneath alluvial deposits at depth soil and SOC stocks. The different types of volcanic sediments, below 1 m and up to 3.5 m (Blazejewski et al., 2009). Buried SOC in from ash to pumice to lava, are expected to affect the permeability saturated riparian areas can take thousands of years to decompose of overlying deposits, the isolation of buried SOC from surface due to low oxygen availability (Blazejewski et al., 2009, 2005) but conditions, and the rate of weathering. The amount of SOC in may become vulnerable to decomposition if drained through hu- volcanic soil profiles that include multiple buried horizons is man or geomorphic alterations to hydrologic flow. In some areas, affected by soil age and mineral composition (Basile-Doelsch et al., riparian zone alluvial and outwash areas have high rates of mi- 2005), although the specific mechanisms influencing SOC crobial activity, indicating that buried soils in riparian areas can be 256 N.T. Chaopricha, E. Marín-Spiotta / Soil Biology & Biochemistry 69 (2014) 251e264

important hotpots of C mineralization and of denitrification (Gurwick et al., 2008a, 2008b).

le. Burial Typical SOC values of paleosols in alluvial sediments reported fi in the literature range from 3 to 35 mg/g at depths commonly up Biryukova et al., 2008 Huang et al., 2002 Karlstrom et al., 2008 Zimov et al., 2006a,b Reference Feggestad et al., 2004 to 4 m but also as deep as 8 m (Table 3). Our review found that SOC stored under alluvial deposits tended to be more recently buried, within the Holocene, than SOC under volcanic and loess 750

e deposits, possibly because soils buried under alluvial sediments

Altitude (m.a.s.l.) are prone to continued disturbance, exposure, and reburial in areas where fluvial processes continue to operate. Colluvium, or the accumulation of loose, eroded material dropped at the base of hillslopes by mass wasting processes, can (mm) quickly bury vegetation and soils. In landscapes prone to frequent successive debris flows such as areas above the treeline or other C) MAP high-altitude ecosystems, buried soils are characterized by thin organic layers, as they do not have time to accumulate between sedimentary events (Matthews et al., 1999). Colluvial paleosols are often prone to disturbance after burial due to subsequent erosion by the same processes that created them in the first place (Temme et al., 2012). The rate of burial by colluvial activities can be a lot faster than that by other depositional processes, partic-

les or sites reported in the same study. We report mean and standard error ularly those driven by wind erosion. fi ed by summing SOC stocks for all buried soil layers within a soil pro

meters above sea level. 3.4. Glacial deposits ¼

Soil burial rates by alpine and mountain glacier depositional processes operate on a range of time scales due to the diversity of delivery mechanisms and types of sediments deposited, ranging from glacial till to meltwater. Burial can be instantaneous on oc- casions when a proglacial lake ice dam ruptures and sends sediment-laden waters flooding downstream. Soils buried by Western Siberia, Russia Nebraska, USA Shortgrass prairie 9.7 495 sediment from a proglacial lake in Norway that drained 200e300

years before present; m.a.s.l. years ago showed SOC values of 77e6 mg/g between 1.5 and 1.8 m ¼ depth (Harris et al., 1987). Soils buried under glacial deposits can . exist at the edge of currently glaciated landscapes as well as in much wider geographic regions with a diversity of temperatures but with a common glacial history. Till, glaciofluvial, and glacio- 400 650 150 400

lacustrine deposits from up to 60 kyr BP have buried paleosols at 11,500 cal YBP S. Loess Plateau, China Agriculture 12 629 600 e multiple depths containing very high SOC contents of 150e 750 mg/g (Mahaney et al., 2001). Sometimes, multiple deposi- Burial time or soil age (y) Location Current vegetation MAT ( to 38,900 YBP to 6600 Schuur et al. (2008) tional processes bury soils at one site (Table 4).

3.5. Anthropogenic deposits and structures Marín-Spiotta et al. (in review)

Total SOC stocks (Mg C/ha) The field of archeology has focused on buried soils to facilitate the discovery of cultural artifacts and provide insight into past ) 2 human environments. Some information on SOC buried by pre- historic human constructions exists from burial mounds and their 2.1 118.40.9 Upper Pleistocene 1.511.5 263.8 105.0 3100 8465 Pleistocene Northest Siberia, Russia Forest tundra Tallgrass prairie 19.3 843 360 SE SOC stocks (kg C/m underlying soils in remains from the Bronze Age in Denmark (Thomsen et al., 2008) and from steppe kurgans in the central ) for loess paleosols from ) for yeodema loess paleosols from ) 3 3 2 Russian Plain (Demkina et al., 2010). Studies of buried soils from

