SOIL CARBON DYNAMICS ACROSS A WINDTHROW DISTURBANCE SEQUENCE IN SOUTHEAST ALASKA

2230

Ecology, 85(8), 2004, pp. 2230–2244q 2004 by the Ecological Society of America

SOIL CARBON DYNAMICS ACROSS A WINDTHROW DISTURBANCESEQUENCE IN SOUTHEAST ALASKA

MARC G. KRAMER,1 PHILLIP SOLLINS,2 AND RONALD S. SLETTEN3

1Ecosystem Science and Technology Branch, Earth Science Division, NASA AMES, Mail Stop 242–4,Moffett Field, California 94035 USA

2Department of Forest Science, Oregon State University, Corvallis, Oregon 97331 USA3Quaternary Research Center/Space Sciences, University of Washington, Seattle, Washington 98195-1310 USA

Abstract. Few studies have examined the influence of natural disturbances, such aswindthrow, on soil organic matter formation, stabilization, and loss in soils. In shallow(,1 m) mountain forest soils, windthrow activity may result in the redistribution and mixingof mineral and organic soil horizons down to bedrock. We studied the patterns of soilcarbon, the dominant constituent of soil organic matter, in watersheds along a windthrowdisturbance sequence in a mountainous temperate rain forest in southeast Alaska. Ourobjectives were (1) to evaluate the influence of windthrow and illuviation on the accu-mulation of soil organic carbon in mineral horizons and (2) to compare the forms of soilorganic matter that have accumulated. Soils were described, and the thickness of the majororganic and mineral horizons was measured, every 5 m along transects in three watershedswith contrasting windthrow histories. A subset of the soil description sites was randomlyselected and then sampled to determine the quantity and quality of soil carbon. Mineralsoil samples were physically fractionated based on particle density. Total carbon (C) andnitrogen (N), natural 15N and 13C abundance, micromorphologic analysis (using scanningelectron microscopy), and solid-state 13C nuclear magnetic resonance (NMR) were used tocompare soil organic matter pools in the three watersheds. The Bh horizon heavy-fraction(.1.65 g/cm3) C pools decreased from 25 Mg/ha in the least disturbed watershed to 3 Mg/ha in the most disturbed watershed; whereas light-fraction C pools were similar (4–6 Mg/ha) across all watersheds. Soil C stocks decreased from 216 Mg/ha in the least disturbedwatershed to 157 Mg/ha in the most disturbed watersheds. Evidence was found that mobileorganic carbon transported in soil water accumulated in mineral horizons principally throughsorption to mineral particles. Strong association with mineral particles was greater in thethicker, presumably older, illuvial horizons. We found light amorphous material in someof these thicker illuvial horizons, suggesting that mobile organic carbon may have beenimmobilized through flocculation of metal-bearing organic acids. Watersheds that experi-enced more intense soil mixing from windthrow had lower levels of strongly humified soilorganic matter, and more in a partially decomposed particulate form.

Key words: 13C solid-state NMR; humification; illuviation; lateral flow; mineral soil carbon;natural isotope abundance; podzolization; soil mixing; soil organic matter; temperate rain forest; treeuprooting; windthrow disturbance.

INTRODUCTION

Soil organic matter (SOM), which averages ;55%carbon (C) worldwide (Nelson and Sommers 1982),exerts strong control on ecosystem dynamics (soil ero-sion rates, nutrient cycling, and ecosystem productiv-ity), and its decomposition represents a possibly verylarge positive feedback mechanism for global warming.Yet the mechanisms of SOM formation, stabilization,and loss remain poorly understood. Even less well doc-umented are SOM pools in mountain forest watershedsand the processes that influence their formation. Wa-tershed-scale processes that may interact to stronglyinfluence stores of SOM include disturbances from hu-mans, fire, or windthrow, and topographic controls on

Manuscript received 22 November 2002; revised 26 August2003; accepted 3 September 2003; final version received 16 De-cember 2003. Corresponding Editor: R. A. Dahlgren.

hillslope erosion, and the transport and storage of SOMthrough lateral and vertical movement of soil water.Other factors include the quality and quantity of forestlitter, precipitation (Shuur et al. 2001), soil mineralogy(Torn et al. 1997), and temperature. In this study, weexamine processes of soil C accumulation and loss, andforms of SOM, in watersheds along a windthrow dis-turbance sequence in a mountainous temperate rain for-est in southeast Alaska.

Large quantities of SOM accumulate in organic andmineral horizons in southeast Alaska (Alexander et al.1989, Van Cleve and Powers 1995) both through in situdeposition and illuviation (i.e., the accumulation oftranslocated carbon, iron, and aluminum; Ugolini andMann 1979, Lundstrom et al. 2000a). Soil organic mat-ter amounts may also be strongly influenced by wind-throw (Kramer et al. 2001), which may, in some cases,redistribute and mix mineral and organic soil horizons

August 2004 2231SOIL CARBON AND WINDTHROW

FIG. 1. Scanning electron microscopy images. Top left: Intact old soil horizon in which thick organic and mineral soilhorizons have developed (Oie, Oa, Bh, Bs). Top right: Soil profile disturbed after windthrow in which developed soil horizonshave been disrupted and redistributed resulting in Oie (organic) and Bs (mineral) soil horizons. Bottom: Large root wadassociated with windthrow that can cause the disruption and redistribution of organic and mineral soil horizons.

down to bedrock (Fig. 1; Schaetzl 1986). Other dis-turbances, such as fire or widespread insect outbreak,are largely absent from this chronically wet and coolregion (Alaback 1996).

The illuviation of soil C in southeast Alaska resultsfrom chemical–physical interactions between dissolvedand suspended soil C and mineral soil, a process knowncollectively as podzolization. Podzolization typically

occurs where thick organic horizons (Oi, Oe, Oa) ac-cumulate in wet cool climates. The water passingthrough organic horizons leaches large amounts ofhighly reactive fulvic and humic acids that chelate ironand aluminum from upper mineral horizons, resultingin the formation of a white silicon-rich horizon (E) atthe surface of the mineral soil. Dissolved metal-bearingorganic acids then percolate deeper in the soil, where

2232 MARC G. KRAMER ET AL. Ecology, Vol. 85, No. 8

they eventually accumulate or decompose. The mech-anisms of metal and C immobilization remain unclear,especially for aluminum, and are still the subject ofconsiderable research (Farmer 1999). For dissolvedcarbon, proposed mechanisms of immobilization in-clude (1) flocculation through organo-mineral inter-actions, (2) microbial processing and polymerizationof organics, and (3) physical sorption, principally tomineral particles (Lundstrom et al. 2000a). Weaker in-teractions include ligand exchange, van der Wallsbonding, hydrogen bonding, and cation bridging (Dahl-gren and Marrett 1991). Immobilization results in theilluviation of mobile organic carbon, iron, and alumi-num to various mineral horizons below (Bh, Bs; Ug-olini and Dahlgren 1987). Although illuvial carbon inmineral soil may decompose over time (Lundstrom etal. 2000b), it does so slowly and may persist in the soilfor .6000 yr (Theng et al. 1992).

