Climatic factors influencing fluxes of dissolvedorganic carbon from the forest floor in acontinuous-permafrost Siberian watershed1

A.S. Prokushkin, T. Kajimoto, S.G. Prokushkin, W.H. McDowell, A.P. Abaimov,and Y. Matsuura

Abstract: Fluxes of dissolved organic carbon (DOC) in forested watersheds underlain by permafrost are likely to varywith changes in climatic regime that increase soil moisture and temperature. We examined the effects of temporal andspatial variations in soil temperature and moisture on DOC fluxes from the forest floor of contrasting north- and south-facing slopes in central Siberia. DOC fluxes increased throughout the growing season (June–September) on both slopesin 2002 and 2003. The most favorable combination of moisture content and temperature (deepest active soil layer) oc-curred in September, and we believe this was the primary driver of increased DOC concentrations and flux in autumn.Total DOC flux for June–September was 12.6–17.6 g C·m–2 on the south-facing slope and 4.6–8.9 g C·m–2 on the north-facing slope. DOC concentrations in forest floor leachates increased with increasing temperature on the north-facingslope, but were almost unaffected by temperature on the south-facing slope. Our results suggest that water input inmidseason from melting of ice or precipitation events is the primary factor limiting DOC production. Significant posi-tive correlations between amounts of precipitation and DOC flux were found on both slopes. Dilution of DOC concen-trations by high precipitation volumes was observed only for the forest floor leachates collected from the north-facingslope. Our results suggest that global warming will result in increased DOC production in forest floors of permafrostregions, and that precipitation patterns will play an important role in determining the magnitude of these changes inDOC flux as well as its interannual variability. However, the longer-term response of soils and DOC flux to a warmingclimate will be driven by changes in vegetation and microbial communities as well as by the direct results of tempera-ture and moisture conditions.

2140Résumé : Les flux de carbone organique dissout (COD) dans les bassins versants couverts de forêts sur un pergélisolont des chances d’être modifiés à la suite des changements dans le régime climatique qui entraînent une augmentationde l’humidité et de la température du sol. Nous avons examiné les effets des variations temporelles et spatiales dans latempérature et l’humidité du sol sur les flux de COD dans la couverture morte sur des pentes exposées au nord ou ausud dans le centre de la Sibérie. Les flux de COD ont augmenté tout au long de la saison de croissance (juin à sep-tembre) sur les deux versants en 2002 et 2003. La combinaison la plus favorable de contenu en eau et de température(dans la plus profonde couche de sol actif) est survenue en septembre et nous croyons que c’est la principale caused’augmentation de la concentration et du flux de COD à l’automne. Le flux total de COD de juin à septembre était de12,6–17,6 g C·m–2 sur la pente exposée au sud et de 4,6–8,9 g C·m–2 sur la pente exposée au nord. La concentrationde COD dans le lessivat de la couverture morte augmentait avec la température sur la pente exposée au nord mais ellen’était presque pas affectée par la température sur la pente exposée au sud. Nos résultats indiquent que l’apport d’eau àmi-saison provenant de la fonte de la glace ou des précipitations est le principal facteur qui limite la production deCOD. Des corrélations positives significatives entre la quantité de précipitation et le flux de COD ont été observées surles deux versants. La dilution de la concentration de COD par de forts volumes de précipitation a été observée seule-ment dans le lessivat de la couverture morte provenant de la pente exposée au nord. Nos résultats indiquent que le ré-chauffement global a entraîné une augmentation de la production de COD dans la couverture morte dans les régions où

Can. J. For. Res. 35: 2130–2140 (2005) doi: 10.1139/X05-150 © 2005 NRC Canada

2130

Received 30 November 2004. Accepted 30 June 2005. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on18 October 2005.

A.S. Prokushkin,2 S.G. Prokushkin, and A.P. Abaimov. V.N. Sukachev Institute of Forest SB RAS, Krasnoyarsk, 660036,Akademgorodok, Russia.T. Kajimoto. Kyushu Research Center, Forestry and Forest Products Research Institute, Kurokami 4-11-16, Kumamoto, 860-0862,Japan.W.H. McDowell. Department of Natural Resources, University of New Hampshire, Durham, NH 03824, USA.Y. Matsuura. Soil Resources Evaluation Laboratory, Department of Forest Environment, Forestry and Forest Products ResearchInstitute, Matsunosato 1, Kukizaki, Ibaraki 305-8687, Japan.