b the Neolithic settlement region in the Lower Rhine area in Ger- a a a many have contributed to reinterpretations of ancient human Mean SOC stocks (kg C/m land use and history of soil erosion (Gerlach et al., 2012). Humans move massive amounts of soil and sediment that can rival the effect of geologic events (Haff, 2010; Wilkinson and 1.90 0.734.9016.0 2.6 37.0 52.9 7.26 55.0 2580 8.3 8250 Forest tundra 8.00 3.9 3.106.753.55 0.61 1.05 5.3 0.34 42.05 6.05 10,250 6600 e e e e e e e e McElroy, 2007). Human activities such as tillage during soil cultivation, soil movement during construction, and direct and (cm) indirect alterations to geomorphic processes can lead to the burial of SOC transported with the eroding material as well as photo- synthesized in situ at the depositional site. The effects of agri- cultural practices on local erosional transport and burial of C are better quantified than the effects of other types of human activ-

Estimated using soil bulk densityEstimated (1.21 using g/cm soil bulk density (1.65 g/cm ities, although uncertainties still exist in the spatial and temporal 1) Buried soil 2 1.70 Milford loess1) Yeodema 1 2.05 2) Yeodema 2 1.00 3.00 Minusinsk 7.15 2) Buried soil 33) Brady soilNanguanzhuang 2.60 1.25 5.50 Buried soil Depth range a b Table 2 Examples of soil organic carbon (SOC) stocks in aeolian-buried soil horizons below 1 m depth. Numbers (1, 2) indicate buried soils from different pro (SE) SOC stocks for buried soilstimes/soil in cases ages when are vertical based sequences of on multiple dates buried of soils were the reported burial by deposits the authors. or Total buried SOC stocks horizons were as calculat reported by the authors. YBP extent of C losses versus gains at the landscape and regional scales Table 3 Examples of soil organic carbon (SOC) concentrations of alluvially-buried soil horizons below 1 m depth. Numbers (1, 2) indicate buried soils from different profiles or sites reported in the same study. Letters (a, b) indicate multiple buried soil horizons within a vertical profile. Burial times and soil ages are based on dates of the burial deposits or buried horizons as reported by the authors. YBP ¼ years before present; m.a.s.l. ¼ meters above sea level.

Buried soil horizon(s) Depth (m) SOC (mg C/g soil) Burial time Location Current MAT ( C) MAP (mm) Altitude (m.a.s.l.) Reference or soil age vegetation ea 4.5e7 a Late Holocene Southern Plains, Prairie (Carter et al., 2009) 1a) Ab3 2.3 3.4 251 (2014) 69 Biochemistry & Biology Soil / Marín-Spiotta E. Chaopricha, N.T. 1b) ABb3 3.4e3.5a 3a Oklahoma, U.S.: 1c) Bkb3 3.5e4.0 a 1e2a 1) Carnegie Canyon a a 1d) Cb3 4.1 0.5 2) Lizard Site 2a) A2b 1.3e1.8 a 7e8a 2b) Btssb2/Btb2 1.8e2.5 a 3e8a 2c) BCb2 2.5e3.3 a 2e3a Four buried A horizons 2e3.3 a 15e35 a,b Late Holocene Rio Indio floodplain, (Clark et al., 2003) under coarse overbank Vega Baja, Puerto Rico deposits in karst area 1a) 2Bkb1 1.07e1.35 5.13 Late Holocene Upper Prairie 500 (Daniels, 2003) 1b) 2Bkb2 1.35e1.60 4.99 Republican River 1c) 2Bkb3 1.60e2.25 7.22 Basin, Central 1d) 2Bw 2.25e3.25 4.04 Great Plains, U.S. 1e) 2BC 3.25e4.00 1.01 1) Nichols Site 2) State Site 2a) 4Akb1 0.95e1.12 8.82 2b) 4Akb2 1.12e1.41 10.12 2c) 4ABkb 1.41e1.70 7.22 2d) 4Ckb1 1.70e1.95 7.22 2e) 4Ckb2 1.95e2.15 5.12 2f) 4Ckb3 2.15e2.40 4.16 a (Hill and Cardaci, 2004) Interbedded sand and mud 1.6e2.7 21.9 Near Toronto, Ontario, Mixed forest, 205 stream channel sediments Canada marsh (river channel migration) a Buried Mollisols in floodplains 1.0e5.1 5e40 Since 735 B.C. Platte River system, Agriculture (Knox, 2001)