While the general processes of illuviation and mixingdue to windthrow are relatively well understood(Schaetzel et al. 1990, Lundstrom et al. 2000a, Ulanova2000), their influence on soil C dynamics in forestedwatersheds has not been well studied. In mountainoussoutheast Alaska and elsewhere, illuviation may resultfrom both lateral and vertical water flow (Hornbergeret al. 1994, Ranville and Macalady 1997, Sommer etal. 2000; M. G. Kramer, unpublished data). Soil dis-turbance is common on steeper slopes, often in asso-ciation with soil mixing due to windthrow (Schaetzland Follmer 1990, Normann et al. 1995, Kramer et al.2001). Soil mixing can lead to both import and exportof mineral soil C (Beatty and Stone 1986, Cremeansand Kalisz 1988). Fine root turnover contributes bothdissolved and particulate C directly into mineral ho-rizons (Joslin and Henderson 1987, Brewer 1994).Macrofauna are not likely to be a major source of Cin mineral soil horizons of these acid (mor) soils sincethey are relatively scarce (Schaefer and Schauermann1990) and confined largely to surface organic horizons.

Our first objective was to examine the influence ofwindthrow and illuviation on the accumulation of soilC in mineral horizons of these mountain forest soils.Previous workers (e.g., Sollins et al. 1983, Christensen1992, Barrios et al. 1996, Kramer et al. 2003) haveseparated two distinct pools of soil C in mineral soil,a heavy fraction and a light fraction. Carbon in theheavy fraction is strongly sorbed on mineral particles,while the light-fraction organic matter retains more ofits initial biological structure and is less associated withmineral matter. Few studies have investigated factorscontrolling light-fraction vs. heavy-fraction C accu-mulation in forest soils (Sollins et al. 1983, 1996).Heavy-fraction C accumulation in illuvial (Bh) hori-zons might result from chelation of iron and aluminumby dissolved organic carbon, and its subsequent pre-cipitation or sorption onto mineral soil particles (Zu-nino and Martin 1977). Conversely, light-fraction Caccumulation could result from mineral and organic

particle mixing or root inputs, since it is in particulateform and has weaker chemical and physical associationwith mineral particles. Our second objective was tocompare the forms in which SOM accumulates in majororganic and mineral horizons in these watersheds withcontrasting disturbance histories.

METHODS

Three forested watersheds with contrasting degreesof disturbance were selected for the study. Soil profileswere described, and the thickness of the major organicand mineral horizons was measured, every 5 m alonga grid in each watershed. A subset of these horizonswas randomly selected and sampled to determine thequantity and quality of carbon present. Carbon-en-riched (Bh) illuvial samples were physically fraction-ated based on particle density. Some of the sampleswere dispersed using an ultrasonic probe prior to frac-tionation to determine if aggregation influenced par-ticle density. Total C and N, natural 15N and 13C abun-dance, solid-state 13C nuclear magnetic resonance(NMR), and scanning electron microscopy (SEM) wereused to characterize and compare soil C pools.

Study site

This work was conducted in the middle of the Ton-gass National Forest, a vast region of pristine coastaltemperate rain forest in southeast Alaska. The forestsare distributed throughout the Alexander Archipelagoon 7 3 106 ha located on .1000 islands with diversegeology and topography (Alaback 1996). Soils arecharacteristically shallow (,1 m in depth), due to re-cent glaciation. Podzolization is common largely as aresult of year-round precipitation and a cool maritimeclimate (Heilman and Gass 1974, Alaback 1996).

Six conifer species dominate the region (Pawuk andKissinger 1989); their distribution is strongly con-trolled by soil drainage. On well-drained sites, pro-ductive western hemlock, Tsuga heterophylla (Raf.)Sarg., and Sitka spruce, Picea sitchensis (Bong.) Carr.,forests are common. On less well-drained but still pro-ductive sites, hemlock and spruce still dominate withsome mixtures of less productive Alaska yellow cedar,Chamaecyparis nootkatensis (D. Don) Spach, andwestern red cedar, Thuja plicata Donn ex D. Don. Athigher elevations (above 400 m), less productive moun-tain hemlock, Tsuga martensiana (Bong.) Carr., typi-cally replaces western hemlock. Low-productivitymixed conifer–scrub forests often dominated by lodge-pole pine, Pinus contorta Dougl. ex Loud. var. contorta,occur extensively on the landscape, along with muskegplant communities on lower site hydric soils or wet-lands (Pojar and MacKinnon 1994, Alaback 1996).

Extratropical cyclones pass through southeast Alaskaevery four or five days during winter (Shumacher andWilson 1986). Associated with these storms are windsup to, and occasionally in excess of, 40 m/s, persistentcloud cover, and up to 13 m of precipitation annually


FIG. 2. Map of High Island and vicinity showing watershed locations.

in the coastal mountains. The trajectory for these lowpressure systems, referred to as the North Pacific StormTrack, is largely determined by the location andstrength of three semipermanent atmospheric features:the Aleutian low pressure and Siberian high pressuresystems in autumn, winter, and spring, giving way tothe east Pacific high pressure system in summer. Largeinterannual changes in storm frequency, intensity, size,and position occur as a consequence of El Nino, whichpenetrates poleward into the Gulf of Alaska generallyintensifying storm activity in the region (Schumacherand Wilson 1986).

