Climatic factors influencing fluxes of dissolved organic carbon from the ...

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Climatic factors influencing fluxes of dissolved organic carbon from the forest floor in a continuous-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 vary with changes in climatic regime that increase soil moisture and temperature. We examined the effects of temporal and spatial variations in soil temperature and moisture on DOC fluxes from the forest floor of contrasting north- and southfacing slopes in central Siberia. DOC fluxes increased throughout the growing season (June–September) on both slopes in 2002 and 2003. The most favorable combination of moisture content and temperature (deepest active soil layer) occurred 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 northfacing slope. DOC concentrations in forest floor leachates increased with increasing temperature on the north-facing slope, but were almost unaffected by temperature on the south-facing slope. Our results suggest that water input in midseason from melting of ice or precipitation events is the primary factor limiting DOC production. Significant positive correlations between amounts of precipitation and DOC flux were found on both slopes. Dilution of DOC concentrations by high precipitation volumes was observed only for the forest floor leachates collected from the north-facing slope. Our results suggest that global warming will result in increased DOC production in forest floors of permafrost regions, and that precipitation patterns will play an important role in determining the magnitude of these changes in DOC flux as well as its interannual variability. However, the longer-term response of soils and DOC flux to a warming climate will be driven by changes in vegetation and microbial communities as well as by the direct results of temperature and moisture conditions. Résumé : Les flux de carbone organique 2140 dissout (COD) dans les bassins versants couverts de forêts sur un pergélisol ont des chances d’être modifiés à la suite des changements dans le régime climatique qui entraînent une augmentation de l’humidité et de la température du sol. Nous avons examiné les effets des variations temporelles et spatiales dans la température et l’humidité du sol sur les flux de COD dans la couverture morte sur des pentes exposées au nord ou au sud dans le centre de la Sibérie. Les flux de COD ont augmenté tout au long de la saison de croissance (juin à septembre) 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 cause d’augmentation de la concentration et du flux de COD à l’automne. Le flux total de COD de juin à septembre était de 12,6–17,6 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 concentration de COD dans le lessivat de la couverture morte augmentait avec la température sur la pente exposée au nord mais elle n’était presque pas affectée par la température sur la pente exposée au sud. Nos résultats indiquent que l’apport d’eau à mi-saison provenant de la fonte de la glace ou des précipitations est le principal facteur qui limite la production de COD. Des corrélations positives significatives entre la quantité de précipitation et le flux de COD ont été observées sur les deux versants. La dilution de la concentration de COD par de forts volumes de précipitation a été observée seulement dans le lessivat de la couverture morte provenant de la pente exposée au nord. Nos résultats indiquent que le réchauffement global a entraîné une augmentation de la production de COD dans la couverture morte dans les régions où Received 30 November 2004. Accepted 30 June 2005. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on 18 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 Research Institute, Matsunosato 1, Kukizaki, Ibaraki 305-8687, Japan. 1

This article is one of a selection of papers published in the Special Issue on Climate–Disturbance Interactions in Boreal Forest Ecosystems. 2 Corresponding author (e-mail: [email protected]). Can. J. For. Res. 35: 2130–2140 (2005)

doi: 10.1139/X05-150

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le pergélisol est présent et que le patron de précipitation jouera un rôle important dans la détermination de l’ampleur de ces changements dans les flux de COD de même que dans leurs variations interannuelles. Cependant, la réaction à plus long terme des sols et du flux de COD au réchauffement du climat sera contrôlée par les changements dans la végétation et les communautés microbiennes autant qu’elle sera le résultat direct des conditions de température et d’humidité. [Traduit par la Rédaction]