Southwestern Wisconsin e 264 (Hinderman Site 1) a a Floodplain soil 1e5.3 (core AKIII) 2e13 1250e2900 cal YBP Adigrat, Tigray Plateau, Grassland 619 2493 (Terwilliger et al., 2011) Ethiopia & Eritrea 1) Ab 1.975 83 Pawcatuck Watershed, Riparian forests (Gurwick et al., 2008a,b) 2) Ab 1.005 126 Rhode Island, USA 3) Ab 2.025 94 a Values were approximated from figures. b Values were obtained from organic matter amounts using the formula OC ¼ OM/2. 257 258

Table 4 Examples of soil organic carbon (SOC) concentrations in soil horizons buried below 1 m depth under glacial, erosional, and multiple types of deposits. Letters (a, b) indicate multiple buried soil horizons within a vertical profile. Numbers (1, 2) indicate buried soils from different profiles or sites reported in the same study. Burial times and soil ages are based on dates of the burial deposits or buried horizons as reported by the authors. YBP ¼ years before present; m.a.s.l. ¼ meters above sea level.

Burial processes Buried soil horizon(s) Depth (m) SOC Burial time or soil age Location Current vegetation MAT ( C) MAP (mm) Altitude (m.a.s.l.) Reference (mg C/g soil) ..Capih,E aí-pot olBooy&Bohmsr 9(04 251 (2014) 69 Biochemistry & Biology Soil / Marín-Spiotta E. Chaopricha, N.T. Till, glaciofluvial, Vertical sequence 8.6e13.9a 0e750 a Wisconsinan/ Northern Venezuelan Agriculture: 10 100 3550 (Mahaney and glaciolacustrine of sandy peat paleosols Weichselian Andes potatoes, grazing et al., 2001) deposition without clay transformation Glaciation or aeolian deposition (up to 60,000 YBP) Colluvium from a) Bkb3 1.05e1.35 1 Holocene Middle Park, Big sagebrush, 12.9 to 5.7 255 2350 (Mayer hillslope erosion b) 2Brkb3 (dark 1.35e1.85þ 1 Colorado, USA grasses (max/min) et al., 2008) in intermontane basin carbonate-rich horizons) a a Colluvium (erosional Aggraded sandy loam 1 10 Since 1955 Grand Falls, New Agriculture: (VandenBygaart sediment) deposition cropland soil below plow Brunswick, Canada potatoes, forage et al., 2012) line (Orthic Humo-Ferric

Podzol): B horizon a Loess, fluvial, and a) Paleosol 4 1.0 6.9 3000e7000 YBP Stampede site, Aspen forest 1238 (Klassen, 2004) a volcanic tephra b) Paleosol 5A 1.3 24.8 Cypress Hills, Alberta, a deposition c) Paleosol 6 1.6 11.1 Canada with thin, a d) Paleosol 7 1.7 16.0 weakly developed a e) Paleosol 8 1.75 6.1 paleosols f) Paleosol 10 2a 11.1 g) Paleosol 11/12 2.1a 7.1 h) Paleosol 13 2.35a 9.0 i) Paleosol 14 2.9a 3.2 j) Paleosol 15 3.1a 9.1 k) Paleosol 16 3.45a 10.7 Loess, fluvial, a) Cumulic Ab1 1.00e1.47 7 1263 YBP S. Dakota White Semi-arid mixed 10.3 400 950 (Rawling and colluvial b) Ab1 (darker) 1.47e1.52 8 2003e2036 YBP River Badlands, USA grass ecosystem et al., 2003) deposition in c) Cb1 1.52e1.62 7 2355 YBP

aeolian cliff-top d) Ab2 1.62e1.80 7 4152e4219 YBP e 264 deposits e) Cb2 1.80e1.90 5 6665e8588 YBP f) Ab3 1.90e2.05 6 g) Cb3 2.05e2.15 5 h) ABb4 2.15e2.70 4 i) BCb4 2.70e2.80 3 j) Gravel 2.80e2.98 2 k) Sand 2.98e3.20 1 a Values were approximated from figures. N.T. Chaopricha, E. Marín-Spiotta / Soil Biology & Biochemistry 69 (2014) 251e264 259

