Permafrost‐affected soils of the northern circumpolar region represent 50% of the terrestrial soil organic carbon (SOC) reservoir and are most strongly affected by climatic change. There is growing ...concern that this vast SOC pool could transition from a net C sink to a source. But so far little is known on how the organic matter (OM) in permafrost soils will respond in a warming future, which is governed by OM composition and possible stabilization mechanisms. To investigate if and how SOC in the active layer and adjacent permafrost is protected against degradation, we employed density fractionation to separate differently stabilized SOM fractions. We studied the quantity and quality of OM in different compartments using elemental analysis, ¹³C solid‐phase nuclear magnetic resonance (¹³C‐NMR) spectroscopy, and ¹⁴C analyses. The soil samples were derived from 16 cores from drained thaw lake basins, ranging from 0 to 5500 years of age, representing a unique series of developing Arctic soils over time. The normalized SOC stocks ranged between 35.5 and 86.2 kg SOC m⁻³, with the major amount of SOC located in the active layers. The SOC stock is dominated by large amounts of particulate organic matter (POM), whereas mineral‐associated OM especially in older soils is of minor importance on a mass basis. We show that tremendous amounts of over 25 kg OC per square meter are stored as presumably easily degradable OM rich in carbohydrates. Only about 10 kg OC per square meter is present as presumably more stable, mineral‐associated OC. Significant amounts of the easily degradable, carbohydrate‐rich OM are preserved in the yet permanently frozen soil below the permafrost table. Forced by global warming, this vast labile OM pool could soon become available for microbial degradation due to the continuous deepening of the annually thawing active layer.
► Managers are looking to natural disturbance regimes to model harvest guidelines. ► We review field methods for sampling gaps and common gap models. ► We discuss the attributes of natural and ...man-made gaps. ► We make recommendations for future gap research.
As silvicultural objectives have changed over the last several decades, managers are increasingly designing cutting regimes that mimic natural disturbance with the hopes that such systems will restore forests to a more natural condition while optimizing harvest yield. Treefall gaps, canopy openings caused by the death of one or more trees, are the dominant form of disturbance in many forest systems worldwide. These gaps play an important role in forest ecology by helping to maintain bio- and pedo-diversity, influencing nutrient cycling, and preserving the uneven-age nature of late-successional forests. In gap literature, there are inconsistencies with regard to gap terminology, methods for identifying and studying gaps, and modeling gap disturbances. From the papers reviewed, the size of treefall gaps ranges widely from 10 to >5000
m
2; we suggest that the maximum gap size should be set at 1000
m
2. Larger openings tend to have microclimates and return intervals significantly different than smaller treefall gaps. Two main definitions of treefall gaps exist:
canopy gap: a ‘hole’ in the forest through all levels down to an average height of 2
m above ground and
extended gap: canopy gap plus the area that extends to the bases of surrounding canopy trees. Although researchers have assumed a variety of gap shapes to simplify measuring gap size, gaps are often irregularly shaped and so we recommend that gap areas and shapes be determined from detailed field measurements. Gap age may be determined from tree ring analysis of released trees in or near the gap edge, the spacing of whorls on released saplings, or from decomposition of gap-making trees. Windthrow is the main cause of canopy gaps in a variety of ecosystems; other causes include insects, diseases, acidic deposition, drought, and climate change. Treefall-gap models have been developed to predict the following processes during gap making or infilling: (i) gap abundance, (ii) forest structure, (iii) spatial and temporal variations in light levels, (iv) canopy dynamics, and (v) soil nutrient and water regimes. We recommend a protocol for gap studies and identify future research topics.
