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Estimating organic carbon storage in temperate wetland profiles in Northeast China
Geoderma 146 (2008) 311–316
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Geoderma j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a
Estimating organic carbon storage in temperate wetland profiles in Northeast China Wen-Ju Zhang a,b, He-Ai Xiao a, Cheng-Li Tong a, Yi-Rong Su a, Wan-sheng Xiang a, Dao-You Huang a, J. Keith Syers c, Jinshui Wu a,⁎ a b c
Hunan Key Laboratory for Agro-ecological Engineering, Institute of Subtropical Agriculture, CAS, Changsha, Hunan 410125, China Institute of Agricultural Resources and Regional Planning, CAAS, Beijing 100081, China School of Science, Mae Fah Luang University, Chiang Rai 57100, Thailand
A R T I C L E
I N F O
Article history: Received 11 March 2008 Received in revised form 31 May 2008 Accepted 4 June 2008 Available online 15 July 2008 Keywords: Temperate wetland Organic carbon distribution Depth of accumulation Organic carbon storage
A B S T R A C T Wetlands contain a large carbon (C) stock and play an important role in global C cycling. To improve knowledge on organic carbon (OC) storage in the temperate wetlands of China, sixteen profiles (0–200 cm) from peat fen (PF, six profiles), humus marsh (HM, five profiles), and marshy meadow (MM, five profiles) from the Sanjiang Plain region of Northeast China were sampled and investigated. The OC content in the upper part of the organic horizon (specific gravity, SG b 1.0) in the profiles varied from 330 to 470 g kg− 1 for PF, from 180 to 450 g kg− 1 for HM, and from 40 to 110 g kg− 1 for MM, respectively. For all of three types of wetlands, the proportion of OC in the light fraction (SG ≤ 1.7), which is considered to be a more decomposable fraction, was significantly correlated with the content of OC in the organic horizon. In the lower illuvial horizon (SG N 1.0), the content of OC was considerably smaller (b 10 g kg− 1) and also little was presented in the light fraction, despite differences in wetland type. OC storage in temperate wetlands is significantly underestimated if only the top 1 m of the profile is considered, because the upper organic horizon could vary considerably in depth. We conclude that considering the organic horizon plus the top 1 m of illuvial horizon gives a more reliable estimate of OC storage in the temperate wetland profiles studied. Using this approach, it was estimated that the mean OC storage was 828 t ha− 1 for PF. The corresponding values were 274 t ha− 1 for HM, and 171 t ha− 1 for MM. © 2008 Published by Elsevier B.V.
1. Introduction Wetland ecosystems on the earth have a total C stock amounting to about 20–25% of the total stock in terrestrial soils, and are considered to play an important role in global C cycling. This directly affects the concentration of atmospheric CO2, with potential implications for global climate change (Gorham, 1995; IPCC, 2000; Lal, 2004). Native and restored wetlands are generally a sink for atmospheric CO2, and therefore make an important contribution to global C sequestration (Inubushi et al., 2003; Moore and Turunen, 2004; Furukawa et al., 2005; Inubushi et al., 2005; Roulet, 2000). However, it has been proposed that about 68% of the total global area of wetlands, and presumably a similar percentage of the C stock, has been lost since the Industrial Revolution because of environmental changes (e.g., a warmer and drier climate) and human activities, such as peat harvesting and converting wetlands to farmland, forestry, and urban areas (Gorham, 1991; Jenkinson et al., 1991; Larson, 1995; IPCC, 2000; Neher et al., 2003). Thus, there are substantial uncertainties concerning the C stock of wetlands at local, national, regional, and global ⁎ Corresponding author. Institute of Subtropical Agriculture, CAS, Changsha, Hunan 410125, China. Tel.: +86 731 4615224; fax: +86 731 4612685. E-mail addresses: [email protected] (J. Wu). 0016-7061/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.geoderma.2008.06.006