(Quinton et al., 2010). The redistribution of SOC by agricultural 4.1. Effects of climate on buried SOC activities is significant: during the last 50 years, an estimated 16e 21 Pg C of SOC are thought to have been buried on agricultural lands Climate during soil pedogenesis and at the time of burial can (Van Oost et al., 2007). While much of this erosion leads to shallow have an important influence on the content, composition, and burial, repeated events over time can lead successive layers of persistence of OC in deep buried horizons. Climate can affect the sediment. In Belgian croplands under continuous cultivation for at magnitude and frequency of specific burial processes; for least 250 years, up to 2.5 m of soil accumulated in depositional example, extremely wet and dry periods promote alluvial and areas on the landscape, contributing to up to 6 times greater SOC aeolian deposition, respectively. In the U.S. Central Great Plains, stocks than in eroding areas (Doetterl et al., 2012). Landscape higher accumulations of SOC in a paleosol of a multiple sequence destabilization by human removal of vegetation or alteration of have been attributed to periods of higher primary production at river channels and floodplains can also trigger or magnify the ef- the time of soil formation during the PleistoceneeHolocene fects of other geomorphic processes (Knox, 2001). A recent analysis transition, while subsequent increasing aridity led to its burial of OC burial in soil sediments in agricultural landscapes from under successive events of loess deposition (Feggestad et al., Central Europe reported mean OC stocks of 3.1 1.0 kg C/m2 and 2004). Changes in climate that lead to multiple freezeethaw cy- 5.0 1.3 kg C/m2 for hillslope and floodplain soils, respectively cles are known to influence the rate of incorporation of surface (Hoffmann et al., 2013). SOC into deeper permafrost layers by vertical mixing (Becher Few studies have quantified SOC storage under urban environ- et al., 2013; Koven et al., 2009). In buried permafrost loess and ments. One calculation of C storage in U.S. urban soils down to 1 m peat soils, frozen temperatures currently preserve deep SOC, but assumed that SOC pools were negligible below that depth, despite thawing due to future atmospheric temperature are predicted to recognizing that this method ignored the SOC in buried A horizons accelerate microbial decomposition and lead to large OC loss in fill soils (Pouyat et al., 2006). A study of the U.K. city of Leicester, (Schuur et al., 2008; Tarnocai et al., 2009; Zimov et al., 2006a, which is 15% covered by pavement, buildings, and other impervious 2006b). capped surfaces, found no significant difference in SOC stocks in Deep soils in well-drained profiles tend to have a smaller range capped vs. greenspace soils at equivalent depths down to 1 m and of temperatures and lower levels of moisture than surface horizons, found higher SOC under capped soils than in nearby arable fields which commonly leads to low microbial activity (Rumpel and (Edmondson et al., 2012). The authors conclude that soils in urban Kögel-Knabner, 2011; Salomé et al., 2010). Low soil moisture environments may continue to be influenced by vegetation and inhibited respiration rates of buried soils in arid loess deposits in lose less SOC over time while conventional agricultural practices the U.S. Central Great Plains (Chaopricha, 2013) and of paleosols tend to deplete soils of OC. It is unknown how construction and collected under burial mounds in the dry Russian steppe (Demkina other soil disturbance events that are common practice in urban- et al., 2010). The response of paleosols buried 1700 and 4000 years ized environments contribute to the long-term preservation of SOC to moisture treatments did not differ, but the magnitude of their in soils under anthropogenic impervious surfaces. response was lower than that of modern surface soils (Demkina et al., 2010). In contrast, waterlogging within a 3330-year-old 4. State factors affecting SOC accumulation and stability in burial mound in Denmark resulted in marked differences in the buried soils microbial biomass and chemical composition of SOC in anaerobic and aerobic microsites (Thomsen et al., 2008). The preservation of undecomposed wood dated tens of 1000s of The depth and structure of overlying deposits affect the rate of years old in deep buried soils (e.g., Orlova and Panychev, 2006) gas exchange with the atmosphere as well as the transport of provides supporting evidence that burial can contribute to long- water, nutrients, and microorganisms down to a buried horizon. term persistence of OC and is consistent with current perspec- Most paleosols can be influenced by current surface conditions tives on the importance of environmental conditions in regulating through slow leaching and translocation. Preferential flow-paths SOC turnover time (Schmidt et al., 2011). The specific processes can deliver particulate and dissolved OC from surface organic contributing to slow turnover times and old radiocarbon ages of horizons to mineral subsoils in shrink-swell soils (Marín-Spiotta SOC in deep soils have been the subject of numerous reviews et al., 2011), and aquifer advection can transport young, surface- (Rumpel and Kögel-Knabner, 2011; Salomé et al., 2010; Schmidt derived OC to soil horizons as deep as 11 m (Mailloux et al., et al., 2011; Torn et al., 2009; Trumbore, 2009): depressed micro- 2013). Post-burial changes in groundwater levels can transform bial biomass and activity due to low rates of oxygen diffusivity, low paleosols, as evidenced by redoximorphic features in Roman soil moisture and nutrient availability; the microbial recycling of paleosols in the ancient ruins of Pompeii (Vogel and Märker, 2011). aged C compounds; and aging during the slow, downward migra- These changes in redox can accelerate pedogenesis as well