1This article is one of a selection of papers published in the Special Issue on Climate–Disturbance Interactions in Boreal ForestEcosystems.

2Corresponding author (e-mail: prokushkin@ksc.krasn.ru).

le pergélisol est présent et que le patron de précipitation jouera un rôle important dans la détermination de l’ampleurde ces changements dans les flux de COD de même que dans leurs variations interannuelles. Cependant, la réaction àplus long terme des sols et du flux de COD au réchauffement du climat sera contrôlée par les changements dans la vé-gétation et les communautés microbiennes autant qu’elle sera le résultat direct des conditions de température etd’humidité.

[Traduit par la Rédaction] Prokushkin et al.

Introduction

High latitude forests of central Siberia are dominated bylarch (Larix gmelinii (Rupr.) Rupr.) that is established onsoils characterized by the presence of continuous permafrost(Abaimov et al. 2000). The low temperatures of these soilsand their poor nutrient availability result in a relatively largeallocation of fixed carbon to root production (Van Cleve andYarie 1986; Kajimoto et al. 2003) and in considerable accumu-lations of organic matter in the forest floor between fire eventsbecause of slow rates of litter decomposition (Pozdnyakov 1963;Trumbore and Harden 1997). Organic matter in the forestfloor serves as a primary source of dissolved organic carbon(DOC) in forest ecosystems (Cronan and Aiken 1985;Michalzik and Matzner 1999), and the importance of dis-solved organic matter in belowground carbon cycling is nowwell established (Neff and Asner 2001; Park et al. 2002).Understanding the influence of permafrost on the factorscontrolling the production, mobilization, and, likely, storageof DOC in the frozen ground is important for understandingthe factors controlling carbon balance and DOC flux of taigawatersheds under global climate change (MacLean et al. 1999).

Laboratory and field studies have shown that the tempera-ture and moisture content of organic soil horizons are impor-tant factors in the control of DOC production (Christ andDavid 1996; Michalzik et al. 2001). Other important factorsinclude quality of the organic material in the forest floor(e.g., C/N and lignin) and total amount and frequency ofprecipitation, (Gödde et al. 1996; Tipping et al. 1999;Michalzik et al. 2001). These factors together influence soilmicrobial activity, rates of abiotic leaching, and thus the netrelease of DOC from the forest floor into soil solution(McDowell and Wood 1984; Kaiser et al. 2001).

In mountainous terrain, variations in topography can af-fect watershed-scale variability in tree cover, biogeochemicalprocesses, and soil physicochemical properties. At high lati-tudes, aspect can be equally important in regulating thesevariables. High-latitude sites with a northern aspect receivesignificantly lower inputs of solar radiation than those withsouthern aspects, resulting in cooler soils, slower thawingrates, and a shallower active layer. Consequently, plant com-position and soil development differ dramatically amongslopes of different aspects (Pozdnyakov 1963; Van Cleveand Yarie 1986; Yershov 1995). Warming of arctic climates,which results in increased river discharge and in deepeningof the active layer in permafrost regions, has already oc-curred over the last several decades (Peterson et al. 2002;Frauenfeld et al. 2004; Chen et al. 2003). Because the over-all effects of climate warming are predicted to increase soiltemperature and moisture with significant melting of perma-frost (Zhang et al. 1997; Osterkamp and Romanovsky 2002),comparative analysis of biogeochemical processes on slopes

of different aspects provides an excellent opportunity to studythe effect of a general climatic warming on dissolved or-ganic matter (DOM) fluxes in permafrost regions. In thisstudy, our objective was to measure DOC flux in forest floorleachates from opposite south- and north-facing slopes withwell-expressed microscale topography (mounds and troughs;Kajimoto et al. 2003). Specifically, we addressed (i) the rolethat temporal changes in hydroclimatic conditions play indriving DOC production during the summer growing season,and (ii) the potential impact of warming on DOC export inpermafrost regions.

Material and methods

Study siteThe watershed of Kulingdakan Stream (64°17′N, 100°11′E)

is located about 5 km northeast of Tura (Evenkia Autono-mous District) in central Siberia (Fig. 1). The watershed area(ca. 4100 ha), with elevation ranging from 132 to 630 ma.s.l., represents the central part of the Syverma Plateau. Theregion has a cold continental climate. The average tempera-ture of January (the coldest month) is –36 °C, and that ofJuly (the warmest month) is 16 °C. The average annual tem-perature is –9.5 °C, and average annual precipitation for theregion is 300–350 mm. About 30%–40% of annual precipi-tation falls as snow.