scales, and its impacts on global C cycling (Gorham, 1991; Roulet, 2000; Bai et al., 2005). It has been estimated that the global C stock down to 1 m in wetlands is 225 Pg (1 Pg = 1015 g) (IPCC, 2000). This value compares favorably with the estimated range (180–249 Pg) given in earlier work (Post et al., 1982; Armentano and Menges, 1986). Gorham (1991) proposed that the C stock of wetlands was largely underestimated in early work, which used 1 m as the mean depth, as used for terrestrial soil. He estimated that the average depth of the peat layer in boreal and sub-arctic peatlands was 230 cm, and that the total organic C storage down to this depth was 445 Pg, a value much larger than that for global wetlands given by earlier estimates. Other studies suggested that the C storage of wetlands varied considerably for the different climatic zones and was affected by vegetation type and landscape and hydrologic conditions (Gorham, 1991; Tolonen et al., 1992; Roulet, 2000; Bockheim et al., 2003; Moore and Turunen, 2004; Bai et al., 2005). Temperate wetlands are particularly important in this regard as it has been estimated that most of the wetland area and associated C storage is in peatlands in temperate and boreal regions, with roughly 10–30% occurring in the tropics (IPCC, 2000). To reduce the uncertainties in the estimates of the C stock in wetlands, there is an urgent need to obtain more reliable information and this requires that an appropriate sampling depth be used in
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making the necessary calculations for wetlands in different climatic regions. This paper reports on the contents and distribution of OC in profiles of the three major types of temperate wetlands in Northeast China. The aim was to provide factual knowledge as a basis for obtaining a better understanding of the major mechanisms determining C storage in temperate wetlands. 2. Materials and methods 2.1. Sites and sampling The Sanjiang Plain region (129°11′–135°05′N, 43°50′–48°27′E) is a river basin in Northeast China, consisting of typical inland wetlands spreading over hillocks, river terraces, and floodplains. This region has a temperate climate, with annual average temperatures ranging from 1.9 to 3.9 °C and rainfall from 500 to 650 mm. It currently has a total wetland area of about 20, 000 km2; of which 20% is peat fen (PF), 43% is humus marsh (HM), and 37% is marshy meadow (MM) (Wang et al., 2003). Sixteen sites were selected for representative profiles under PF (six profiles), HM (five profiles), and MM (five profiles) in the Sanjiang Plain region, with the general information for each site as described in Table 1. In order to minimize disturbance to the sediment profiles, sampling was done during the frozen period (April) in 2002 and 2003. Sediment profiles were hand-dug down to a depth of 120–200 cm, and triplicate samples were taken from each 5-cm section of the profiles and sealed in plastic bags. After the removal of fresh roots and plant residues by hand picking, samples were dried in an oven at 60 °C and sieved (b1 mm and N0.15 mm) for further analyses. 2.2. Analyses Total organic C, referred to as OC, in the samples was determined using an automated carbon analyzer (VARIO MAX CN, Germany), and dry matter by weighing after 48 h oven-drying (105 °C).