High Island (;200 ha) is located in the middle ofthe Alexander Archipelago on state forest land, ;160km from the mainland and 20 km south of the town ofKake, Alaska (Fig. 2). Maximum elevation on the is-land is 150 m. The parent material is fractured basalt.Mean annual precipitation is 1.9 m, with the wettestmonths occurring during fall and winter. Cloud cover,precipitation, cool ambient air temperatures (4–108C),and high relatively humidity (.80%) are characteristicthroughout the year (M. G. Kramer, unpublished data).Extreme high and low temperatures are infrequent dueto the maritime influence.

Three small watersheds (0.1–0.6 ha in size; WS1,WS3, and WS4) were selected for this study because

they have similar topography, parent material, climate,and plant species composition, and differ principallyin disturbance history. These watersheds were selectedfrom four previously mapped watersheds (WS1–WS4),which were monitored in a separate study of hydro-chemical properties (Kramer 2001). WS2 was not in-cluded in this study because our sampling resourceswere limited and because WS2 had a disturbance his-tory very similar to WS1. Elevation ranges from 3 mto 60 m above sea level. Only western hemlock andSitka spruce are found in the watersheds.

High Island was struck by a catastrophic cyclonicstorm in ;1905. Forests on the south and west portionof the island show evidence of greater damage fromthe storm than north to northeast portions (Fig. 2).WS1, located on the west side, experienced completestand loss from the storm. Storm effects visible nowinclude many large windthrow mounds (rising to 2 mabove the forest floor but more typically ;1 m) andpits (sometimes 1 m below the forest floor but moretypically ;0.5 m) throughout the watershed as well asan even-aged forest that regenerated after the storm.Prior to the 1905 windstorm, limited logging (16 trees/ha) occurred in WS1 (Fig. 2). WS3, located on the northportion of the island, shows evidence of partial dis-turbance (;30%) from the 1905 storm. WS4, located


on the northeast portion, shows little evidence of dam-age from the 1905 storm, such as downed stems or pit-and-mound microtopography. The forest in WS4 ap-pears to be older and shows evidence of gap-phaseforest development. It is difficult to infer the distur-bance history of these three watersheds prior to 1905,but work by Kramer et al. (2001) suggests that topo-graphically exposed portions of High Island may neverfully recover from one catastrophic windstorm beforethe next storm cycles the forest back to an early seralstage. Three major soil types occur on High Island;Spodosols (Lithic or Typic Humicryods), Histosols(Lithic or Typic Cryosaprists), and Inceptisols (Lithicor Typic Dystrochrepts). The soils were very hetero-geneous in each watershed due to hillslope position,soil water status, and disturbance dynamics. WS1 isdominated by Inceptisols, WS3 contains a mix of allthree, and WS4 is dominated by Histosols and Spo-dosols.

Damage from past windstorms

The extent of windstorm damage in each watershedwas determined by measuring tree age and stem size,and quantifying the area occupied by prominent pit-and-mound topography. Tree ages were determined bycoring a random sample (.50%) of overstory trees ineach watershed and counting annual growth rings witha dissecting microscope. Cores that were difficult tocount were mounted, sanded, and then counted.

Field measurements and sampling

Each watershed was divided into 5 3 5 m grid cells.Soils profiles were determined at each grid intersection(every 5 m) across the entire watershed at 251, 147,and 36 grid points in WS1, WS3, and WS4, respec-tively. Thickness of major organic and mineral hori-zons (.1 cm) was measured at each grid point (Oi, Oe,Oa, A, E, Bh, Bhs, Bs). Horizons were identified basedon color and field estimation of organic vs. mineralcontent (by judging the grittiness of the soil; Soil Sur-vey Staff 1998). For Bs horizons, thickness was de-termined by measuring depth to bedrock from the topof the Bs with a 2-m steel probe rod. The probe wasused four times per soil pit, to increase the chance thattrue bedrock was reached. For the most part, these fourdepth measures were in good agreement with one an-other, suggesting the probe had not hit a rock. In pre-liminary digging we found that BC and C horizons werethin (,10 cm) and did not comprise a significant por-tion of the total profile.

Samples (between 12% and 50%) of each major min-eral and organic soil horizon were collected from ran-domly selected grid points with a 200-mL, 10-cm di-ameter corer. Soils were weighed, dried for 48 h at608C, then weighed again to determine wet and drybulk density. Oven-dried samples were sieved (using2-mm mesh) to remove large organic and mineral par-ticles and to estimate stone content. A 1-mm sieve was

used to further pick out large organic particles includ-ing roots, bark, and other identifiable plant parts. Thesamples were then shipped to Oregon State Universityfor physical and chemical characterization.

Density fractionation

For Bs and Bh mineral soil horizons, a 10-g drysample of the ,2 mm fraction (i.e., the soil fractionthat passed through a 2-mm sieve) was physically sep-arated based on particle density. Samples of dry min-eral soil were ground with a mortar and pestle to passa 0.425-mm sieve. For density fractionation, 10 g ofsoil were placed in a 110 mL centrifuge tube and sus-pended in 30 mL of sodium polytungstate (NaPT) at adensity of 1.65 g/mL (Strickland and Sollins 1987).After mixing for 12 h using a low speed shaker, sampleswere centrifuged for 1 min, and allowed to settle for48 h. The suspended light fraction was removed bygentle aspiration through a Tygon hose. Remaining so-lution was then aspirated and discarded, leaving theheavy fraction in the bottom of the tube. The light andheavy fractions were placed on a No. 52 Whatmanpaper filter (Whatman, Clifton, New Jersey, USA) andrinsed five times with 300 mL of deionized water.Heavy and light fractions were oven-dried overnight(608C), then weighed. Fraction weights were reportedas percentage of dry soil weight.

Ultrasonic dispersion (1050 J/mL; see Christensen1992) was performed on 10 samples to estimate theinfluence that remaining small aggregates (,0.425mm) had on particle density. Dispersion should freelight organic particles that may be trapped in aggre-gates (and possibly microaggregates) in the heavy frac-tion (Christensen 1992). After ultrasonication, the samedensity fractionation procedure was applied. Loss-on-ignition (LOI) was measured on a subset of the coarse(.2 mm) fraction from light and heavy Bh samples.

Elemental analysis

Dry samples (,2 mm) were ground finely with azirconium mortar and pestle, and loaded into tin boats.Each sample was analyzed for total C, N, d15N, andd13C using a Europa 20/20 ANCA GSL continuous-flow isotope-ratio mass spectrometer (PDZ Europa,Cheshire, UK) at the Rosenstiel School of Marine Sci-ence, University of Miami, Florida, USA. One standardwas run for every 10 unknowns, and two blanks andstandards were included at the beginning and end ofeach run. Analysis of internal standards indicated ananalytical error of ,5% for N and ,2% for C.