as the tion of C that escapes mineralization from upper soil horizons. The mobility of SOC (Hall and Silver, 2013). Future changes in precip- first two mechanisms contribute to the long-term persistence of itation projected with climate change for many regions of the SOC present in the soil at the time of burial, while the third affects world can alter current hydrologic patterns that can both recon- incorporation of OC following burial. Processes that contribute to nect or disconnect buried soil horizons from surface biological slow turnover times for SOC in surface soils and prevent its loss processes (Holden, 2005). from the soil profile, such as the incorporation of OC into soil ag- gregates and associations with mineral surfaces, cations or metals 4.2. Biotic controls on buried SOC (Lorenz et al., 2011; Six et al., 2002; Torn et al., 2009; Von Lützow et al., 2006), also apply to buried mineral soils. Here, we focus on Plants and soil micro- and macroorganisms, such as animals, how the state factors of soil formationdclimate, organisms, relief, bacteria, and fungi, affect the quantity of SOC in soils before and parent material, and time, sensu Jenny (1941)daffect the potential after burial by influencing rates of OC inputs, decomposition and of different burial processes to contribute to the accumulation and mobilization. High rates of primary production during periods of long-term persistence of deep SOC. The stability of SOC in a buried warming and increased precipitation resulted in the buildup of SOC horizon is directly influenced by the burial process as well as by the in soils that were subsequently buried by loess during periods of properties of both the buried soil and the overlying depositional low vegetative cover that destabilized dune sources (Feggestad material. et al., 2004). High amounts of SOC in buried volcanic soils can 260 N.T. Chaopricha, E. Marín-Spiotta / Soil Biology & Biochemistry 69 (2014) 251e264 also be attributed to high fertility soils typical of many volcanic 4.4. Effects of parent material on SOC persistence in buried soils parent materials (Torn et al., 1997). Bioturbation by cicadas, earthworms, and small mammals can Buried SOC content and persistence are affected by the type of transport fresh OM to the mineral subsoil (Don et al., 2008; parent material of the buried soil and of the overlying and under- Wilkinson et al., 2009), where the potential for stabilization in- lying layers within a profile. A soil’s source material and mineralogy creases. Bioturbation can also increase the exchange of soil gases affect its particle size distribution and texture-related mechanisms and water through the promotion of preferential flow paths, thus for SOC stabilization (Fabrizzi et al., 2008; Hassink et al., 1997; potentially reducing the stability of deep SOC by reconnecting Jenny, 1941; Krull et al., 2003). Clay content is strongly correlated buried soils with modern surface conditions. Post-burial additions with SOC storage in deep soil layers (Jobbágy and Jackson, 2000). of OC from plant roots extending into deep soils (Davidson et al., SOC content and radiocarbon age are strongly influenced by type of 2011) can facilitate microbial decomposition of ancient SOC mineral composition, which can vary with parent material as well (Fontaine et al., 2007) and alter the composition of buried soils as weathering stage (Torn et al., 1997). Buried tropical volcanic ash (Gocke et al., 2010). soil horizons with poorly crystalline minerals stored more SOC with The activity of microbial decomposers in deep and buried soils is slower turnover rates than similar horizons containing more crys- poorly understood. Isolation from fresh OC inputs, low nutrient talline minerals, such as feldspars and gibbsite (Basile-Doelsch concentrations, and the great spatial patchiness of substrates can et al., 2005). lead to reduced microbial biomass and a decrease in functional The depth, structure, and composition of material above and diversity with increasing soil depth (Ekschmitt et al., 2008; Fierer below buried soils can affect the diffusivity of oxygen and soil et al., 2003). Microbial biomass from soils deep within Bronze moisture conditions. Soil texture also affects water retention of Age burial mounds and an underlying paleosol several meters deep soils (Sotta et al., 2007), influencing soil CO2 efflux and mi- below the surface were 10e20 times lower than those in a nearby crobial activity. The presence of an impermeable Fe layer under a arable topsoil (Thomsen et al., 2008). Respiration rates of incubated paleosol buried under a 7-m tall 3330-year-old burial mound in soils from burial mounds responded to addition of a labile C sub- Denmark led to the formation of a water-logged basin within its strate (glucose) with a two to three week lag compared to modern interior, affecting the decomposition of buried SOC (Thomsen et al., surface soils (Thomsen et al., 2008). 2008). Soil development of overlying sediment over time, espe- Microbial transformations of SOC post-burial can also cially in coarser sediments such as loess deposits, can alter soil contribute to the persistence of buried SOC. The role of microbial hydraulic properties (Beerten et al., 2012) and the connectivity of processing in the formation of stable SOC, through preferential the underlying buried soil with the atmosphere. sorption of microbial products to mineral surfaces or through oc- clusion within micropores, is increasingly being recognized 4.5. Timescale effects on buried SOC (Knicker, 2011; Miltner et al., 2012; Schmidt et al., 2011), especially in deep soils (Liang and Balser, 2008). A study of burial mounds in SOC storage in buried soils is affected by the timescale of the the central Russian Plain steppe found that the contribution of burial process as well as the amount of time since burial. The burial microbial biomass