Soils of the watershed are characterized by coarse texture(high gravel content), shallow (20–40 cm) depths, and lowor medium clay contents (Yershov 1995) and have slight orneutral acidity. The valley bottom consists of deep alluvialsilt and gravels over bedrock. The overstory is dominated bylarch (L. gmelinii), which regenerated after a fire that oc-curred in 1902, with small amounts of birch (Betula pubes-cence L.) and spruce (Picea obovata L.) on the south-facingslopes. Forest understory typically consists of Dushekiafrutucosa (closely related to Alnus spp.), which is abundantalong stream edges and well-drained sites on the slopes.

The ground vegetation consists of mosses (Pleurozium schre-beri, Hylocomium splendens, and Aulocomnium turgidum)and patches of lichens (Cladina spp., Cetraria spp.), whichform an acidic forest floor (pH 3.8–5.0) that is 4–9 cm (3.6 ±0.8 cm (mean ± SD), n = 10) thick on south-facing slopesand 7–15 cm (9.4 ± 1.2 cm, n = 10) thick on north-facingslopes. In the valley bottom Sphagnum mosses form a peat-like litter layer with a thickness of 35–52 cm (41.5 ± 3.2 cm,n = 15).

Field and analytical methodsWe located two plots on north- (NFS) and south-facing

(SFS) slopes separated by the stream valley to compare DOCfluxes from the forest floor with those from the mineral soil

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Prokushkin et al. 2131

during the growing seasons of 2002 and 2003. Plots weresimilar in vegetative cover and other attributes (Table 1).The dominant moss covering about 50% of both plots wasPleurozium schreberi. Both experimental plots were 10 m ×10 m. The depth of the “active layer” (the depth to which per-mafrost melts) and microscale topography (trough or mound)were measured in a grid at 1-m intervals within these plots(n = 121 positions). The active-soil depth was measuredwith a steel pole (1 m length, 1 cm diameter) at weekly in-tervals in June and once in mid-September, when thawingdepths were maximal (Pozdnyakov 1963). Air and soil tem-perature were recorded continuously at 1-h intervals by water-resistant thermo recorders (TR-51A, T&D Co., Matsumoto,Japan). The soil water suction of the upper 5 cm mineral soilwas measured on 20 June, 25 July, 30 August, and 15 Sep-tember in mounds and troughs using tensiometers with porouscups (DIK-3151, Daiki Corp., Saitama, Japan) (n = 3). Pre-cipitation was collected by rain gauges (n = 6). Leachatesfrom the indigenous forest floor covered by Pleuroziumschreberi were collected by zero-tension lysimeters (Hongveet al. 2000) placed in both mounds and troughs within theseplots (n = 3 for each topographic position, depth = 18 cm).Samples were collected weekly in June, August, and Sep-tember during 2002; a single sample was collected in July. In2003 a similar sampling regime was followed except that nosamples were collected in September. Rain gauges were lo-cated near each lysimeter to estimate the net input of precipita-tion (throughfall) to the forest floor. Additional rain gaugeswere placed near the plot in a flat clear-cut area for estima-tion of total rainfall at the site.

Leachate samples were collected in acid-washed 500–1000 mL polyethylene bottles. Samples were transported tothe Experimental Station of the V.N. Sukachev Institute ofForestry and filtered (0.45-µm glass fiber filter MFOS-2,Vladipor, Moscow, Russia). Samples were stored frozen (–3 °C)prior to analysis. After their storage, several samples in whichsome DOC precipitated required shaking prior to analyses,and therefore DOC concentrations may be underestimated.

Samples of the moss layer and forest floor were collectedin June 2002 from 10 locations near the study plots to esti-mate spatial heterogeneity. Live roots >0.5 mm and large de-bris (>10 mm) were removed from the forest floor on theday of sampling and then air-dried. Water-extractable or-ganic carbon (WEOC) was measured by extracting the entirelitter and moss layer with distilled water (at ratio of 1:10m/v) during a 24-h incubation at 20 °C; samples were thenfiltered (0.45 µm). The forest floor was separated into spe-cific horizons (dead moss, Oi, combined Oe and Oa) beforedrying and analysis for C and N content.