Light fraction organic matter (SG ≤ 1.7) was isolated by a procedure slightly modified from Janzen et al. (1992). A portion (1–5 g, according to litter content) of the sample was weighed into an 80ml centrifuge tube and 40 ml NaI solution (SG 1.7) added. The suspension was dispersed with an ultrasonic vibrator (SB5200, 20–40 kHz, 300 W) for 10 min and then centrifuged at 4200 r min− 1 for 15 min, and the suspended light fraction organic matter was collected on a 420-mesh sieve. The residue was dispersed and centrifuged four more times. The light fraction organic matter collected was washed with 0.01 M CaCl2 and distilled water to completely remove residual NaI, dried at 60 °C for 48 h, weighed, and then ground to pass 100-mesh. Dry matter and OC contents of the light fraction organic matter were determined. 2.3. Estimation of organic C storage in the wetland sediments Because there were no visible air bubbles, the air volume of the frozen sediments can be taken as being negligible. Therefore, the mean density (Di) of a sediment section (referred to as i) can be expressed as follows: Di ¼
1 ðWw =Dw þ Wm =Dm þ Wo =Do Þ
where Ww, Wm, and Wo (all in g g− 1) indicate the contents of water, mineral material, and organic matter in the section, respectively; and Dw, Dm, and Do (all in g cm− 3) indicate the densities for water (frozen), mineral, and organic components, respectively. The bulk density of dry matter (Dm, excluding water) in the section is calculated as: Dm ¼ Di ð1−Ww Þ
Profile Type of no. wetlandsa
Landscapeb
Predominant vegetation
Sampling depth (cm)
1
PF
River floodplain (P)
200
2
PF
River floodplain (P)
3
PF
River floodplain (P)
4 5
PF PF
River floodplain (P) River floodplain (P)
6
PF
River floodplain (P)
7 8 9
HM HM HM
River floodplain (P) Hillock (P) River floodplain (P)
10
HM
River floodplain (P)
11
HM
River floodplain (P)
12 13 14
MM MM MM
Terrace of river Terrace of river Terrace of river
15
MM
Terrace of river
16
MM
Hillock (S)
Carex pseudocuraica and Deyeuxia angustifolia Carex pseudocuraica and Deyeuxia angustifolia Carex lasiocarpa and Carex pseudocuraica Deyeuxia angustifolia Carex pseudocuraica and Deyeuxia angustifolia Carex pseudocuraica and Deyeuxia angustifolia Carex meyeriana Carex lasiocarpa Carex meyeriana and Deyeuxia angustifolia Carex meyeriana and Carex pseudocuraica Phragmites australis and Deyeuxia angustifolia Deyeuxia angustifolia Deyeuxia angustifolia Salix brachyboda and Deyeuxia angustifolia Betula platy phyla and Deyeuxia angustifolia Deyeuxia angustifolia
a
200 200 200 200 200 160 160 200 150 160 200 200 120 120 150
PF refers to peat fen (or fen in the European classification), HM to humus marsh (or marsh in the European classification), and MM to marshy meadow. b Letter P in the parentheses refers to permanently waterlogged and S to seasonally waterlogged.
ð2Þ
Taking Di from Eq. (1), then: Dm ¼
Table 1 Description of sampling sites for the wetland profiles in the Sanjiang Plain region
ð1Þ
ð1−Ww Þ ðWw =Dw þ Wm =Dm þ Wo =Do Þ
ð3Þ
Density of OC (Ci, g cm− 3) for the section is calculated as: Ci ¼ Dm Wc
ð4Þ
where Wc indicates OC content of dry matter (g g− 1). Thus, OC storage (TC, t ha− 1) of the sediment profile from the top down to the j section can be calculated as: j
TC ¼ ∑ Ci Hi
ð5Þ
i¼1
where Hi indicates the thickness (5 cm) of the i layer. For the calculation of TC, the densities of the water (frozen) and mineral components were assumed to be 0.9 and 2.6 g cm− 3, respectively (Boyd, 1995). For the organic horizon where specific gravity was smaller than 1.0 g cm− 3, the mean density of the organic matter was assumed to be 1.0 g cm− 3. For the illuvial horizon (SG N 1.0, as proposed in this study), the mean density of the organic matter was assumed to be 1.25 g cm− 3, as proposed in previous studies for mineral soils (Boyd, 1995; Avnimelech et al., 2001). 3. Results 3.1. Bulk density For each of the six profiles from PF, bulk density (excluding water) was small (0.1–0.3 g cm− 1) in the top 50–75 cm (Fig. 1). In
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ing 3 profiles from PF, OC content declined smoothly from 70 cm down to 120–150 cm. In the 5 sediment profiles from HM, OC content increased substantially from values of 180–290 g kg− 1 in the top section (0–5 cm) to 310–450 g kg− 1 at a depth of 15–25 cm (Fig. 2). In 4 of these 5 profiles, OC content declined sharply to a level smaller than 5 g kg− 1 by 25–45 cm, with only one exception in which OC content declined gradually between 15 and 75 cm. For the MM profiles, OC content declined sharply from a value within the range of 40–110 g kg− 1 in the top to levels below 5 g kg− 1 at 15–50 cm (Fig. 2). 3.3. Light fraction organic carbon contents In PF, light fraction OC varied from 1.4 to 90.3% of total OC (with a mean value of 48.7 ± 36.4%) in the organic horizon (Fig. 3). In the top 60 cm of this wetland type, light fraction OC accounted for about 70% of total OC. For HM, light fraction OC in the upper 0–20 cm was
Fig. 1. Bulk density of wetland profiles from the Sanjiang Plain of Northeast China. PF, HM, and MM refer to peat fen, humus marsh, and marshy meadow, respectively; the numbers refer to the profiles as listed in Table 1.