Natural isotope ratios were reported as

R 2 Rsample standardd (‰) 5 3 1000sample 1 2Rstandard

where Rsample and Rstandard are the heavy and light isotoperatios of sample and standard, respectively.

Stores of light and heavy C fractions in soil horizonswere calculated for each 25-m2 cell across the water-


TABLE 1. Tree size and age characteristics within three watershed areas in the Tongass NationalForest, Alaska, USA.

Water-shed

Standdensity

(stems/ha)

Stem size (cm)

Mean(1 SE)

Standarddeviation

Stem age (yr)

Mean(1 SE)

Standarddeviation

Prominent pitand mound

topography (%)

WS1WS3WS4

663408360

31.4 (0.81)31.6 (1.39)48.4 (7.17)

16.219.843.0

68.8 (2.33)127.9 (5.06)218.5 (28.6)

37.970.2

171.6

1150

sheds by estimating soil horizon thickness from soilpits dug in each cell, along with the average quantityof each type of SOM found in soil samples. Cell quan-tities were added up for each watershed, divided byarea of the watershed, and then expressed as Mg/ha.For thick Bs horizons (.30 cm), we assumed that theC concentration in the portion of the profile beyond 30cm was half the measured value for the portion of theBs horizon that was sampled:

2Soil C (Mg/ha) 5 [horizon thickness (m) 3 25 mOi

33 ,2 mm bulk density (g/m )

3 (1 2 volume rock) 3 C (%)

63 Mg/10 g]

4 [watershed area (ha)].

Solid-state 13C nuclear magnetic resonance (NMR)spectroscopy and microscopy

Solid-state 13C nuclear magnetic resonance (NMR)allows approximate quantification of three groups ofcompounds: O-alkyl (carbohydrate), alkyl, and aro-matic (Baldock and Preston 1995). Fresh litter material(needles, wood, and fungal mat), light-fraction andheavy-fraction Bh samples, and samples from Oie andOa organic horizons were characterized for functionalgroups using 13C NMR. We used a Bruker AF-300 sol-id-state Pulse NMR spectrometer (Bruker Biospin,Rheinstetten, Germany) at the University of Washing-ton with cross polarization and magic angle spinning,equipped with a double-tuned, single-coil probe withan external lock and an Andrew-Beams type spinningapparatus. The magnetic field was 300 MHz (7.0 T)giving a 13C resonance frequency of 75.46 MHz. Typ-ical parameters were: proton 908 pulse, 5 mS; contacttime, 1 ms; recycle delay, 1 s; spinning speed, 3.5 kHz;number of scans, 3600–5000. Due to the formation ofsidebands, which is greater on 300 MHz than on 100MHz NMRs, oxygen substituted alkyl carbon forms(carbohydrates) are over represented while aromaticcarbon forms are under represented. However, this didnot affect the internal comparison between samples.Select samples were also examined under a scanningelectron microscope (Amray, Bedford, Massachusetts,USA) at the Oregon State University, Department ofBotany, in order to study morphological features oforganic detritus.

RESULTS

Damage from past windstorms

Recent (,100-yr-old) prominent pit-and-mound to-pography was most extensive in WS1, which appearedto have experienced the most damage from the 1905storm (Table 1). Eleven percent of the soil surface hadpronounced pit-and-mound topography. WS3 had few-er pits and mounds (5% of the area) and trees wereolder, but WS3 nonetheless showed evidence of partialdamage and a distinct pulse of tree recruitment fromthis storm. WS4 had a wide range of tree ages and noevidence of recent (since 1905) pits and mounds; fallenlogs were highly decomposed (Class IV). Trees in WS1were smaller and higher in density, and most had es-tablished after or near the time of the storm (Table 1).In WS3, trees were older, more variable in age, andcomprised at least two distinct age classes (Fig. 3).WS4 showed evidence of gap-phase forest develop-ment, and no distinct pulse of recruitment could bedetected. Small-scale, low-intensity disturbances likelyexplain the present WS4 stand attributes (age, size, anddensity).

Soil horizon thickness and composition

Soils were shallow on average (,0.73 6 0.02 m[mean 6 1 SE]), and bedrock was often reached whileprofiling the major organic and mineral soil horizons.Frequency distributions of the thickness of major soilhorizons from each watershed are shown in Fig. 4.Mean horizon thickness was not reported because ho-rizons were not found at all grid points, and many ofthe horizon thickness frequency distributions were non-normal (negative exponential or uniform) (Fig. 4). Ourestimates of Bs horizon thickness likely included someBC and C material. Presence of horizons thicker than1 cm and the number of soil samples collected, per-centage of ,2 mm fraction, and corrected bulk density(wet and dry) from each horizon in each watershed arereported in Table 2. Coarse fragment content was cal-culated by multiplying the corrected bulk density bypercentage of .2 mm fraction and assuming a stonedensity of 2.65 g/cm3. Stone content was likely un-derestimated with the small core used to collect sam-ples (Table 2). Bh horizon particles .2 mm were mostabundant in the two more disturbed watersheds (WS1and WS3; Table 2). Soil water content was higher inthe older less disturbed watersheds (WS3 and WS4;Table 2).


FIG. 3. Tree age distribution of the three wa-tersheds.

FIG. 4. Frequency distributions of thickness of organic and mineral soil horizon within the three watersheds.

Density fractionation/elemental analysis

Results from the density fractionation and C, N, andisotope analyses are summarized in Table 3. On av-erage, d13C and d15N increased while C:N ratio valuesdecreased with depth across all watersheds (Table 3; P, 0.01). d13C values were low (229.0 6 0.2‰ [mean6 1 SE]) in fresh organic material and forest litter(227.35 6 0.1‰), and only slightly higher in deepmineral soil horizons (225.5 6 0.1‰). d15N valueswere low in fresh organic material (23.8 6 0.2‰) andvery high in mineral soil samples (6.5 6 0.2‰), show-

ing greater isotopic discrimination for N than for Cbetween fresh organic source material and Bs soil sam-ples. Forest litter (Oie) d15N values were considerablylower in WS4 than in either WS3 or WS1 (21.2 60.1‰, 1.98 6 0.2‰, and 2.07 6 0.28‰, respectively).Deeper major soil horizons were on average slightlymore depleted in d15N in WS1 than in WS3 or WS4(5.04 6 0.3‰, 8.71 6 0.39‰, and 7.32 6 0.55, re-spectively). The C:N ratio of forest litter (Oie) waslower in WS1 than in WS3 or WS4 (31.8 6 1.06, 47.46 5.73, and 38.7 6 2.35, respectively).