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Introduction High latitude forests of central Siberia are dominated by larch (Larix gmelinii (Rupr.) Rupr.) that is established on soils characterized by the presence of continuous permafrost (Abaimov et al. 2000). The low temperatures of these soils and their poor nutrient availability result in a relatively large allocation of fixed carbon to root production (Van Cleve and Yarie 1986; Kajimoto et al. 2003) and in considerable accumulations of organic matter in the forest floor between fire events because of slow rates of litter decomposition (Pozdnyakov 1963; Trumbore and Harden 1997). Organic matter in the forest floor 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 dissolved organic matter in belowground carbon cycling is now well established (Neff and Asner 2001; Park et al. 2002). Understanding the influence of permafrost on the factors controlling the production, mobilization, and, likely, storage of DOC in the frozen ground is important for understanding the factors controlling carbon balance and DOC flux of taiga watersheds under global climate change (MacLean et al. 1999). Laboratory and field studies have shown that the temperature and moisture content of organic soil horizons are important factors in the control of DOC production (Christ and David 1996; Michalzik et al. 2001). Other important factors include quality of the organic material in the forest floor (e.g., C/N and lignin) and total amount and frequency of precipitation, (Gödde et al. 1996; Tipping et al. 1999; Michalzik et al. 2001). These factors together influence soil microbial activity, rates of abiotic leaching, and thus the net release of DOC from the forest floor into soil solution (McDowell and Wood 1984; Kaiser et al. 2001). In mountainous terrain, variations in topography can affect watershed-scale variability in tree cover, biogeochemical processes, and soil physicochemical properties. At high latitudes, aspect can be equally important in regulating these variables. High-latitude sites with a northern aspect receive significantly lower inputs of solar radiation than those with southern aspects, resulting in cooler soils, slower thawing rates, and a shallower active layer. Consequently, plant composition and soil development differ dramatically among slopes of different aspects (Pozdnyakov 1963; Van Cleve and Yarie 1986; Yershov 1995). Warming of arctic climates, which results in increased river discharge and in deepening of the active layer in permafrost regions, has already occurred over the last several decades (Peterson et al. 2002; Frauenfeld et al. 2004; Chen et al. 2003). Because the overall effects of climate warming are predicted to increase soil temperature and moisture with significant melting of permafrost (Zhang et al. 1997; Osterkamp and Romanovsky 2002), comparative analysis of biogeochemical processes on slopes

of different aspects provides an excellent opportunity to study the effect of a general climatic warming on dissolved organic matter (DOM) fluxes in permafrost regions. In this study, our objective was to measure DOC flux in forest floor leachates from opposite south- and north-facing slopes with well-expressed microscale topography (mounds and troughs; Kajimoto et al. 2003). Specifically, we addressed (i) the role that temporal changes in hydroclimatic conditions play in driving DOC production during the summer growing season, and (ii) the potential impact of warming on DOC export in permafrost regions.

Material and methods Study site The watershed of Kulingdakan Stream (64°17′N, 100°11′E) is located about 5 km northeast of Tura (Evenkia Autonomous District) in central Siberia (Fig. 1). The watershed area (ca. 4100 ha), with elevation ranging from 132 to 630 m a.s.l., represents the central part of the Syverma Plateau. The region has a cold continental climate. The average temperature of January (the coldest month) is –36 °C, and that of July (the warmest month) is 16 °C. The average annual temperature is –9.5 °C, and average annual precipitation for the region is 300–350 mm. About 30%–40% of annual precipitation falls as snow. Soils of the watershed are characterized by coarse texture (high gravel content), shallow (20–40 cm) depths, and low or medium clay contents (Yershov 1995) and have slight or neutral acidity. The valley bottom consists of deep alluvial silt and gravels over bedrock. The overstory is dominated by larch (L. gmelinii), which regenerated after a fire that occurred in 1902, with small amounts of birch (Betula pubescence L.) and spruce (Picea obovata L.) on the south-facing slopes. Forest understory typically consists of Dushekia frutucosa (closely related to Alnus spp.), which is abundant along stream edges and well-drained sites on the slopes. The ground vegetation consists of mosses (Pleurozium schreberi, Hylocomium splendens, and Aulocomnium turgidum) and patches of lichens (Cladina spp., Cetraria spp.), which form 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 slopes and 7–15 cm (9.4 ± 1.2 cm, n = 10) thick on north-facing slopes. In the valley bottom Sphagnum mosses form a peatlike litter layer with a thickness of 35–52 cm (41.5 ± 3.2 cm, n = 15). Field and analytical methods We located two plots on north- (NFS) and south-facing (SFS) slopes separated by the stream valley to compare DOC fluxes from the forest floor with those from the mineral soil © 2005 NRC Canada