three of the PF profiles, bulk density increased up to 1.0 g cm− 1 at a depth of about 80 cm, but for the other three profiles not until a depth of between 115 and 155 cm. For profiles from HM, bulk density reached this value at a depth varying from 25 to 75 cm (Fig. 1). In MM profiles, bulk density increased sharply and reached a value of 1.0 g cm− 1 in the top 15–40 cm (Fig. 1). As the depth increased by a further 10–15 cm, bulk density for all profiles from the three types of wetlands increased to 1.3–1.8 g cm− 1 and then remained relatively constant. 3.2. Organic carbon contents and distribution OC contents in the top 50–70 cm of the PF profiles varied from 330 to 470 g kg− 1 (Fig. 2). For 3 PF profiles, OC content declined sharply from 60 or 80 cm to an amount smaller than 5 g kg− 1, and below this depth it remained essentially constant. For the remain-
Fig. 2. Organic C content of wetland profiles from the Sanjiang Plain. See Fig. 1 for the description of abbreviations and numbers.
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Fig. 3. Percentage of light fraction organic C to the total organic C in selected wetland profiles of peat fen (○, profile no. 1), humus marsh (●, profile no. 9), and marsh meadow (△, profile no. 15) from the Sanjiang Plain.
more than 70% of total OC. From 20 to 40 cm, this percentage decreased to 1%. Light fraction OC varied from 1 to 86% (mean of 46.0 ± 35.3%) in the organic horizon of the HM. However, in MM, light fraction OC accounted for only 4 to 18% (with a mean value of 13.2 ± 5.2%) of total OC in the organic horizon. Hardly any light fraction organic matter could be separated by this fractionation method from the illuvial horizon. There was a highly significant correlation ( R 2 = 0.96 ⁎⁎ ) between light fraction OC and total OC for the three types of wetlands (Fig. 4). This relationship also showed that the content of light fraction OC changed only slightly when total OC was low (less than 200 g kg− 1). However, when total OC was more than 200 g kg− 1, light fraction OC increased considerably, according to the content of total OC. 3.4. Organic carbon density The distribution of OC density in the PF profiles showed a dual pattern in that the upper layer had high but variable OC density, and the lower layer had a low but more uniform OC density (Fig. 5). Also, the thickness of the upper, high OC density layer (80–155 cm) was almost the same as that of the layer with a low bulk density (Fig. 1) and high OC content (Fig. 2). For the six PF profiles, OC density of the upper layer varied from 36.7 to 164.8 kg m− 3, with a mean of 71.5 kg m− 3 and a coefficient of variation of about 21% (Fig. 5). OC density of the upper layer varied in the range of 24 to 74 kg m− 3 (with a mean of 47.0 kg m− 3), and 29 to 45 kg m− 3 (with a mean of 38.7 kg m− 3) for the HM and MM profiles, respectively. For all the
Fig. 5. Organic C density of wetland profiles from the Sanjiang Plain. See Fig. 1 for the description of abbreviations and numbers.
PF, HM, and MM profiles, OC density in the lower layer varied from 2.4 to 48.8 kg m− 3, with a mean of 8.3 kg m− 3. 4. Discussion 4.1. Distribution of organic C in the wetland profiles
Fig. 4. Correlation between light fraction organic C and total organic C at selected wetland profiles (no. 1, 9, 15, Table 1) from the Sanjiang Plain.