TABLE 2. Corrected bulk density and ,2 mm fraction fromsamples collected.

Watershedand soilhorizon

No. sam-ples

collected

Bulk density,,2 mm(g/cm3)

Watercontent(g/cm3)

Percent-age

,2 mm

WS1OeOaBhBs

344032

125

0.10 (0.01)0.17 (0.01)0.28 (0.02)0.23 (0.01)

0.29 (0.02)0.50 (0.02)0.45 (0.02)0.31 (0.01)

······

67 (5)49 (2)

WS3OeOaBhBs

29695546

0.10 (0.01)0.16 (0.01)0.31 (0.02)0.26 (0.02)

0.35 (0.02)0.63 (0.07)0.75 (0.15)0.39 (0.02)

······

79 (2)62 (3)

WS4OeOaBhBs

12273821

0.08 (0.00)0.12 (0.00)0.31 (0.03)0.33 (0.04)

0.40 (0.02)0.55 (0.02)0.70 (0.03)0.55 (0.02)

······

84 (4)82 (3)

Note: Numbers in parentheses represent 61 SE.

TABLE 3. Results from density fractionation and analyses of carbon, nitrogen, and natural abundance isotopes of bothcarbon and nitrogen.

Source of sample No. samples C (%) d13C (‰) N (%) d15N (‰) C:N ratio

Hemlock needlesHemlock woodSpruce needlesSpruce woodFungal mat

252111

49.4 (0.01)51.648.650.850.4

229.2 (0.23)228.9226.3227.3229.3

1.02 (0.01)0.411.101.090.48

23.83 (0.27)25.7520.5121.9726.19

49.5 (1.39)128

43.946.6

104.4

Watershed and soil horizonWS1

OieOaBh, lightBh, heavyBs

1421322957

44.0 (0.74)39.9 (1.11)34.8 (0.67)16.4 (1.07)12.7 (0.60)

227.2 (0.12)226.3 (0.10)226.3 (0.09)226.1 (0.08)225.3 (0.07)

1.41 (0.06)1.34 (0.07)1.39 (0.05)0.75 (0.05)0.61 (0.03)

2.07 (0.28)2.13 (0.48)5.01 (0.27)5.68 (0.16)5.04 (0.30)

31.8 (1.06)31.9 (2.25)26.2 (1.13)22.4 (0.81)21.3 (0.73)

WS3OieOaBh, lightBh, heavyBs

1348525244

47.9 (0.51)40.9 (1.09)31.1 (0.81)14.6 (0.79)14.2 (0.89)

227.5 (0.2)226.9 (0.12)227.1 (0.10)226.4 (0.06)225.6 (0.10)

1.14 (0.09)1.04 (0.06)1.06 (0.04)0.63 (0.04)0.55 (0.04)

1.98 (0.29)5.05 (0.24)6.58 (0.25)7.18 (0.29)8.71 (0.39)

47.4 (5.73)49.9 (4.64)31.1 (1.37)25.5 (1.21)28.2 (2.48)

WS4OieOaBh, lightBh, heavyBs

1423

73710

44.2 (0.82)43.1 (0.75)31.6 (1.35)16.2 (0.80)12.9 (1.42)

227.3 (0.14)226.6 (0.12)226.7 (0.30)226.1 (0.05)225.8 (0.09)

1.21 (0.09)1.45 (0.06)1.49 (0.10)0.87 (0.05)0.6 (0.04)

21.18 (0.76)1.89 (0.48)2.8 (1.73)6.65 (0.22)7.32 (0.55)

38.7 (2.35)31.0 (1.54)22.0 (2.28)19.1 (0.50)21.3 (1.24)

Note: Numbers in parentheses represent 61 SE.

The whole-soil C concentration in heavy-fraction Bhsamples was higher in the less disturbed watersheds(6.3 6 1.0%, 9.4 6 0.6%, and 13.8 6 0.5% in WS1,WS3, and WS4, respectively) while light-fraction BhC concentrations were lower (8.5 6 0.7%, 5.0 6 0.4%,and 3.1 6 0.7% in WS1, WS3, and WS4, respectively;Fig. 5). Total C in the light and heavy fractions is shownin Fig. 6. Watershed-wide total soil C pools were slight-ly higher (157 6 6 Mg/ha [mean 6 95% CI], 194 619 Mg/ha, and 216 6 33 Mg/ha in WS1, WS3, and

WS4, respectively) in the less disturbed watersheds(Fig. 7). However, C stores in a given horizon differedconsiderably among watersheds. Bh C pools were 7 62 Mg/ha in WS1 (most disturbed), 17 6 2 Mg/ha inWS3, and 31 6 6 Mg/ha in WS4 (least disturbed).Carbon in the Oa horizon decreased with increasingdisturbance (WS4 5 58 6 7 Mg/ha, WS3 5 46 6 8Mg/ha, WS1 5 17 6 3 Mg/ha). Ultrasonication priorto density fractionation of Bh samples resulted in re-covery similar to that from samples fractionated with-out ultrasonication (85 6 3% [mean 6 1 SE] recovery).Mass loss-on-ignition of .2 mm fraction samples was,1% (60.2).

Solid-state 13C nuclearmagnetic resonance (NMR) spectroscopy

Chemical shifts for major types of carbon compoundare shown in Table 4. NMR spectra from select samplesacross soil horizons and from fresh litter (Oie organichorizon, needles, and fungal mats) were clustered basedon O-alkyl, aliphatic, aromatic, and carboxyl/carbonylcontent using a Chebychev distance metric (Fig. 8;Michalski et al. 1981). The O-alkyl group decreasedand aliphatic, carboxyl/carbonyl, and aromatic groupsincreased with depth (Fig. 9). Heavy-fraction Bh sam-ples across all watersheds, and all four of the light-fraction Bh samples from WS3, were richest in ali-phatic, carbonyl, and aromatic groups.