The content of DOC in aqueous samples (10–20 mL, threereplicates) after drying at 40 °C was determined by dichromatedigestion (4.9 g·dm–3 K2Cr2O7 in H2SO4, 1:1 m/m) withcolorimetric detection of the reduced Cr3+ (Kaurichev et al.1977; Khomutova et al. 2000). The absorbance of the digestedsolution was measured with a KFK-3 colorimeter (ZOMZ,Zagorsk, Russia) at a wavelength of 590 nm (Prokushkin etal. 2001) and calibrated against a standard sucrose solution.The average standard deviation of samples with concentra-tions ranging from 30 to 120 mg C·L–1 was 4.8%; for stan-dards (0.1–1.0 mg C, n = 10, three replicates), the averagestandard deviation was 0.3% (R2 = 0.9984, p < 0.001). Su-crose O/C ratio (0.92) among sugars was closest to DOCO/C ratio (0.98) shown in an earlier study (Kracht and Gleixner2000).

Electrical conductivity and pH were measured by pHmeter – conductivity meter Anion-7152 (Infraspac-analit,Novosibirsk, Russia). Nitrate and ammonium were measuredby an ion chromatograph (Tsvet-100, Tsvet Ltd., Dzerzhinsk,Russia) with a detection limit of 0.1 mg·L–1 for both ions.Total C and N in solid matter were determined by elementalanalyzer Vario EL (Elementar Analysensysteme GmbH,Hanau, Germany).

Statistical calculationsStatistical analysis was performed using Sigma Plot ver-

sion 4.0 for MS Windows. The significance of differences

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2132 Can. J. For. Res. Vol. 35, 2005

N

100o00'E 100o30'E

64o20'N

64o40'N

5 km

Fig. 1. Location of study area. The inset shows the topographic position and boundary of Kulingdakan Watershed. Black dots are ex-perimental plots on south- and north-facing slopes.

between DOC concentrations and DOC fluxes of north- andsouth-facing slopes was estimated using Student’s t test.

Results

Soil conditionsThe depth of the active layer increased faster and reached

a greater final value on the SFS plot than on the NFS plot(Fig. 2). The maximum depth of thawing occurred in Sep-tember on both plots and ranged from 100 cm on the SFS toabout 60 cm on the NFS. This difference in depth of thawingwas associated with a thicker forest floor on the NFS andcolder temperatures at the mineral soil surface throughoutthe year (Fig. 3). The annual temperature of the mineral soilsurface was –3.3 °C (SD = 8.6 °C) on the SFS and –6.0 °C(SD = 8.9 °C) on the NFS. When average temperatures werebelow zero, mounds and troughs on both NFS and SFS hadvery similar temperatures (Fig. 3). During the frost-free pe-riod, however, there were striking differences between thetemperature regimes of mounds and troughs. The averagesummer temperature on the SFS was 11.3 °C on mounds and8.2 °C in troughs. In contrast, on the NFS plot the tempera-ture was 8.3 °C on mounds and 4.8 °C in troughs. The ef-

© 2005 NRC Canada

Prokushkin et al. 2133

2

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Date (dd–mm)

18–6 2–7 16–7 30–7 13–8 27–8

4–6 18–6 2–7 16–7 30–7 13–8 27–8

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fects of microtopography and aspect appear to balance eachother on an annual basis, as the temperature regimes and an-nual means of the NFS mounds and the SFS troughs werevery similar. However, the length of the frost-free period intopsoil in 2002–2003 was 159 days on the SFS and only 129days on the NFS.

The interception of rain water by the tree canopy rangedfrom 7% on the NFS, with its sparse tree cover, to about12% on the SFS (i.e., throughfall was 93%–88% of incomingprecipitation on the two plots). Recovery of this throughfallin the zero-tension lysimeters varied by plot aspect as wellas by microtopography. Most of the precipitation enteringthe forest floor in both troughs and mounds was recoveredas soil solution on the NFS. On the SFS, most of the inci-dent throughfall was also recovered as soil solution on themounds, but soil solution was more than double the incom-ing throughfall in the troughs (Fig. 4). This observation couldbe explained by the channeling of runoff water into troughsfrom areas adjacent to the troughs.

Although the amounts of precipitation on the two plotswere almost identical, soil moisture content as measured bysoil suction was greater on NFS than on SFS, and greater introughs than on mounds (Table 2). Litter (data not presentedhere) also showed similar patterns in water content duringthe whole growing season. On the NFS, water flow in troughsoccurred repeatedly after rainfall events.