lipids to the bulk SOC pool increased with of a surface soil by a depositional material can happen instanta- buried soil depth and age of the burial mound (Khomutova et al., neously, such as during a volcanic eruption or glacial lake dam 2011). rupture. Soil burial can occur repeatedly over long periods of time, such as with sequences of volcanic eruptions (Sedov et al., 2001), 4.3. Influence of topographic relief on soil C burial processes gradual loess deposition over thousands of years (Feggestad et al., 2004; Zech et al. 2013), and river overbank deposits from A landscape’s geomorphology and erosional patterns affect the repeated floods (Daniels and Knox, 2005). distribution of deep SOC as sediment accumulates in different The rate of sediment accumulation can affect the number of landscape positions, creating spatial variability in SOC distribution buried horizons in a profile. Very slow deposition rates are more (Rosenbloom et al., 2006; VandenBygaart et al., 2012). Erosion likely to contribute to the formation of thicker surface soil A hori- removes C from surface soils and buries it in another location where zons (Daniels, 2003), while soil burial is more likely to occur when it may become stabilized (Berhe and Kleber, 2013; Berhe et al., rates of sediment aggradation exceed that of soil development. A 2007). Studies of SOC in erosional and depositional sites along vertical sequence of multiple buried soils is representative of in- topographic gradients report differences in the distribution of SOC termediate frequency of sediment deposition, which allow for pools associated with clay surfaces and with soil aggregates, sug- pedogenesis in between burial events. gesting geomorphic and pedogenic controls on the physical pro- The type of burial process can also affect the time scales of SOC tection of residual and accumulating SOC (Berhe et al., 2012; persistence and preservation. A study comparing the reliability of Doetterl et al., 2012). radiocarbon dating of buried SOC concluded that risk of OC Topography can also affect the relative frequency and extent contamination was greatest in loess deposits due to more gradual of different burial processes. Steep topographic regions tend to accumulation than in alluvial and flood deposits where burial can have more erosion, leading to soil burial under colluvium. occur relatively quickly and where an individual event will tend to Different types of buried soils are likely to be found in specific deposit thicker sediments (Orlova and Panychev, 2006). In a study landscape positions, such as volcanic soils downslope of volcanic of Japanese Andisols, estimates of past OC inputs based on phyto- peaks (Zech, 2006), loess deposits downwind of regional source liths and other SOC measurements suggested that the high levels of areas (Feggestad et al., 2004), alluvial soils in old river banks C buried by successive tephra deposits had not substantially (Knox, 2006), colluvial soils in downslope topographic positions decreased over the thousands of years since burial (Inoue et al., where eroded material is deposited (Mayer et al., 2008), and 2000). glacial deposits at high elevations or northern latitudes near currently or formerly glaciated landscapes (Mahaney et al., 5. Vulnerability of buried SOC to environmental change 2001). The spatial distribution and thickness of loess sediments overlying buried soils is affected by landform type and slope Alteration or disruption of the specific mechanisms contributing (Jacobs et al., 2012). to the protection of buried SOC from mineralization or from loss via N.T. Chaopricha, E. Marín-Spiotta / Soil Biology & Biochemistry 69 (2014) 251e264 261 leaching can lead to the reduction of SOC pools in buried soils. in land use and agricultural practices or hydrology that distribute Changes in environmental properties regulating the decomposition fresh C down the soil profile can prime microbial activity and and persistence of SOC in buried soils can occur through changes in stimulate the decomposition and release of deep ancient SOC the structure of overlying horizons that alter some of the mecha- (Fontaine et al., 2007). Buried soil horizons, where microbes are nisms that had previously isolated the buried soil from surface often C-starved, may show greater responses to priming than conditions or by exposure to the atmosphere through lateral and modern surface soils (Zhuravleva et al., 2012). More research is vertical erosion. Gully erosion in the Drakensberg escarpment needed on how land use and other environmental changes affect foothills in South Africa has revealed a sequence of paleosols in deep and buried SOC under different ecosystems (Harper and colluvial sediment (Temme et al., 2012). The existence of multiple Tibbett, 2013). sequences of buried horizons in loess deposits extending across a large area of the U.S. Central Great Plains (Miao et al., 2007) became 6. Implications of soil C burial for modeling and management known to researchers through exposure from road construction. The sensitivity of buried SOC to changing environmental con- A more explicit incorporation of processes contributing to the ditions varies with the properties of the mineral matrix and the delivery, accumulation, and turnover of SOC in the subsoil is organic matter, but recent research suggests that deep, ancient SOC fundamental for improving predictions of belowground SOC pools can respond to disturbance on annual to decadal time-scales to environmental change (Schmidt et al., 2011). A growing body of (Devine et al., 2011; Harrison et al., 2011; Veldkamp et al., 2003). evidence suggests that SOC in deep and buried soils can become a In buried soils where OC has been preserved due to isolation from large source of ancient C to the atmosphere if reconnected to the conditions favorable for microbial respiration, exposure to the at- atmosphere, and some