Scanning electron microscopy

Scanning electron microscopy (SEM) images of se-lect samples showed identifiable plant debris in light-


FIG. 5. Soil carbon as a percentage of whole soil in theBh horizons. Error bars represent 95% confidence intervals.

FIG. 6. Carbon stores in light fractions (LF) and heavyfractions (HF) from Bh horizons in the three watersheds. Errorbars represent 95% confidence intervals.

fraction Bh samples (Fig. 10a). In contrast, we couldnot discern recognizable plant debris in most of thematerial in four of the light-fraction Bh sample thatwere rich in aromatic and carboxyl groups, or in anyof the heavy-fraction Bh samples that we examined(Fig. 10b). These same samples showed micromorpho-logic similarities to SEM images of precipitated humicacid (Fig. 10c; e.g., amorphous masses with distinctcracked, but otherwise smooth, surfaces.)

DISCUSSION

Evidence for redistribution and loss of soil organiccarbon by windthrow

Our results suggest that both windthrow and illuvi-ation strongly influence the accumulation of carbon (C)in mineral soil horizons in forest soils of southeastAlaska. Partially decomposed organic debris (lightfraction) was found in all Bh (i.e., C rich) mineralhorizons sampled, and the proportion of total C in thisform declined with increasing horizon thickness (Fig.6). These thicker, more developed, Bh horizons areprobably also older since they were found in water-sheds with old trees (.200 yr) and less evidence ofrecent soil disturbance. Soil mixing and soil creep dueto hillslope processes were observed in all of thesesteeply sloping watersheds. The organic debris in Bhhorizons was likely the result of tree uprooting duringwindthrow and subsequent slope redistribution pro-cesses (Schaetzl and Follmer 1990, Normann et al.1995). In the most disturbed watershed (WS1), pro-nounced pit-and-mound topography occupied 11% ofthe soil surface. However, the area of soil disturbance(estimated to be in excess of 40% of the watershedarea) was larger than that of pronounced pit-and-moundtopography. Many pits were likely filled, and mounds

smoothed, from slope movement and redistribution ofmaterials.

In situ decomposition of root and mycelial fragments(biotic processes) is an alternative mechanism that canlead to the presence of organic debris in mineral soilhorizons. Macrofauna, such as earthworms, are notlikely to be a major source of C in this mineral soilbecause they are relatively scarce and inactive in acid(mor) podzols (Shaefer and Schauermann 1990, Beyerand Irmler 1991), and were never observed at HighIsland. Spiers et al. (1986) did find earthworms in morhumus of the coastal temperate rain forests of NorthAmerica, but activity was restricted to organic hori-zons. In situ root mortality may contribute C to bothdeep and shallow mineral soil horizons (McClaughertyand Aber 1982, Vogt et al. 1983, Nepstad et al. 1994).Earthworms and root death alone are not likely to haveresulted in the large accumulations of light-fractionorganic matter in Bh horizons or in bare Bs horizonsexposed from recent windthrow. We rarely could dis-cern root fragments in the Bh horizons, and SEM andNMR analysis of the Bh soil organic matter (SOM)suggested that fine roots were not a major constituent.We conclude, therefore, that soil mixing from wind-throw and subsequent hillslope redistribution processesmost likely caused the large accumulations of partiallydecomposed organic debris in the subsoil. However,we recognize that root-derived C was not well quan-tified.

Redistribution and loss of soil organic carbon bywindthrow across watersheds

Our results suggest that soil mixing from windthrowresulted in the redistribution and overall loss of soil Cacross entire watersheds and a decrease in the soil or-ganic matter C:N ratio. In addition to redistributingentire soil profiles (organic and mineral soil materialsin some cases even down to bedrock), windthrow canwarm soil, thus increasing microbial activity (Schaetzlet al. 1989). In newly formed pits, increased litter ac-cumulation and soil moisture can occur, resulting inaccelerated podzolization (Schaetzl et al. 1989). Soil


FIG. 7. Distribution of total carbon (C) in major soil or-ganic matter pools in the three watersheds. Error bars rep-resent 195% confidence limits.

TABLE 4. Chemical shift assignments of 13C CP/MAS solid-state NMR of natural organic materials.

Shift (ppm) Assignment

0–4545–110

110–160160–220

alkyl-CO-Alkyl-Caromatic-Ccarbonylic-C/carboxylic-C

Note: See Baldock and Preston (1995).

drainage can improve on newly formed mounds, re-sulting in drier soils with higher rates of decomposition(Schaetzl et al. 1989). The low litter (Oie) C:N ratioin WS1 suggests that the quality of litter increased afterwindthrow (Table 3), perhaps due to increased decom-position rates of litter due to increased aeration andwarmer soil temperatures. Total C stocks were lower(by up to 59 Mg/ha) in the two more disturbed water-sheds (Fig. 7). Soil C occurred principally in organic(Oie) or deep (Bs) horizons in the more disturbed wa-tersheds, likely due to respiration of the recalcitrantpools (Oa and heavy-fraction Bh) and redistribution ofthis material to upper and lower horizons during mixing(Fig. 7). The decrease in total soil C observed in themore disturbed watersheds (up to 59 Mg/ha) was duemainly to a decrease in Bh and Oa horizon C pools(Fig. 7). Overall the total soil C stocks in each water-shed (157–216 Mg/ha) were very close to those re-ported previously for southeast Alaska (Alexander etal. 1989, Van Cleve and Powers 1995). The light-frac-tion Bh pool (;5 Mg/ha) was similar in size across allthree watersheds (Fig. 6). This suggests that particulateC in this illuvial horizon is relatively constant and nei-ther accumulates nor declines with disturbance.

Evidence for illuviation

More heavy-fraction C was found in the less-dis-turbed watersheds (Fig. 6). SEM images and NMRcharacterization suggest that this heavy-fraction C ac-cumulated through illuviation. SEM images of heavy-fraction material found in Bh soil horizons show crackson the surface of large particles (Fig. 10b). Such crackshave been shown elsewhere to result from dehydrationof gelatinous, hydrated organo-mineral complexes that

were transported in soil water to mineral horizons fromoverlying organic horizons (Deconnick 1980, Pusto-voitov and Targulian 1996). Our NMR analysis showsthat much of the heavy-fraction organic matter is com-posed of carboxyl and carbonyl groups, characteristicof dissolved fulvic and humic acids (Beyer 1996, Parfittet al. 1999). In the two less disturbed, older watersheds,three differences in illuvial (Bh) mineral horizons wereobserved: (1) horizons were thicker (Fig. 4), (2) theconcentration of C in the heavy fraction was higher(Fig. 5), and (3) amount of heavy-fraction soil organicmatter was higher. All of these differences can be ex-plained by illuvial processes.