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

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

N

64o40'N

5 km

64o20'N

100o00'E

during the growing seasons of 2002 and 2003. Plots were similar in vegetative cover and other attributes (Table 1). The dominant moss covering about 50% of both plots was Pleurozium schreberi. Both experimental plots were 10 m × 10 m. The depth of the “active layer” (the depth to which permafrost 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 measured with a steel pole (1 m length, 1 cm diameter) at weekly intervals in June and once in mid-September, when thawing depths were maximal (Pozdnyakov 1963). Air and soil temperature were recorded continuously at 1-h intervals by waterresistant thermo recorders (TR-51A, T&D Co., Matsumoto, Japan). The soil water suction of the upper 5 cm mineral soil was measured on 20 June, 25 July, 30 August, and 15 September in mounds and troughs using tensiometers with porous cups (DIK-3151, Daiki Corp., Saitama, Japan) (n = 3). Precipitation was collected by rain gauges (n = 6). Leachates from the indigenous forest floor covered by Pleurozium schreberi were collected by zero-tension lysimeters (Hongve et al. 2000) placed in both mounds and troughs within these plots (n = 3 for each topographic position, depth = 18 cm). Samples were collected weekly in June, August, and September during 2002; a single sample was collected in July. In 2003 a similar sampling regime was followed except that no samples were collected in September. Rain gauges were located near each lysimeter to estimate the net input of precipitation (throughfall) to the forest floor. Additional rain gauges were placed near the plot in a flat clear-cut area for estimation of total rainfall at the site. Leachate samples were collected in acid-washed 500– 1000 mL polyethylene bottles. Samples were transported to the Experimental Station of the V.N. Sukachev Institute of Forestry 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 which some DOC precipitated required shaking prior to analyses, and therefore DOC concentrations may be underestimated.

100o30'E

Samples of the moss layer and forest floor were collected in June 2002 from 10 locations near the study plots to estimate spatial heterogeneity. Live roots >0.5 mm and large debris (>10 mm) were removed from the forest floor on the day of sampling and then air-dried. Water-extractable organic carbon (WEOC) was measured by extracting the entire litter and moss layer with distilled water (at ratio of 1:10 m/v) during a 24-h incubation at 20 °C; samples were then filtered (0.45 µm). The forest floor was separated into specific horizons (dead moss, Oi, combined Oe and Oa) before drying and analysis for C and N content. The content of DOC in aqueous samples (10–20 mL, three replicates) after drying at 40 °C was determined by dichromate digestion (4.9 g·dm–3 K2Cr2O7 in H2SO4, 1:1 m/m) with colorimetric detection of the reduced Cr3+ (Kaurichev et al. 1977; Khomutova et al. 2000). The absorbance of the digested solution was measured with a KFK-3 colorimeter (ZOMZ, Zagorsk, Russia) at a wavelength of 590 nm (Prokushkin et al. 2001) and calibrated against a standard sucrose solution. The average standard deviation of samples with concentrations ranging from 30 to 120 mg C·L–1 was 4.8%; for standards (0.1–1.0 mg C, n = 10, three replicates), the average standard deviation was 0.3% (R2 = 0.9984, p < 0.001). Sucrose O/C ratio (0.92) among sugars was closest to DOC O/C ratio (0.98) shown in an earlier study (Kracht and Gleixner 2000). Electrical conductivity and pH were measured by pH meter – conductivity meter Anion-7152 (Infraspac-analit, Novosibirsk, Russia). Nitrate and ammonium were measured by 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 elemental analyzer Vario EL (Elementar Analysensysteme GmbH, Hanau, Germany). Statistical calculations Statistical analysis was performed using Sigma Plot version 4.0 for MS Windows. The significance of differences © 2005 NRC Canada

Note: Values are means ± SD. Values for the forest floor and active layer depth are given for mounds (first value) and troughs (second value). The canopy in both plots is larch. Active layer depth (depth of soil melting) is that for 14 September 2002.

31, 34 51, 61 34.3±2.3, 37.0±2.0 26.0±1.5, 23.7±1.5 1040±44, 1256±69 1325±78, 1405±90 3.6±0.8, 5.9±1.1 9.4±1.2, 10.8±1.6 0.2 0.1 215 198 South North

12 13

83–100 85–99

10.9±2.1 8.0±1.6

8.3±1.4 6.6±2.2

C/N N (g·m–2) C (g·m–2) Aspect

Forest floor

Thickness (cm) Crown projection DBH (cm) Height (m)

Stand canopy

Age (years) Slope angle (°) Elevation (m a.s.l.)