A continuous time series of annual soil thaw records, extending from 1994 to 2009, is available for comparison with the records of thaw obtained from the Biocomplexity Experiment (BE) for the period ...2006–2009. Discontinuous records of thaw at Barrow from wet tundra sites date back to the 1960s. Comparisons between the longer records with the BE observations reveal strong similarities. Records of permafrost temperature, reflecting changes in the annual surface energy exchange, are available from the 1950s for comparison with results from measurement programs begun in 2002. The long‐term systematic geocryological investigations at Barrow indicate an increase in permafrost temperature, especially during the last several years. The increase in near‐surface permafrost temperature is most pronounced in winter. Marked trends are not apparent in the active‐layer record, although subsidence measurements on the North Slope indicate that penetration into the ice‐rich layer at the top of permafrost has occurred over the past decade. Active‐layer thickness values from the 1960s are generally higher than those from the 1990s, and are very similar to those of the 2000s. Analysis of spatial active‐layer observations at representative locations demonstrates significant variations in active‐layer thickness between different landscape types, reflecting the influence of vegetation, substrate, microtopography, and, especially, soil moisture. Landscape‐specific differences exist in the response of active‐layer thickness to climatic forcing. These differences are attributable to the existence of localized controls related to combinations of surface and subsurface characteristics. The geocryological records at Barrow illustrate the importance and effectiveness of sustained, well organized monitoring efforts to document long‐term trends.
Soils with mountain permafrost occupy 3.5 million km2 worldwide, with 70% in central Asia. High-mountain environments have “warm” permafrost, with surface permafrost temperatures of -0.5 to -2 °C and ...deep active layers (2 to 8 m). From a global database of 41 sites and 312 pedons, alpine soils with permafrost are strongly acid (pH = 5.0 to 5.5), have intermediate cation-exchange capacities (20 to 25 cmolc/kg) and base saturation (44% to 85%), and commonly have an isotic mineral class. Soil organic carbon is concentrated in the upper 30 to 40 cm, with profile density averaging 15.2 ± 1.3 kg m-2 (range = <1.0 to 88.3 kg m-2), which is comparable to temperate grasslands (13 kg m-2) but substantially less than moist arctic tundra (32 kg m-2). Mountain soils with permafrost contain 66.3 Pg of soil organic carbon (SOC), which constitutes 4.5% of the global pool. In contrast, the SOC pool in the Arctic is 496 Pg (33% of the global pool). Alpine soils with deep active layers contrast strongly with high-latitude soils in areas of continuous permafrost. Permafrost in the upper 2 m induces cryoturbation in the profile, acts as a barrier to water movement, and generates cooler temperatures resulting in greater SOC levels. High-elevation and high-latitude soils are experiencing warming of air temperature and permafrost and a thickening of the active layer.
Satellite images and high resolution air photos, coupled with field examinations, were used to examine 24 rock glaciers/debris-covered glaciers and 25 gelifluction sheets, collectively referred to as ...viscous-flow features, in the McMurdo Dry Valleys, Antarctica. Debris-covered glaciers are the dominant form and are longer (mean length=2.5km), wider (mean width=0.8km), and less steep (mean slope=12°) than similar features reported in most arctic and alpine environments. The catchment areas tend to be large, averaging over 9km2. Most of the debris-covered glaciers are tongue-shaped, and where excavation was possible, the ice core was readily observable. Gelifluction sheets primarily occur at the base of valley sidewalls below talus on slopes ranging from 5 to 30° (average=13°) and contain a very thin active layer (normal range 20 to 40cm). Both viscous-flow forms occur on the north- and south-facing slopes of the east–west trending valleys and are concentrated in the inland mixed zone and stable upland microclimatic zone; these lobes were not found in the coastal thaw zone. Gelifluction sheets result from the melting of snow high on the valley walls, subsurface flow of meltwater on top of the permafrost, and slow movement downslope. They are readily observable from nonsorted polygons that are stretched into rectangles that are perpendicular to the slope and contain raised polygon rims upslope. The movement of gelifluction sheets can be detected from upturned stones containing carbonate coatings. Rates of horizontal surface flow of the viscous-flow features are comparable to those reported elsewhere in Antarctica and in the alpine and arctic regions of the world. Some of the viscous-flow features appear to be inactive, possibly reflecting the recession of alpine glaciers in high elevation cirques.