Illuviation due both to lateral and vertical soil watertransport in watersheds

At our site, the thicker black soil horizons (Oa andBh) could not be explained by vertical translocation(illuvial) processes alone: (1) white, elluvial soil ho-rizons were thinner and often altogether absent aboveunderlying Bh horizons; (2) the elluvial horizons wereconcentrated in the upper edges of the watershed, oftenin the absence of an underlying Bh horizon (notshown); and (3) lateral flow on top of organic (Oa) andmineral (Bh) soil horizons could be observed regularlyin soil pits under wet conditions. While lateral flowwas observed in all three watersheds, it was observedto decrease in volume and occur deeper with increaseddisturbance (M. G. Kramer, unpublished data). Evenin the most recently disturbed watershed, however, wesaw lateral flow through organic and mineral horizons.

Although both lateral and vertical soil water move-ment may lead to translocation of carbon to mineralhorizons, their relative contributions remain unclear.Translocation of materials via lateral flow may depleteor enrich entire mineral soil profiles down to bedrock,or deposit translocated material in surface organic ho-rizons, depending on factors such as hillslope position,fractured vs. unfractured bedrock, and soil permeability(Scatena and Lugo 1995, Newmann et al. 1998, Ha-gedorn et al. 2000, Sommer et al. 2000). Many of thethicker, wetter Oa (black muck) horizons at our sitecontained high concentrations of inorganic material(20–50%). The source of this inorganic material maybe laterally or even upward flowing organo-mineralcomplexes that were immobilized after transport tothese organic horizons.


FIG. 8. Clustering of nuclear magnetic resonance (NMR) data based on aliphatic, O-alkyl, aromatic, and carboxyl groups.Clustering of NMR spectra resulted in four distinct groups that varied in the extent of decomposition and generally coincidedwith different soil horizons (abbreviations: Bh LF, light fraction; Bh HF, heavy fraction). Representative NMR spectra ofindividual samples are shown. Units on the x-axis are ppm.

Interactions between windthrow and illuviation

Soil mixing resulting from tree uprooting and pod-zolization may be viewed as opposing processes (John-son et al. 1987). In fact, Schaetzl et al. (1990) havereferred to soil mixing as a process of regressive pod-zolization. In this view, a succession of soil developmentprocesses beginning with podzolization may shift anecosystem toward a climax condition, unless soil dis-turbance resets this sequence (Ugolini and Mann 1979,Bormann et al. 1995). This view of the interaction be-tween disturbance and soil development is similar toplant succession in which community succession is resetafter catastrophic disturbance—a punctuated equilibri-um view of ecosystem change (Pickett and White 1985).Many studies in New Zealand have reported the presenceof brown Inceptisols where Spodosols would be ex-pected (Campbell and Mew 1986, Schaetzel et al. 1990,Stewart et al. 1993, Mew and Ross 1994), and attributedthe presence of these younger soils to soil mixing fromwindthrow. On a watershed scale, we found that the twowatersheds that experienced greater windthrow con-tained younger soils (Inceptisols) with less evidence of

soil horizonation. It is possible that soils in these moredisturbed watersheds never reach an advanced stage ofpodzolization because they are susceptible to repeatedstorms (Kramer et al. 2001).

Not all C in Bh horizons originated from illuviation;some was the result of windthrow-driven soil mixing.In thin, poorly developed illuvial horizons, most C waslight-fraction, particulate debris, and likely originatedfrom windthrow that occurred during the 1905 wind-storm (Figs. 5 and 6, Table 1). In WS3, which experi-enced partial, less recent windthrow, particulate organicdebris still comprised 33% of the total C present. In theoldest watershed, which lacked evidence of recent(,200 yr) windthrow, there was little light-fraction C.Most C in the older thicker illuvial horizons was in theheavy fraction, and likely originated predominantly fromimmobilization of mobile organic C (Fig. 6).

Origin of light- and heavy-fraction carbonin Bh soil horizons

In the two older, less disturbed watersheds, most Cin the illuvial horizon was in the heavy fraction. The


FIG. 9. Qualitative summary of the distribution of soilorganic matter (SOM) types from major pools. LF representslight fraction; HF represents heavy fraction.

FIG. 10. (a) Scanning electron microscope (SEM) imagesof light-fraction Bh horizon, showing partially humified or-ganic particles; (b) SEM images of heavy-fraction Bh horizon,showing cracked coatings; and (c) NaOH-extracted humicacid from Bh horizon, showing cracking from dehydration ofthe hydrated organo-mineral complex.

cracked surfaces of these materials, as revealed throughSEM and optical images, suggest that they are likelycoatings that originated through illuviation (Deconnick1980). NMR analysis shows abundant carboxyl andcarbonyl groups in this heavy-fraction C (Fig. 9), whichcan complex with hydroxylated mineral surfaces. In-teractions including ligand exchange and weaker in-teractions such as cation bridging, anion exchange (out-er-sphere complexation), and van der Walls forces mayalso play a role in the formation of heavy-fraction C(Jardine et al. 1989).

Thin illuvial horizons were dominated by light(,1.65 g/mL) organic debris with discernible plantfragments (Figs. 6 and 10a). Microbial secretions arethought to bind clay and small mineral particles to thelarger organic particles (Golchin et al. 1998). This formof organo-mineral association (principally physical)can be observed in the SEM images of light-fractionmaterial in the illuvial horizon (Fig. 10a). Overall,these C pools have weak chemical association withmineral particles, are particulate, and contain identi-fiable plant and microbial matter.