Table 1. Characteristics of south- and north-facing slopes used in this study.

112±7, 86±19 64±11, 45±22

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Active layer depth (cm)

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Fig. 2. Dynamics of active layer depth measured on north(A) and south-facing (B) slopes during the growing season of 2002.

Date (dd–mm) 4–6 18–6 2–7 16–7 30–7 13–8 27–8

2

Date (dd–mm) 4–6 18–6 2–7 16–7 30–7 13–8 27–8

2

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

Results Soil conditions The 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 September on both plots and ranged from 100 cm on the SFS to about 60 cm on the NFS. This difference in depth of thawing was associated with a thicker forest floor on the NFS and colder temperatures at the mineral soil surface throughout the year (Fig. 3). The annual temperature of the mineral soil surface 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 were below zero, mounds and troughs on both NFS and SFS had very similar temperatures (Fig. 3). During the frost-free period, however, there were striking differences between the temperature regimes of mounds and troughs. The average summer temperature on the SFS was 11.3 °C on mounds and 8.2 °C in troughs. In contrast, on the NFS plot the temperature was 8.3 °C on mounds and 4.8 °C in troughs. The ef© 2005 NRC Canada

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

fects of microtopography and aspect appear to balance each other on an annual basis, as the temperature regimes and annual means of the NFS mounds and the SFS troughs were very similar. However, the length of the frost-free period in topsoil in 2002–2003 was 159 days on the SFS and only 129 days on the NFS. The interception of rain water by the tree canopy ranged from 7% on the NFS, with its sparse tree cover, to about 12% on the SFS (i.e., throughfall was 93%–88% of incoming precipitation on the two plots). Recovery of this throughfall in the zero-tension lysimeters varied by plot aspect as well as by microtopography. Most of the precipitation entering the forest floor in both troughs and mounds was recovered as soil solution on the NFS. On the SFS, most of the incident throughfall was also recovered as soil solution on the mounds, but soil solution was more than double the incoming throughfall in the troughs (Fig. 4). This observation could be explained by the channeling of runoff water into troughs from areas adjacent to the troughs. Although the amounts of precipitation on the two plots were almost identical, soil moisture content as measured by soil suction was greater on NFS than on SFS, and greater in troughs than on mounds (Table 2). Litter (data not presented here) also showed similar patterns in water content during the whole growing season. On the NFS, water flow in troughs occurred repeatedly after rainfall events.

Organic matter The accumulation of organic C in the moss layer on the NFS was twice as much as that on the SFS (232 ± 46 and 114 ± 16 g C·m–2 (mean ± SD), respectively), and litter organic C stock was 1.3 times larger on the NFS (Table 1). Although there was no significant difference in the C content in mosses, organic C in litter ranged from 33%–42% on the SFS to 46%–49% on the NFS. In contrast with the NFS, where the C content of forest floor organic matter remained constant, the SFS showed a decrease in C concentration from live moss tissues to Oe–Oa horizon (Fig. 5). Nitrogen content ranged from 0.3%–0.5% in the moss layer to 1.0%– 1.2% in the Oe–Oa horizon (Fig. 5). Higher N concentrations were generally found in the forest floor of the SFS than in the forest floor of the NFS, resulting in consistently higher C/N ratios in the entire forest floor (troughs and mounds) 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 of DOC, was similar on both slopes. Total WEOC averaged 11.6 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 by slope. The WEOC content of the O horizon on the SFS was more 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 © 2005 NRC Canada

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

Fig. 5. Carbon and N contents in mosses and forest floor layers on the north- (A) and south-facing (B) slopes. 1.5 1.2 0.9 0.6

2

0.3 2

Oe-Oa 1.5 1.2 0.9 0.6

2

0.3 2

Oe-Oa

Table 2. Soil water suction measured in upper organogenic horizon of the mineral soil (0–5 cm) on the south- and north-facing slopes at various dates in 2003. Soil water suction (kPa) 25 July