of it may already be cycling more actively mosphere and inoculation with microbial biomass is expected to than previously considered (Devine et al., 2011; Fontaine et al., lead to large losses of SOC. Wetting of paleosols collected below 1 m 2007; Gurwick et al., 2008b; Trumbore, 1997). Despite the large under 4000-year old burial mounds resulted in comparable respi- stocks of OC in deep soils and their potential sensitivity to climate ration rates to those of modern surface soils in the arid Russian and land use change, most biogeochemical models include only steppe (Demkina et al., 2010). Deep SOC pools have been shown to surface or shallow soil C dynamics. Recent improvements to be more responsive to increased temperature during experimental biogeochemical process models include a vertical dimension to C manipulations than surface soil horizons (Schwendenmann and pools and transformations, the consideration of vertical mixing Veldkamp, 2006). On the other hand, buried SOC that may have such as bioturbation, and the input of OC at various depths, rather been biochemically altered before or during the burial process than the trickle down of OC from surface horizons (Koven et al., (such as during a fire) or since burial may not respond to envi- 2013). ronmental change as rapidly (Chaopricha, 2013). Given the importance of burial to long-term SOC storage, the The effects of climate change on buried SOC are complex and incorporation of depositional processes across the landscape into poorly understood. Changing precipitation patterns coupled with models of SOC should improve predictions of whole-soil profile C landscape disturbance could alter hydrologic flowpaths and in- dynamics as well as regional C budgets. In Central Europe, rates of crease soil erosion. These processes can reconnect deep SOC with SOC burial at the landscape level caused by post-Neolithic agri- biologically active surface soil horizons and lead to the exposure of cultural erosion were estimated to be about 170 times greater than buried SOC to the atmosphere. Buried peatlands, which store regional OC storage in lake and reservoir sediments, but these considerable amounts of OC, are vulnerable to changes in drainage processes are not included in continental C budgets (Hoffmann that accelerate decomposition and increase susceptibility to fire, et al., 2013). especially in areas where fire is used to clear vegetation (Limpens Our understanding of geomorphic controls on SOC has been et al., 2008). Changes in hydrologic processes or erosion rates greatly improved due to recent research quantifying lateral and that disrupt the accumulation of clastic sediments on floodplain vertical C fluxes during erosion across topographic sequences in peat can also minimize the protection of buried peats to fire (Ellery both cultivated and non-cultivated landscapes and measuring et al., 2012). processes leading to the release and protection of C at erosional and The effect of rising temperatures on buried and deep permafrost depositional sites (Berhe et al., 2012; Doetterl et al., 2012; Van Oost C pools has received great attention. Zimov et al. (2006b) estimate et al., 2012; VandenBygaart et al., 2012) coupled with models of that rising temperatures threaten 500 Pg of SOC in Siberian and sediment transport (Pelletier et al., 2011; Yoo et al., 2005). To Alaskan loess permafrost. Estimates of C release from thawing accurately represent buried SOC dynamics, the following additional northern permafrost vary widely, partly due to uncertainties in SOC information should be incorporated into vertically-resolved soil stocks, thawing rates, and the temperature sensitivity of decom- biogeochemistry models: the type of depositional process (e.g., position (Tarnocai et al., 2009). Some authors estimate that alluvial, colluvial, volcanic, aeolian, anthropogenic, etc.), the rate of permafrost SOC, which has accumulated over tens of 1000s of burial, the amount of time since burial, the amount of SOC trans- years, may be released to the atmosphere within a century (Zimov ported in the sediment, the mineral and particle-size composition et al., 2006b). Others suggest that increased cryoturbation in of the aggrading sediment, and depths of the soil and sediment thawing permafrost could lead to greater stabilization of SOC, in layers. This information provides a first approximation of the types part by transporting OC to deeper, colder layers with depressed of mechanisms contributing to the accumulation or loss of SOC at microbial activity (Bockheim, 2007; Kaiser et al., 2007). different depths. Inclusion of a process-based understanding of Less is known about the response of buried SOC to changes in how environmental properties affect SOC decomposition and sta- land management or land use, although recent work suggests that bilization in different soil layers in soil C models (Schmidt et al., deep SOC stocks are more sensitive than previously thought and 2011) will improve predictions of the response of deep and can show opposite responses than surface horizons (Baker et al., buried SOC to future landscape disturbances driven by changes in 2007; Li et al., 2010; Veldkamp et al., 2003). The conversion of climate or human activities. The incorporation of physical mixing of conventionally tilled agricultural land to managed successional SOC into deeper soil horizons simulating cryoturbation in a land forests in the U.S. state of Georgia affected total SOC content only in surface model improved estimates of high-latitude SOC stocks the top 5 cm of soil but resulted in an accumulation of C in par- down to 3 m (Koven et al., 2009). Even still, lower model pre- ticulate form in soils up to 2 m depth (Devine et al., 2011). Changes dictions than other permafrost pool estimates (Tarnocai et al., 262 N.T. Chaopricha, E. Marín-Spiotta / Soil Biology & Biochemistry 69 (2014) 251e264