In some of the thickest and best developed illuvialsoil horizons, light (,1.65 g/mL), heavily processed(based on d15N values and NMR analysis) organic mat-ter was found (Fig. 9, Table 3). Identifiable plant andmicrobial fragments could not be discerned in thislight-fraction material from either optical or SEM im-ages (Fig. 10). NMR, isotopic, and SEM analysis sug-gest strong resemblance of this material to organo-min-eral coatings found in the heavy fraction of illuvialhorizons. The heavily processed light material alsostrongly resembled (based on SEM) light NaOH-ex-tracted humic acid, which had been precipitated, thensuspended in a 1.65 g/mL solution (Fig. 10b, c). Theorigin of this illuvial light material is unclear. Its den-sity, composition, and distinct fracturing suggest thatit originated by flocculation or precipitation. This lightmaterial was found only in thicker well-developed il-


luvial horizons from the older, less disturbed water-sheds. In these illuvial horizons, the concentration ofheavy-fraction C is high (Fig. 5). Other environmentalfactors that might increase precipitation, such as lowpH, high aluminum and iron concentrations in solution,and a high carbon:metal ratio, are likely present in thesewell-developed, thick illuvial horizons.

Forms of soil organic matter (SOM): accumulationand humification

An abundance of diverse SOM forms was foundacross the watersheds studied. The source materialsexamined (needles, wood, and a fungal mat) all haddistinct C:N ratios, functional group chemistry, andisotopic signatures (Table 3). The chemical, physical,and micromorphologic properties of samples collectedfrom major organic and mineral soil horizons show thateach horizon contained distinct SOM types and that allhorizons were enriched with heavy C and N isotopesrelative to source materials (Table 3, Fig. 10a, b). Thisenrichment may be due to the degradation of thesesource materials in the forest floor. That the d15N valuesfor litter and soil in WS4 were lower than in otherwatersheds may reflect greater contributions from fun-gal-mat material, which had a very low d15N value(26.19‰; Table 3). During field sampling, more fun-gal-mat material was observed in the Oie horizon ofWS4 than in the other watersheds.

In the absence of windthrow, our results suggestSOM becomes increasingly aliphatic and saturated inboth organic and mineral soil horizons (Fig. 9, Table2). The increase in aliphatic:O-alkyl ratio and heavyisotope enrichment, and decrease in C:N ratio withdepth, all suggest that humification of SOM increasedwith soil depth in each watershed (Fig. 9, Table 3; seealso Baldock and Preston 1995, Webster et al. 2000,Kramer et al. 2003). Heavy-fraction Bh and Bs horizonorganic matter was composed largely of aliphatics andwas heavily enriched in 15N (Table 3), suggesting thatorganic matter humification in these deeper horizons(and the concomitant stabilization of soil C) resultedfrom microbial alteration of organics rather than fromaccumulation of recalcitrant compounds (Kramer et al.2003). These results suggest that the forms of organicmatter in soil reflect not only the addition of new ma-terials and subsequent processes, such as illuviation,and an increased retention of water that may influencetheir accumulation throughout the soil profile, but alsothe ongoing alteration of these materials through mi-crobial processing.

Concluding remarks

Windthrow and other disturbance processes canstrongly influence SOM formation and loss. Thus, ac-counting for such effects could substantially improveestimates of global terrestrial C budgets. At our sitesin southeast Alaska, windthrow greatly influenced soilC pools, in part through effects on illuviation. Mobile

organic carbon transported in soil water appears to haveaccumulated in mineral horizons principally throughsorption to mineral particles, and the extent of strongassociation (sorption) with mineral particles was great-er in the thicker, presumably older, illuvial horizons.In some of these thick illuvial horizons we found light(,1.65 g/mL) amorphous material suggesting that mo-bile organic carbon may be immobilized through pre-cipitation of metal-bearing organic acids.

The accumulation of C in illuvial soil horizons ap-pears to be disrupted and reinitiated by windthrow.Some soil profiles at our sites were mixed and redis-tributed down to bedrock by windthrow. In the mostdisturbed watershed the illuvial horizon was thin,weakly developed, and presumably postdated the 1905windstorm. The C in illuvial horizons that have de-veloped after disturbance can be attributed both to or-ganic C burial from windthrow and subsequent pod-zolization, although the proportion of partially humi-fied particulate C is lower in these horizons. Windthrowalso appears to have resulted in a lower total soil C(up to 57 Mg/ha less than in undisturbed watersheds).

Watersheds that experienced more intense soil mix-ing from windthrow also had lower levels of stronglyhumified soil organic matter and higher levels in a par-tially decomposed particulate form. Such a shift in theC forms may exert strong control on ecosystem dy-namics. The redistribution of O horizon materialthroughout mineral soil may increase its rate of de-composition, but it may also stabilize and shift theforms of C in soil. Redistribution may also increasenet ecosystem productivity by creating more favorableenvironmental conditions for tree growth, due to anincrease in soil pH, water permeability, and lower soilbulk density. Other significant ecosystem changes in-clude watershed-wide shifts in stream hydrologic be-havior and increased interaction of rainwater with min-eral soil (Kramer 2001).

C accumulation in mineral soil as a result of wind-throw and illuviation likely depends on factors such asthe quality and quantity of forest litter, precipitation,temperature, soil texture and mineralogy, and the fre-quency, scale, and severity of disturbance. Once or-ganic matter (OM) enters the mineral soil, as litter,roots, or in solution, the quantities and forms of OMthat accumulate are determined by relative rates of sta-bilization (via changes in recalcitrance, interactions,and accessibility) and loss (through CO2 evolution andleaching). In these southeast Alaska soils much OM istransported into mineral soil layers, and in the absenceof windthrow or landslides, the environmental factors(low soil temperatures, near-saturated soil) favor theaccumulation of large, diverse, and persistent stores ofmineral soil OM. In forest soils that experience a lon-ger, warmer, dry season, the environmental factors fa-vor greater loss of C (through enhanced microbial activ-ity), resulting in less total soil in C. In these drier orwarmer forest ecosystems the effects of windthrow and


illluviation on soil C dynamics may still be significant,but the effects may not be as pronounced, because lessC would be expected to accumulate in mineral soilhorizons even in the absence of disturbance.

ACKNOWLEDGMENTS

Major financial support for this project was provided bythe USDA. Additional support was provided by NSF grantDEB-9632122. Field assistance was provided by Andy Neil,Mathew Lupes, and Tim Norris. Stable isotope analyses wereprovided generously by Peter Swart and Amel Saied at theRosenstiel School of Marine Sciences (RSMAS), Universityof Miami. Kermit Cromack, Bernard Bormann, John Harri-son, and Christopher Potter provided helpful comments andsuggestions on earlier drafts of this manuscript.

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SOIL CARBON DYNAMICS ACROSS A WINDTHROW DISTURBANCE SEQUENCE IN SOUTHEAST ALASKA

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