30 August

15 September

South-facing slope Mound –3.4±1.6 Trough –0.8±0.5

20 June

–5.5±1.4 –2.1±0.7

–3.6±0.8 –1.8±0.7

–1.0±0.2 –0.5±0.3

North-facing slope Mound –0.5±0.2 Trough –0.0±0.2

–2.2±0.5 –0.4±0.3

–1.1±0.4 –0.5±0.3

–0.5±0.2 –0.2±0.2

Note: Values are means ± SD.

were almost equal (6.0 ± 1.2 and 6.3 ± 1.8 g·m–2, respectively). Concentrations of DOC in the forest floor seepage waters of both slopes demonstrated some interannual and interseasonal

variation (Table 3). Among individual collection dates, concentrations on the NFS ranged from 16 to 52 mg C·L–1 in 2002, and from 23 to 71 mg C·L–1 in 2003. On the SFS DOC concentrations were significantly higher ranging from 37 to 150 mg C·L–1 in 2002 and from 29 to 129 mg C·L–1 in 2003. In 2002 the highest concentrations of DOC in percolated waters were found at the end of the growing season (August–September), when the highest average daily precipitation was measured (Table 4). In contrast, earlier in the growing season we found relatively low DOC concentrations in forest floor solutions of both slopes. In 2003 fewer samples were collected, making it difficult to describe seasonal patterns. Concentrations of DOC in soil solution percolating from the forest floor varied with both temperature and throughfall amounts on the NFS, but were unrelated to either parameter on the SFS (Figs. 6 and 7). On both plots, DOC concentrations tended to decline with increasing amounts of net precipitation (throughfall), but the relationship was only statistically significant for the NFS plot (Fig. 7A). Concentrations of © 2005 NRC Canada

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Table 3. Seasonal patterns of organic solutes leached from forest floors of south- and north-facing slopes during growing season of 2002 and 2003. Season

DOC (mg C·L–1)

South-facing slope June 2002 56.0±6.5 2003 45.1* July–August 2002 48.7±16.2 2003 56.8±6.1 September 2002 80.7±27.7 2003† nd North-facing slope June 2002 32.5* 2003 29.3* July–August 2002 28.7±7.3 2003 49.2±11.8 September 2002 36.5±11.5 2003† nd

pHH2O

Conductivity (µS·cm–1)

6.4±0.3 nd

73.3±23.3 nd

6.0±0.2 nd nd 5.4±0.1 nd

65.7±10.5 nd nd 53.9±11.2 nd

6.8 nd

60.5 nd

6.0±0.2 nd

29.6±4.1 nd

5.8±0.1 nd

34.5±7.1 nd

Note: Values are means ± SD. DOC, dissolved organic carbon; nd, not determined. *One percolation event occurred. † No sampling was performed in September 2003.

DOC increased with mineral soil temperature on the NFS plot, but showed no response to temperature on the SFS plot (Fig. 6B). DOC export from the organic horizon during individual percolation events ranged from 0.02 g C·m–2 to about 5 g C·m–2 on the SFS and from 0.015 to 2.5 g C·m–2 on the NSF and was positively correlated with the volume of percolating soil solution (Fig. 8). In general, DOC flux from the organic horizon to the underlying mineral soil on the SFS was approximately twofold higher than on the NFS (Table 4). The impacts of microtopography on DOC export varied with slope. On the SFS, export of DOC from troughs and mounds varied considerably, 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 export was 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 for each growing season was 17.6 g C·m–2 on SFS and 8.9 g C·m–2 on the NFS in 2002 (Table 4). In 2003 the overall flux was lower because sampling was not conducted during September, but flux from the SFS plot was over twice that of the NFS plot (12.6 vs. 4.6 g C·m–2; Table 4). Total DOC export from the organic horizon during the growing season presented about 150% of the total WEOC stock in moss and litter on the SFS plot, and only 70% of that on the NFS plot. These DOC fluxes from the organic horizon represent about 0.5% 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 SFS and from 5.8 to 6.8 on the NFS (Table 3). Conductivities