2009) were attributed by the authors to the exclusion of alluvial Soil burial can isolate OC from the atmosphere, creating envi- and loess depositional processes which lead to the accumulation of ronmental conditions unfavorable to microbial decomposition, substantial deep SOC (Koven et al., 2009). Burial processes are such as low moisture or low oxygen, which can lead to the better represented in biogeochemical models of oceans, where persistence of SOC relatively unchanged for thousands of years. environmental conditions play an important role in the fate of Once exposed to ambient conditions, soil microbial activity can organic matter deposited in marine sediments (Kriest and Oschlies, recover to rates comparable to surface soils. In other buried soils, 2013). biochemical transformations from fire before burial or microbial From a management perspective, buried SOC pools can be processing post burial lead to the accumulation of SOC with old protected through practices that inhibit their exposure to the at- radiocarbon ages. mosphere. Alternatives to soil tillage and drainage processes that Accelerated landscape disturbance by climate change and hu- threaten currently buried SOC pools and that are less disruptive to man activities can increase the vulnerability of SOC that has been the soil profile may prevent mobilization and losses of deep and protected from mineralization in deep horizons and increase C buried SOC. Soil micro-environmental conditions and hydrology losses to the atmosphere. At the same time, increases in erosion and could also be managed to stabilize SOC by considering crop choice, shifts in depositional patterns, resulting from changing climates rooting depths, and crop residue retention. Assessments of man- and extreme weather patterns, could bury higher amounts of OC in agement practices on SOC need to consider the potential presence the future. Our results highlight the need for landscape-level of large buried SOC pools and the likelihood that the mechanisms evaluations on the effects of geomorphic processes on mecha- contributing to their persistence could be disrupted under different nisms of SOC stabilization and incorporation of soil burial processes land use and climatic scenarios. The inclusion of geomorphic pro- into soil biogeochemical models. cesses into knowledge of a site’s history can help predict the po- tential for the presence of buried soils at depth in a landscape. Acknowledgments While exposure of buried soils can result in SOC losses to the atmosphere initially from increased microbial respiration The authors would like to thank Joe Mason, Steve Ventura, (Demkina et al., 2010), exposed organic horizons can become an Randy Jackson, Rob Beattie, Michelle Haynes, and three anonymous important source of nutrients for local primary production. The reviewers for providing helpful feedback on earlier versions of this establishment of vegetation on the exposed soil surfaces can lead to manuscript; Patrick Chaopricha for sketching the figure illustra- SOC retention, following the successional model of Vitousek and tions; and the Wisconsin Alumni Research Foundation for sup- Reiners (1975). 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