In the permanently waterlogged PF and HM profiles, OC content varied to a much larger extent than that in seasonally waterlogged MM profiles. The hydrological condition is one of the main factors that cause the large amount of OC to be accumulated in PF and HM profiles (Table 1). Permanent waterlogging induces anaerobic conditions, under which OC in litter decomposes only slowly (Esteves et al., 2001). However, with seasonal change in the hydrologic condition in MM, the decomposition environment is aerobic for most of the time and there is no similar restriction. Brinson et al. (1981) suggested that decomposition was optimized where cycles of wetting and drying prevail. It has also been reported that brief flooding, followed by moist but well-aerated
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conditions, causes maximum OC loss (Lockaby and Wailbridge, 1998). Therefore, it is expected that the content of OC in MM will be substantially lower than that in PF and HM and this was found to be the case. The order for OC content is similar to that reported by Ma et al. (1996) for the Sanjiang Plain. However, the OC contents in the three types of wetland are much higher than those reported for river marginal floodplains in China (Bai et al., 2005). The low and rather constant OC content in the illuvial horizon resulted from illuviation of mineral matter under waterlogged conditions. This is usually a characteristic of OC in mineral subsoil below peat (Moore and Turunen, 2004). Wetland profiles usually develop an organic horizon in the upper layer and an illuvial horizon (Neustadt,1984; Gorham,1991; Frolking et al., 2001). However, there has been no clear basis for separating these two layers and this becomes particularly important in establishing the profile depth to be considered in estimating C storage. Here, we propose that the layer of the profile that has a bulk density smaller that 1.0 g cm− 1 should be taken as the organic horizon in the wetland profiles studied in our work. A careful analysis of the raw data for bulk density with increasing depth (Fig. 1) indicates that, in almost all cases, this corresponds very closely with the pattern for OC content (Fig. 2) and OC density (Fig. 5). Thus, there is no advantage in taking a bulk density different from 1.0 g cm− 1 for deciding the depth of the organic horizon. It is worth noting that the thickness of this horizon could possibly change with seasonal fluctuation in water level of the wetlands studied in this work. As indicated by the data presented in Fig. 1, the thickness of the organic horizon differed substantially for the three different types of wetlands in the Sanjiang Plain. Even for the same type of wetland, the thickness of this horizon also varied quite widely, as in the range of 80 to 155 cm for PF, 25–75 cm for HM, and 15–40 cm for HM (Fig. 1). The mean thickness of the organic horizon in PF was 100 cm, which is comparable to that (110 cm) in wetlands in temperate Fennoscandia (Lappaleinen, 1980) but substantially smaller than that (230–250 cm) for peatlands in the Frigid Zone. Also, the thickness of the peat layer determines the amount of OC stored in wetlands. For example, Neustadt (1984) estimated that the mean thickness of the peat layer was 250 cm for peatlands in Russia. Gorham (1991) proposed a mean thickness of 230 cm for the peat layer of boreal and sub-arctic peatlands but in the temperate zone it is clearly smaller. The mean thickness of the organic horizon of HM and MM profiles in the Sanjiang Plain is much thinner than that of the PF profiles. However, the thickness of the organic horizon of HM and MM was in the range of that in drained thaw-lake basins in Arctic Alaska (Bockheim et al., 2004). The developmental history of wetland ecosystems is expected to contribute mainly to these differences, because there is usually a significant linear relationship between the thickness of the organic horizon and 14C age (Bockheim et al., 2004). Data presented in Fig. 5 show that OC density in the organic horizon varied substantially among the three types of wetlands, being much higher in PF than that in agricultural or forest soils in the same general climatic area (Wang et al., 2002a). For HM and MM, OC in the organic horizon was equal to or a little higher than that in paddy topsoils in China (Pan et al., 2003). Although OC contents in the illuvial horizon were quite small for all the three types of wetlands, OC density was reasonably high, due to the high bulk density of this horizon. Moore and Turunen (2004) also suggested that mineral soil beneath peat could accumulate substantial amounts of OC and it was estimated that the subsoil OC pool could be 10 Pg globally, this being derived primarily from the sorption of dissolved OC in the subsoil and preserved under anoxic condition (Turunen et al., 1999). 4.2. Organic C storage in temperate wetlands For the three types of wetlands, OC storage was estimated for the depth of the organic horizon, for a total depth of 1 m (which includes the organic horizon), and for the organic horizon + 1 m of the mineral illuvial horizon (Table 2).