tended to be lower on the SFS than on the NFS and declined through the growing season on both plots (Table 3). All samples 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 to 0.63 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 of taiga soils appears to follow predictable patterns related to aspect and microtopography as a result of the interacting effects of temperature and moisture availability. Our data show that the warmest, best-drained soils (mounds on the southfacing slope) have thinner forest floors with lower C content per unit area. The coldest, most poorly drained soils (troughs on the north-facing slope) have the thickest forest floor and the highest C content. These results are in agreement with earlier results (Kawahigashi et al. 2004), which showed that increasing thickness of the Oea horizons reflects the difference in moisture conditions and thus probably microbial activity among the organic horizons. Because temperature and hydrologic conditions interact with both the production and decomposition of organic matter, it is hard to determine which factors are dominant in producing the spatial mosaic of C accumulation that we have documented. Water stress (i.e., dry conditions), for example, can both decrease moss productivity and accelerate organic matter decomposition, driving the site to lower accumulation of organic C (Hobbie 1996; A. Knorre, personal communication). An increase of forest floor thickness under cold conditions provides a potential positive feedback loop, as a thicker forest floor further insulates the soil during the summer (Fig. 3) and decreases the depth of the active layer (Fig. 2), as suggested previously by Sofronov et al. (2000). The relatively low N and high C contents of the forest floor on the north-facing slope provide further evidence of the inhibition of decomposition on the north-facing slope. The quality of organic material in the forest floor is also tightly coupled to the relatively fine-scale variation in hydroclimatic conditions that we observed in mounds and troughs (Table 1). These differences in microtopography create mosaics of environmental conditions within the slope. Seasonal and topographic patterns in DOC mobilization suggest that increases in biological activity due to increasing temperatures drive some of the spatial and temporal variation in DOC production that we have observed. Temperature regime drives the depth and timing of thawing, as well as microbial activity. As thawing progresses through the summer and autumn, the active layer deepens and the temperature of the forest floor increases. Warmer conditions result in modest increases in DOC concentrations (Fig. 6) and substantial increases in concentrations of inorganic N on the south-facing slope, although not on the north-facing slope. These results are consistent with past work on temperate forest soils, which shows that DOC production and CO2 evolution increase with temperature (Christ and David 1996; Yanagihara et al. 2000; Neff and Hooper 2002). The strongest response to temperature was found on the heat-deficient north-facing slope. Christ and David (1996) found that DOC production has a higher Q10 in the range of 1–5 °C com© 2005 NRC Canada

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2137 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. DOC flux into soil (g C·m–2)

Precipitation (mm) Observation period June 2002 2003 July–August 2002 2003 September 2002 2003† Total 2002 2003

North-facing

South-facing

North-facing

South-facing

Daily

Daily

Daily

Total

Daily

2.2 0.9

Total 28.0* 27.7

Total

Total

2.1 1.0

26.8 31.3

0.06 0.02

0.80 0.71

0.15 0.05

1.95 1.63

1.4 1.9

43.4 117.9

1.3 1.8

40.3 111.2

0.04 0.06

2.34 3.89

0.09 0.18

5.56 10.92

3.3 —

102.3 —

2.9 —

89.9 —

0.25 —

5.79 —

0.44 —

10.04 —

174 146

157 143

8.9 4.6

17.6 12.6

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

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

DOC (mg C·L–1)

DOC (mg C·L–1)

Fig. 6. Dissolved organic carbon (DOC) concentrations versus temperature of mineral soil surface measured on north- (A) and south-facing (B) slopes.

y = 2.7x + 28.4

y = -0.52x + 53.8

R 2 = 0.63

R 2 = 0.54

DOC (mg C·L–1)

DOC (mg C·L–1)

o

o

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Fig. 8. The export of dissolved organic carbon (DOC) to mineral soil per one percolation event depending on the amount of water percolated through forest floor on north- (filled circles) and south-facing (open circles) slopes.

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

DOC (g C·m–2)

DOC (g C·m–2)