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•Debris-covered glaciers are tongue-shaped with visible ice core.•Gelifluction sheets occur below valley sidewalls with a shallow active layer.•Gelifluction sheets moved by subsurface meltwater on top of permafrost•Features comparable to alpine and arctic regions
Compared to mid-latitude deserts, the properties, formation and evolution of desert pavements and the underlying vesicular layer in Antarctica are poorly understood. This study examines the desert ...pavements and the vesicular layer from seven soil chronosequences in the Transantarctic Mountains that have developed on two contrasting parent materials: sandstone–dolerite and granite–gneiss. The pavement density commonly ranges from 63 to 92% with a median value of 80% and does not vary significantly with time of exposure or parent material composition. The dominant size range of clasts decreases with time of exposure, ranging from 16–64
mm on Holocene and late Quaternary surfaces to 8–16
mm on surfaces of middle Quaternary and older age. The proportion of clasts with ventifaction increases progressively through time from 20% on drifts of Holocene and late Quaternary age to 35% on Miocene-aged drifts. Desert varnish forms rapidly, especially on dolerite clasts, with nearly 100% cover on surfaces of early Quaternary and older age. Macropitting occurs only on clasts that have been exposed since the Miocene. A pavement development index, based on predominant clast-size class, pavement density, and the proportion of clasts with ventifaction, varnish, and pits, readily differentiated pavements according to relative age. From these findings we judge that desert pavements initially form from a surficial concentration of boulders during till deposition followed by a short period of deflation and a longer period of progressive chemical and physical weathering of surface clasts. The vesicular layer that underlies the desert pavement averages 4
cm in thickness and is enriched in silt, which is contributed primarily by weathering rather than eolian deposition. A comparison is made between desert pavement properties in mid-latitude deserts and Antarctic deserts.
AIMS: The objective of this study is to determine the effect of treefall gap size on carbon and nitrogen biogeochemistry in the Northern hardwood forest. The size of the gap controls the ...microclimate, particularly solar radiation, soil temperature, and soil moisture, which can alter nutrient cycling. However, in Northern hardwood forests, our understanding of the relation between biogeochemistry and gap size is incomplete. METHODS: Twelve natural treefall gaps ranging in size from 27 to 590 m² and four control plots located under closed canopy were identified in Upper Michigan in May 2008. Concentrations of ammonium and nitrate were measured in throughfall and soil leachates; soil respiration rates were measured in soil; and nutrient pools were measured in vegetative biomass, forest floor (Oe, Oi horizons), mineral soil, and microbial biomass. RESULTS: Gap size was positively correlated with sapling biomass density and throughfall ammonium concentration but negatively correlated with microbial biomass, endomycorrhizal biomass, and soil respiration. Gap size was unrelated to all other parameters measured. CONCLUSIONS: The results of this study suggest that in the Northern hardwood forest, vegetative recovery and a reduction in microbial biomass may limit the influence larger gaps have on nutrient cycling.
Fildes Peninsula (F.P.) and Ardley Island (A.I.) are among the first ice-free areas in Maritime Antarctica. Since the last glacial retreat in this part of Antarctica (8000 to 5000yearsBP), the ...landscape in these areas evolved under paraglacial to periglacial conditions, with pedogenesis marked by cryogenic processes. We carried out a detailed soil and geomorphology survey, with full morphological and analytical description for both areas; forty-eight soil profiles representing different landforms were sampled, analyzed and classified according to the U.S. Soil Taxonomy and the World Reference Base for Soil Resources (WRB).
Soils are mostly turbic, moderately developed, with podzolization and strong phosphatization (chemical weathering of rock minerals and formation of amorphous Al and Fe minerals) in former ornithogenic sites while in areas with poor vegetation show typical features of cryogenic weathering. Nivation, solifluction, cryoturbation, frost weathering, ablation and surface erosion are widespread. The most represented landform system by surface in Fildes Peninsula is the periglacial one, and 15 different periglacial landforms types have been identified and mapped. These features occupy about 30% of the land surface, in which patterned ground and stone fields are the most common landforms. Other significant landforms as protalus lobes, rock glaciers or debris lobes indicate the extensive presence of permafrost. Soil variability was high, in terms of morphological, physical and chemical properties, due to varying lithic contributions and mixing of different rocks, as well as to different degrees of faunal influence.