Organic matterThe accumulation of organic C in the moss layer on the

NFS was twice as much as that on the SFS (232 ± 46 and114 ± 16 g C·m–2 (mean ± SD), respectively), and litter or-ganic C stock was 1.3 times larger on the NFS (Table 1). Al-though there was no significant difference in the C contentin mosses, organic C in litter ranged from 33%–42% on theSFS to 46%–49% on the NFS. In contrast with the NFS,where the C content of forest floor organic matter remainedconstant, the SFS showed a decrease in C concentrationfrom live moss tissues to Oe–Oa horizon (Fig. 5). Nitrogencontent ranged from 0.3%–0.5% in the moss layer to 1.0%–1.2% in the Oe–Oa horizon (Fig. 5). Higher N concentra-tions were generally found in the forest floor of the SFSthan in the forest floor of the NFS, resulting in consistentlyhigher C/N ratios in the entire forest floor (troughs andmounds) of the NFS compared with the SFS (Table 1).

The overall pool of water-extractable organic carbon(WEOC) in mosses and forest floor, a potential source ofDOC, was similar on both slopes. Total WEOC averaged11.6 g·m–2 on the SFS plot and 12.3 g·m–2 on the NFS plot,though the distribution of WEOC across horizons varied byslope. The WEOC content of the O horizon on the SFS wasmore than twice that in live mosses (8.2 ± 1.8 and 3.5 ±0.3 g·m–2 (mean ± SD), respectively), whereas on the NFS,the quantities of WEOC in live mosses and the O horizon

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2134 Can. J. For. Res. Vol. 35, 2005

Fig. 3. Dynamics of temperature of mineral soil surface on north- (A) and south-facing (B) slopes in 2002–2003.

were almost equal (6.0 ± 1.2 and 6.3 ± 1.8 g·m–2, respec-tively).

Concentrations of DOC in the forest floor seepage watersof both slopes demonstrated some interannual and interseasonal

variation (Table 3). Among individual collection dates, con-centrations on the NFS ranged from 16 to 52 mg C·L–1 in2002, and from 23 to 71 mg C·L–1 in 2003. On the SFSDOC concentrations were significantly higher ranging from37 to 150 mg C·L–1 in 2002 and from 29 to 129 mg C·L–1 in2003. In 2002 the highest concentrations of DOC in perco-lated waters were found at the end of the growing season(August–September), when the highest average daily precip-itation was measured (Table 4). In contrast, earlier in thegrowing season we found relatively low DOC concentra-tions in forest floor solutions of both slopes. In 2003 fewersamples were collected, making it difficult to describe sea-sonal patterns.

Concentrations of DOC in soil solution percolating fromthe forest floor varied with both temperature and throughfallamounts on the NFS, but were unrelated to either parameteron the SFS (Figs. 6 and 7). On both plots, DOC concentrationstended to decline with increasing amounts of net precipita-tion (throughfall), but the relationship was only statisticallysignificant for the NFS plot (Fig. 7A). Concentrations of

© 2005 NRC Canada

Prokushkin et al. 2135

2

2

2

2

Fig. 4. Water percolated through forest floor in troughs (filledcircles) and mounds (open circles) on north- (A) and south-facing (B) slopes versus corresponding throughfall values.

Soil water suction (kPa)

20 June 25 July 30 August 15 September

South-facing slopeMound –3.4±1.6 –5.5±1.4 –3.6±0.8 –1.0±0.2Trough –0.8±0.5 –2.1±0.7 –1.8±0.7 –0.5±0.3

North-facing slopeMound –0.5±0.2 –2.2±0.5 –1.1±0.4 –0.5±0.2Trough –0.0±0.2 –0.4±0.3 –0.5±0.3 –0.2±0.2

Note: Values are means ± SD.

Table 2. Soil water suction measured in upper organogenic hori-zon of the mineral soil (0–5 cm) on the south- and north-facingslopes at various dates in 2003.

1.5

1.2

0.9

0.6

0.3

1.5

1.2

0.9

0.6

0.3

Oe-Oa

Oe-Oa

Fig. 5. Carbon and N contents in mosses and forest floor layerson the north- (A) and south-facing (B) slopes.

DOC increased with mineral soil temperature on the NFSplot, but showed no response to temperature on the SFS plot(Fig. 6B).