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Table 2 Storage of organic C estimated for different depths in the three types of wetlands in the Sanjiang Plain region Wetland Numbers type of profile
PF HM MM
6 5 5
Thickness of OH Organic C storagea (t ha− 1) (cm) In OH In 1 m 100 ± 29.8 39 ± 18.8 20 ± 9.4
OH + 1 m IH
715 ± 240 (86.3) 645 ± 86 (77.9) 828 ± 223 179 ± 88 (65.3) 252 ± 93 (92.0) 274 ± 106 79 ± 32 (46.2) 160 ± 23 (93.6) 171 ± 27
a Values in parentheses indicated the percentage of organic C storage in the organic horizon and in 1 m depth to the total organic C storage in the organic horizon plus 1 m illuvial horizon combined; OH and IH denoted the organic horizon and the illuvial horizon, respectively.
The mean OC storage per unit area in the organic horizon for PF was 715 ± 240 t ha− 1, which was about four and nine times larger than that in HM and MM, respectively (Table 2). For a total depth of 1 m, it was 252 ± 93 and 160 ± 23 t ha− 1 for HM and MM, respectively. These values amounted to 92 and 94% of those in the organic horizon + 1 m of the illuvial horizon. However, for PF, OC storage per unit area in 1 m was 645 ± 86 t ha− 1, which was only 78% of that in the organic horizon plus 1 m of the illuvial horizon. The fact that OC storage varied in the different wetland profile types and also with the depth used for calculation (organic horizon, total depth of 1 m, or organic horizon + 1 m of the illuvial soil), shows that the depth used for estimation is particularly important for calculating OC storage in different types of wetlands. Traditionally, OC storage has been reported for the upper 1 m, which is helpful for comparing the results of different researchers (Post et al., 1982). However, the thickness of the organic horizon in PF is sometimes more than 1 m and OC storage per unit area is greatly underestimated if only 1 m is taken as the depth in making the estimate. The data obtained suggest that the organic horizon plus 1 m of the illuvial horizon provides a more reasonable basis for obtaining reliable OC storage data for the temperate wetland profiles studied here. Light fraction OC is mainly composed of plant residues which are incompletely decomposed. The proportion of light fraction OC in wetlands is considered to be sensitive to management and changes in microenvironment (Barrios et al., 1997; Six et al., 2000). The high ratio for light fraction OC to total C in the present study implies that the OC accumulated in the PF and the HM is dominated by the light fraction and that this could be decomposed easily when hydrological conditions changed. The fractionation result for MM clearly shows that seasonal flooding accelerates the decomposition of light fraction OC and this may be the reason why OC content decreases so rapidly as a wetland is drained. In fact, much of the MM peatland on the Sanjiang Plain has been cultivated in recent years, with the area occupied by this wetland type decreasing from 7309 km2 in 1980 to a very low figure in 2000 (Wang et al., 2002b). In the same time period, PF showed no change whereas for HM the corresponding areas had decreased from 8300 to 5228 km2. 5. Conclusion By the use of a bulk density value of 1.0 g cm− 1 it was possible to make a more reliable assessment of the depth of the organic horizon in the wetland profiles in the Sanjiang Plain in temperate Northeast China. The mean thickness of the organic horizon varied from 100 to 39 to 20 cm for the PF, HM, and MM profiles, respectively, which is of particular significance in selecting an appropriate depth for assessing OC storage. For each profile, a total depth of the thickness of the organic horizon plus 1 m of the underlying mineral soil was used. Approximately 828 ± 223, 274 ± 106, and 171 ± 27 t ha− 1 OC have accumulated in profiles of PF, HM, and MM, respectively. Light fraction OC was the main OC fraction accumulated in the organic horizons and