Three soil taxonomy orders were identified, whereas thirty four individual pedons were differentiated. Fildes Peninsula experiences a south–north gradient from periglacial to paraglacial conditions, and apparently younger soils and landforms are located close to the Collins Glacier. Arenosols/Entisols and Cryosols/Gelisols (frequently cryoturbic) are the most important soil classes; Leptosols/Entisols, Gleysols/Aquents and Cambisols/Inceptisols also occur, all with gelic properties, and with varying faunal influences. Both soil classification systems are adequate to distinguish the local pedogenesis processes. The WRB system is broader, since it was designed to be applied in all Polar Regions; the family classes adopted by the ST were effective in separating soils with important differences with regard to texture and gravel content, all important attributes accounting for the ecological succession and periglacial processes. An altitudinal organization of landforms and processes can be recognized from geomorphological mapping. Periglacial features are dominant above 50ma.s.l. although are present at lower altitude.
The direction and magnitude of soil organic carbon (SOC) changes in response to climate change remain unclear and depend on the spatial distribution of SOC across landscapes. Uncertainties regarding ...the fate of SOC are greater in high-latitude systems where data are sparse and the soils are affected by sub-zero temperatures. To address these issues in Alaska, a first-order assessment of data gaps and spatial distributions of SOC was conducted from a recently compiled soil carbon database. Temperature and landform type were the dominant controls on SOC distribution for selected ecoregions. Mean SOC pools (to a depth of 1-m) varied by three, seven and ten-fold across ecoregion, landform, and ecosystem types, respectively. Climate interactions with landform type and SOC were greatest in the uplands. For upland SOC there was a six-fold non-linear increase in SOC with latitude (i.e., temperature) where SOC was lowest in the Intermontane Boreal compared to the Arctic Tundra and Coastal Rainforest. Additionally, in upland systems mineral SOC pools decreased as climate became more continental, suggesting that the lower productivity, higher decomposition rates and fire activity, common in continental climates, interacted to reduce mineral SOC. For lowland systems, in contrast, these interactions and their impacts on SOC were muted or absent making SOC in these environments more comparable across latitudes. Thus, the magnitudes of SOC change across temperature gradients were non-uniform and depended on landform type. Additional factors that appeared to be related to SOC distribution within ecoregions included stand age, aspect, and permafrost presence or absence in black spruce stands. Overall, these results indicate the influence of major interactions between temperature-controlled decomposition and topography on SOC in high-latitude systems. However, there remains a need for more SOC data from wetlands and boreal-region permafrost soils, especially at depths
>
1
m in order to fully understand the effects of climate on soil carbon in Alaska.
► Temperature and soil drainage control soil organic carbon (SOC) contents in Alaska. ► The magnitude of change in SOC is non-linear and non-uniform across climate gradients. ► Biotic–climatic–fire interactions affect upland SOC contents more than lowland SOC. ► Complete analysis will require more SOC data from permafrost and wetland areas.
Arctic soils contain a large fraction of Earth’s stored carbon. Temperature increases in the Arctic may enhance decomposition of this stored carbon, shifting the role of Arctic soils from a net sink ...to a new source of atmospheric CO
2. Predicting the impact of Arctic warming on soil carbon reserves requires knowledge of the composition of the stored organic matter. Here, we employ solid state
13C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared-photoacoustic spectroscopy (FTIR-PAS) to investigate the chemical composition of soil organic matter collected from drained thaw-lake basins ranging in age from 0 to 5500
years before present (y BP). The
13C NMR and FTIR-PAS data were largely congruent. Surface horizons contain relatively large amounts of
O-alkyl carbon, suggesting that the soil organic matter is rich in labile constituents. Soil organic matter decreases with depth with the relative amounts of
O-alkyl carbon decreasing and aromatic carbon increasing. These data indicate that lower horizons are in a more advanced stage of decomposition than upper horizons. Nonetheless, a substantial fraction of carbon in lower horizons, even for ancient thaw-lake basins (2000–5500
y BP), is present as
O-alkyl carbon reflecting the preservation of intrinsically labile organic matter constituents. Climate change-induced increases in the depth of the soil active layer are expected to accelerate the depletion of this carbon.
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