DOC export from the organic horizon during individualpercolation events ranged from 0.02 g C·m–2 to about 5 gC·m–2 on the SFS and from 0.015 to 2.5 g C·m–2 on the NSFand was positively correlated with the volume of percolatingsoil solution (Fig. 8). In general, DOC flux from the organichorizon to the underlying mineral soil on the SFS was ap-proximately twofold higher than on the NFS (Table 4). Theimpacts of microtopography on DOC export varied with slope.On the SFS, export of DOC from troughs and mounds variedconsiderably, with higher export from the much wetter troughs(2.93 ± 0.73 g C·m–2) than from the drier mounds (1.23 ±0.58 g C·m–2; Fig. 9). On the NFS, in contrast, DOC exportwas almost two times higher from mounds than from troughs(1.27 ± 0.48 vs. 0.70 ± 0.08 g C·m–2). Total DOC export foreach growing season was 17.6 g C·m–2 on SFS and 8.9 gC·m–2 on the NFS in 2002 (Table 4). In 2003 the overall fluxwas lower because sampling was not conducted during Sep-tember, but flux from the SFS plot was over twice that of theNFS plot (12.6 vs. 4.6 g C·m–2; Table 4). Total DOC exportfrom the organic horizon during the growing season pre-sented about 150% of the total WEOC stock in moss and lit-ter on the SFS plot, and only 70% of that on the NFS plot.These DOC fluxes from the organic horizon represent about0.5% of the total standing stock of C on the NFS and 1.5%of standing stock on the SFS.

The pH of soil waters ranged from 5.4 to 6.4 on the SFSand from 5.8 to 6.8 on the NFS (Table 3). Conductivities

tended to be lower on the SFS than on the NFS and declinedthrough the growing season on both plots (Table 3). Allsamples of the NFS leachates contained inorganic N (NO3

–

and NH4+) below the detection limits. In the leachates of the

SFS, concentrations of NO3– and NH4

+ ranged from 0.22 to0.63 mg N·L–1 and from 0.07 to 0.51 mg N·L–1, respectively.

Discussion

Accumulation of organic C in mosses and forest floor oftaiga soils appears to follow predictable patterns related toaspect and microtopography as a result of the interacting ef-fects of temperature and moisture availability. Our data showthat the warmest, best-drained soils (mounds on the south-facing slope) have thinner forest floors with lower C contentper unit area. The coldest, most poorly drained soils (troughson the north-facing slope) have the thickest forest floor andthe highest C content. These results are in agreement withearlier results (Kawahigashi et al. 2004), which showed thatincreasing thickness of the Oea horizons reflects the differ-ence in moisture conditions and thus probably microbial ac-tivity among the organic horizons. Because temperature andhydrologic conditions interact with both the production anddecomposition of organic matter, it is hard to determinewhich factors are dominant in producing the spatial mosaicof C accumulation that we have documented. Water stress(i.e., dry conditions), for example, can both decrease mossproductivity and accelerate organic matter decomposition,driving the site to lower accumulation of organic C (Hobbie1996; A. Knorre, personal communication). An increase offorest floor thickness under cold conditions provides a poten-tial positive feedback loop, as a thicker forest floor furtherinsulates the soil during the summer (Fig. 3) and decreasesthe depth of the active layer (Fig. 2), as suggested previouslyby Sofronov et al. (2000). The relatively low N and high Ccontents of the forest floor on the north-facing slope providefurther evidence of the inhibition of decomposition on thenorth-facing slope. The quality of organic material in theforest floor is also tightly coupled to the relatively fine-scalevariation in hydroclimatic conditions that we observed inmounds and troughs (Table 1). These differences in micro-topography create mosaics of environmental conditions withinthe slope.

Seasonal and topographic patterns in DOC mobilizationsuggest that increases in biological activity due to increasingtemperatures drive some of the spatial and temporal varia-tion in DOC production that we have observed. Temperatureregime drives the depth and timing of thawing, as well asmicrobial activity. As thawing progresses through the sum-mer and autumn, the active layer deepens and the tempera-ture of the forest floor increases. Warmer conditions result inmodest increases in DOC concentrations (Fig. 6) and sub-stantial increases in concentrations of inorganic N on thesouth-facing slope, although not on the north-facing slope.These results are consistent with past work on temperate for-est soils, which shows that DOC production and CO2 evolu-tion increase with temperature (Christ and David 1996;Yanagihara et al. 2000; Neff and Hooper 2002). The stron-gest response to temperature was found on the heat-deficientnorth-facing slope. Christ and David (1996) found that DOCproduction has a higher Q10 in the range of 1–5 °C com-