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this is susceptible to decomposition. The variability of OC content and thickness of the organic horizon contribute to the uncertainty of OC storage estimation in wetlands but these must be determined if reliable estimates for C storage are to be obtained. Acknowledgments We are grateful for the financial support from the Chinese Academy of Sciences (KZCX1-SW-01-14), and the Ministry of Science and Technology, China (2002CB412503, 2006BAD05B09). Assistance from the Northeast Institute of Geography and Agro-ecology, CAS, China, is also gratefully acknowledged. References Armentano, T.V., Menges, E.S., 1986. Patterns of change in the carbon balance of organic soil-wetlands of the temperate zone. Journal of Ecology 74, 755–774. Avnimelech, Y., Ritvo, G., Meijer, L.E., Kochba, M., 2001. Water content, organic carbon and dry bulk density in flooded sediments. Aquacultural Engineering 25, 25–33. Bai, J., Ouyang, H., Deng, W., Zhu, Y., Zhang, X., Wang, Q., 2005. Spatial distribution characteristics of organic matter and total nitrogen of marsh soils in river marginal wetlands. Geoderma 124, 181–192. Barrios, E., Buresh, R.J., Kwesiga, F., Sprent, J.I., 1997. Light fraction soil organic matter and available nitrogen following trees and maize. Soil Science Society of America Journal 61, 826–831. Bockheim, J.G., Hinkel, K.M., Nelson, F.E., 2003. Predicting carbon storage in tundra soils of Arctic Alaska. Soil Science Society of America Journal 67, 948–950. Bockheim, J.G., Hinkel, K.M., Eisner, W.R., Dai, X.Y., 2004. Carbon pools and accumulation rates in an age-series of soil in drained thaw-lake basins, Arctic Alaska. Soil Science Society of America Journal 68, 697–704. Boyd, C.E., 1995. Bottom Soils, Sediment, and Pond Aquaculture. Chapman & Hall, New York, p. 348. Brinson, M.M., Lugo, A.E., Brown, S., 1981. Primary productivity, decomposition and consumer activity in freshwater wetland. Annual Review of Ecology and Systematics 12, 123–161. Esteves, F.A., Enrich-Prast, A., Biesboer, D.D., 2001. Potential denitrification in submerged natural and impacted sediments of Lake Batata, an Amazonian lake. Hydrobiologia 444, 111–117. Frolking, S., Roulet, N.T., Moore, T.R., Richard, P.J.H., Lavoie, M., Muller, S.D., 2001. Modeling northern peatland decomposition and peat accumulation. Ecosystems 4, 479–498. Furukawa, Y., Inubushi, K., Ali, M., Itang, A.M., Tsuruta, H., 2005. Effect of changing groundwater levels caused by land-use changes on greenhouse gas fluxes from tropical peat lands. Nutrient Cycling in Agroecosystems 71, 81–91. Gorham, E., 1991. Northern peatlands: role in the carbon cycle and probable responses to climatic warming. Ecological Applications 1, 182–195. Gorham, E., 1995. The biogeochemistry of northern peatlands and its possible responses to global warming. In: Woodwell, G.M., Mackenzie, F.T. (Eds.), Biotic Feedback in the Global Climate System: Will the Warming Feed the Warming? Oxford University Press, New York, pp. 169–186.