2

pared with 10–20 °C. The stronger response of DOC concentrations to temperature on our north-facing slope may be related to this difference in Q10 and to the decline of microbial communities within the permafrost zone of central Siberia at temperatures above 5 °C (Šantruková et al. 2003). Our data show that interactions among temperature, hydrology, and microbiology will drive any response in soil DOC flux to changes in Siberian climate. In a warming climate, we suspect that the initial response in central Siberia will be increases in the loss of DOC from the forest floor. These increases may be substantial, as the projected changes in temperature are as much as 3–5 °C in the next 50 years (Peterson et al. 2002), and the depth of the active layer in Russian permafrost has already increased by 20 cm from 1956 to 1990 (Nikolaev and Fedorov 2004). The longer-term responses of soils and DOC flux to a warming climate are much harder to predict, however. Changes in vegetation will undoubtedly accompany changes in climate, and this will affect the quality and quantity of organic matter entering the soil and thus the production of DOC (Neff and Hooper 2002). Models of vegetation response to warming in permafrost regions of Siberia suggest that the initial stages of warming (1–2 °C) may accelerate moss production and increase the thickness and insulating ability of the forest floor, with cooling of the soil by 0.5–1 °C (Anisimov and Belolutskaya 2004). Continued warming that results in a shift toward more vascular vegetation in central Siberia owing to higher summer air temperatures could lead to an increase of approximately 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 soils is unlikely to have any effects on autumn DOC fluxes in central Siberia. In the larch forests of the study area, despite their deciduous nature, input of fresh litter cannot play a crucial role, since needle fall is continuous from September to November and often occurs when the ground is already frozen. The most favorable combination of moisture content and temperature (deepest active soil layer) occurs in September,

DOC (g C·m–2)

2

and we believe this is the primary driver of increased DOC concentrations and flux in autumn. Within slopes, the mosaic of mounds and troughs with specific microclimatic and hydrologic conditions affects DOC export significantly. Flux of DOC from mounds on the southfacing slope did not differ from flux of DOC from troughs and mounds on the north-facing slope, but troughs on the south-facing slope showed dramatic increases in DOC export. Although these findings can be attributed, in part, to the lower production of DOC (lower DOC concentration) in the colder microsites, the increase in runoff from troughs with southern exposure (almost triple that of the other microsites) drives the export response. The source of this additional runoff appears to be melting of permafrost in the warmer south-facing slope and accumulation of this additional water in low points (troughs) in the landscape. Production of DOC in Siberian soils does not appear to increase as a function of soil C/N ratio, unlike observations from temperate forests soils (Gödde et al. 1996; Aitkenhead © 2005 NRC Canada

Prokushkin et al.

and McDowell 2000; Michalzik et al. 2001). Our results show that both concentration and flux of DOC are higher from the south-facing slope, which has much lower soil C/N, than from the north-facing slope. Whether this higher DOC production from warmer, south-facing soils translates into greater delivery of DOC to streams is an unanswered question. Although the production and release of DOC from the forest floor may be greater in warmer soils, the deeper active layer in these warmer soils also increases contact with mineral soils and thus the likelihood of DOC adsorption (Jardine et al. 1989; Kaiser et al. 2001). In a study of controls on stream DOC flux in central Alaska, MacLean et al. (1999) found that DOC flux was greater from watersheds with more permafrost coverage and attributed this finding to the relatively shallow flow paths through the high-permafrost basin that resulted in little adsorption of DOC on mineral soils. A similar result was also found recently for central Siberia in comparative analyses of small streams along a gradient from discontinuous to continuous permafrost (Kawahigashi et al. 2004).

Conclusions Concentrations and fluxes of DOC in forest floors of larch ecosystems underlain by continuous permafrost have significant spatial and temporal variations. Both concentrations and fluxes of DOC increase during the growing season because of the increase in the depth of the active layer and increased DOC production at higher temperatures. North-facing slopes had much lower DOC production than south-facing slopes, and microtopography further complicated the response to slope and precipitation events. Greater DOC flux from slopes with southern aspect suggests that short-term DOC production will increase in forest floors under conditions of global warming. Changing climate would also trigger complex changes in both the biological nature and chemical properties of the organic matter that serves as DOC source material, and thus long-term changes in DOC production are much more difficult to evaluate. In particular, the depth of the active soil layer and sorption capacity of thawing soil during permafrost degradation are believed to be the key factors controlling losses of DOC from terrestrial ecosystems (MacLean et al. 1999).

Acknowledgments This research was funded by the Russian Fund of Basic Research (grant 03-04-48037) and financial support provided by the Siberian Branch of the Russian Academy of Sciences for young scientists in 2003–2004. We thank to all members of the joint Japanese–Russian team working in Tura experimental station. The US National Science Foundation (DEB0108385) provided support for manuscript preparation.

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