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2136 Can. J. For. Res. Vol. 35, 2005

SeasonDOC(mg C·L–1) pHH2O

Conductivity(µS·cm–1)

South-facing slopeJune

2002 56.0±6.5 6.4±0.3 73.3±23.32003 45.1* nd nd

July–August2002 48.7±16.2 6.0±0.2 65.7±10.52003 56.8±6.1 nd nd

September nd nd2002 80.7±27.7 5.4±0.1 53.9±11.22003† nd nd nd

North-facing slopeJune

2002 32.5* 6.8 60.52003 29.3* nd nd

July–August2002 28.7±7.3 6.0±0.2 29.6±4.12003 49.2±11.8 nd nd

September2002 36.5±11.5 5.8±0.1 34.5±7.12003† nd nd nd

Note: Values are means ± SD. DOC, dissolved organic carbon; nd, notdetermined.

*One percolation event occurred.†No sampling was performed in September 2003.

Table 3. Seasonal patterns of organic solutes leached from forestfloors of south- and north-facing slopes during growing seasonof 2002 and 2003.

© 2005 NRC Canada

Prokushkin et al. 2137

Precipitation (mm) DOC flux into soil (g C·m–2)

North-facing South-facing North-facing South-facing

Observation period Daily Total Daily Total Daily Total Daily Total

June2002 2.2 28.0* 2.1 26.8 0.06 0.80 0.15 1.952003 0.9 27.7 1.0 31.3 0.02 0.71 0.05 1.63

July–August2002 1.4 43.4 1.3 40.3 0.04 2.34 0.09 5.562003 1.9 117.9 1.8 111.2 0.06 3.89 0.18 10.92

September2002 3.3 102.3 2.9 89.9 0.25 5.79 0.44 10.042003† — — — — — — — —

Total2002 174 157 8.9 17.62003 146 143 4.6 12.6

*Precipitation was measured for 14 days (June).†No sampling was performed in September 2003.

Table 4. Average daily and total monthly precipitation input and dissolved organic carbon (DOC)fluxes in 2002 and 2003 for the north- and south-facing plots.

y = 2.7x + 28.4

R2 = 0.63

o

o

DO

C(m

gC

·L)

–1

DO

C(m

gC

·L)

–1

Fig. 6. Dissolved organic carbon (DOC) concentrations versustemperature of mineral soil surface measured on north- (A) andsouth-facing (B) slopes.

y = -0.52x + 53.8

R2 = 0.54

DO

C(m

gC

·L)

–1

DO

C(m

gC

·L)

–1

Fig. 7. Dissolved organic carbon (DOC) concentrations versusprecipitation percolated through forest floor on the north-(A) and south-facing (B) slopes.

pared with 10–20 °C. The stronger response of DOC con-centrations to temperature on our north-facing slope may berelated to this difference in Q10 and to the decline of micro-bial communities within the permafrost zone of central Siberiaat temperatures above 5 °C (Šantru�ková et al. 2003).

Our data show that interactions among temperature, hy-drology, and microbiology will drive any response in soilDOC flux to changes in Siberian climate. In a warming cli-mate, we suspect that the initial response in central Siberiawill be increases in the loss of DOC from the forest floor.These increases may be substantial, as the projected changesin temperature are as much as 3–5 °C in the next 50 years(Peterson et al. 2002), and the depth of the active layer inRussian permafrost has already increased by 20 cm from1956 to 1990 (Nikolaev and Fedorov 2004). The longer-termresponses of soils and DOC flux to a warming climate aremuch harder to predict, however. Changes in vegetation willundoubtedly accompany changes in climate, and this will af-fect the quality and quantity of organic matter entering thesoil and thus the production of DOC (Neff and Hooper 2002).Models of vegetation response to warming in permafrost re-gions of Siberia suggest that the initial stages of warming(1–2 °C) may accelerate moss production and increase thethickness and insulating ability of the forest floor, with cool-ing of the soil by 0.5–1 °C (Anisimov and Belolutskaya2004). Continued warming that results in a shift toward morevascular vegetation in central Siberia owing to higher sum-mer air temperatures could lead to an increase of approxi-mately 30% in the active layer depth.

Unlike the situation in temperate forests (Hongve et al.2000; Kaiser et al. 2001) the input of fresh leaf litter to soilsis unlikely to have any effects on autumn DOC fluxes incentral Siberia. In the larch forests of the study area, despitetheir deciduous nature, input of fresh litter cannot play a cru-cial role, since needle fall is continuous from September toNovember and often occurs when the ground is already frozen.The most favorable combination of moisture content andtemperature (deepest active soil layer) occurs in September,

and we believe this is the primary driver of increased DOCconcentrations and flux in autumn.

Within slopes, the mosaic of mounds and troughs withspecific microclimatic and hydrologic conditions affects DOCexport significantly. Flux of DOC from mounds on the south-facing slope did not differ from flux of DOC from troughsand mounds on the north-facing slope, but troughs on thesouth-facing slope showed dramatic increases in DOC ex-port. Although these findings can be attributed, in part, tothe lower production of DOC (lower DOC concentration) inthe colder microsites, the increase in runoff from troughswith southern exposure (almost triple that of the other micro-sites) drives the export response. The source of this addi-tional runoff appears to be melting of permafrost in thewarmer south-facing slope and accumulation of this addi-tional water in low points (troughs) in the landscape.

Production of DOC in Siberian soils does not appear toincrease as a function of soil C/N ratio, unlike observationsfrom temperate forests soils (Gödde et al. 1996; Aitkenhead

© 2005 NRC Canada

2138 Can. J. For. Res. Vol. 35, 2005

2

2

DO

C(g

C·m

)–

2

Fig. 8. The export of dissolved organic carbon (DOC) to mineralsoil per one percolation event depending on the amount of waterpercolated through forest floor on north- (filled circles) andsouth-facing (open circles) slopes.

DO

C(g

C·m

)–

2D

OC

(gC

·m)

–2

Fig. 9. The export of dissolved organic carbon (DOC) to mineralsoil per one percolation event in troughs (A) and mounds (B) de-pending on amount of water percolated through forest floor onnorth- (filled circles) and south-facing (open circles) slopes.

and McDowell 2000; Michalzik et al. 2001). Our resultsshow that both concentration and flux of DOC are higherfrom the south-facing slope, which has much lower soilC/N, than from the north-facing slope. Whether this higherDOC production from warmer, south-facing soils translatesinto greater delivery of DOC to streams is an unansweredquestion. Although the production and release of DOC fromthe forest floor may be greater in warmer soils, the deeperactive layer in these warmer soils also increases contact withmineral soils and thus the likelihood of DOC adsorption(Jardine et al. 1989; Kaiser et al. 2001). In a study of controlson stream DOC flux in central Alaska, MacLean et al. (1999)found that DOC flux was greater from watersheds with morepermafrost coverage and attributed this finding to the rela-tively shallow flow paths through the high-permafrost basinthat resulted in little adsorption of DOC on mineral soils. Asimilar result was also found recently for central Siberia incomparative analyses of small streams along a gradient fromdiscontinuous to continuous permafrost (Kawahigashi et al.2004).

Conclusions

Concentrations and fluxes of DOC in forest floors of larchecosystems underlain by continuous permafrost have signifi-cant spatial and temporal variations. Both concentrations andfluxes of DOC increase during the growing season becauseof the increase in the depth of the active layer and increasedDOC production at higher temperatures. North-facing slopeshad much lower DOC production than south-facing slopes,and microtopography further complicated the response toslope and precipitation events. Greater DOC flux from slopeswith southern aspect suggests that short-term DOC produc-tion will increase in forest floors under conditions of globalwarming. Changing climate would also trigger complexchanges in both the biological nature and chemical proper-ties of the organic matter that serves as DOC source mate-rial, and thus long-term changes in DOC production aremuch more difficult to evaluate. In particular, the depth ofthe active soil layer and sorption capacity of thawing soilduring permafrost degradation are believed to be the key fac-tors controlling losses of DOC from terrestrial ecosystems(MacLean et al. 1999).

Acknowledgments

This research was funded by the Russian Fund of BasicResearch (grant 03-04-48037) and financial support providedby the Siberian Branch of the Russian Academy of Sciencesfor young scientists in 2003–2004. We thank to all membersof the joint Japanese–Russian team working in Tura experi-mental station. The US National Science Foundation (DEB-0108385) provided support for manuscript preparation.

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