Inubushi, K., Furukawa, Y., Hadi, A., Purnomo, E., Tsuruta, H., 2003. Seasonal changes of CO2, CH4 and N2O fluxes in relation to land-use change in tropical peatlands located in coastal area of South Kalimantan. Chemosphere 52, 603–608. Inubushi, K., Otake, S., Furukawa, Y., Shibasaki, N., Ali, M., Itang, A.M., Tsuruta, H., 2005. Factors influencing methane emission from peat soils: Comparison of tropical and temperate wetlands. Nutrient Cycling in Agroecosystems 71, 93–99. IPCC, 2000. Special Report: Land Use, Land-Use Change, and Forestry. Cambridge University Press, Cambridge. Janzen, H.H., Campbell, C.A., Brandt, S.A., Lafond, G.P., Townley-Smith, L., 1992. Lightfraction organic matter in soils from long-term crop rotations. Soil Science Society of America Journal 56, 1799–1806. Jenkinson, D.S., Adams, D.E., Wild, A., 1991. Model estimates of CO2 emissions from soil in response to global warming. Nature 351, 304–306. Lal, R., 2004. Soil carbon sequestration impacts on global climate change and food security. Science 304, 1623–1627. Lappaleinen, E., 1980. The useful fuel peat resources in Finland. Proceedings of the Sixth International Peat Congree, Duluth, Minnesota, USA. International Peat Society, Helsinki, Finland, pp. 59–63. Larson, D.L., 1995. Effects of climate on numbers of northern prairie wetlands. Climatic Change 30, 169–180. Lockaby, B.G., Wailbridge, M.R., 1998. Biogeochemistry. In: Messina, M.G., Conner, W.H. (Eds.), Southern Forested Wetlands: Ecology and Management. Lewis Publishers, Boca Raton, FL, pp. 149–172. Ma, X., Lu, X., Yang, Q., Yan, M., 1996. Carbon cycle of a marsh in Sangjiang Plain. Scientia Geographica Sinica 16 (4), 323–330 (in Chinese with English Abstr.). Moore, T.R., Turunen, J., 2004. Carbon accumulation and storage in mineral subsoil beneath peat. Soil Science Society of America Journal 68, 690–696. Neher, D.A., Barbercheck, M.E., El-Allaf, S.M., Anasa, O., 2003. Effects of disturbance and ecosystem on decomposition. Applied Soil Ecology 23, 165–179. Neustadt, M.I., 1984. Holocene peatland development. In: Velichko, A.A. (Ed.), Later Quaternary Environments of the Soviet Union. University of Minnesota Press, Minneapolis, Minnesota, USA, pp. 201–206. Pan, G., Li, L., Wu, L., Zhang, X., 2003. Storage and sequestration potential of topsoil organic carbon in China's paddy soils. Global Change Biology 10, 79–92. Post, W.M., Emanuel, W.R., Zinke, P.J., Stangenberger, A.G., 1982. Soil carbon pools and life zones. Nature 298, 156–159. Roulet, N.T., 2000. Peatlands, carbon storage, greenhouse gases, and the Kyoto protocol: prospects and significance for Canada. Wetlands 20, 605–615. Six, J., Merckx, R., Kimpe, K., Paustian, K., Elliott, E.T., 2000. A re-evaluation of the enriched labile soil organic matter fraction. European Journal of Soil Science 51, 283–293. Tolonen, K., Vasander, H., Damman, A.W.H., Clymo, R.S., 1992. Preliminary estimate of long-term carbon accumulation and loss in 25 boreal peatlands. Suo 43, 277–280. Turunen, J., Tolonen, K., Tolvanen, S., Remes, M., Ronkainen, J., Jungner, H., 1999. Carbon accumulation in the mineral subsoil of boreal mires. Global Biogeochemical Cycles 13, 71–79. Wang, S., Zhou, C., Liu, J., Tian, H., Li, K., Yang, X., 2002a. Carbon storage in Northeast China as estimated from vegetation and soil inventories. Environmental Pollution 116, S157–S165. Wang, A., Zhang, S., He, Y., 2002b. Study on dynamic on change of mire in Sanjiang Plain based on RS and GIS. Scientia Geographica Sinica, 22 (5), 636–640 (in Chinese with English Abstr.). Wang, A., Zhang, X., Zhang, B., 2003. A study on the change of spatial pattern of wetland in the Sanjiang Plain. Acta Ecological Sinica, 23 (2), 239–243 (in Chinese with English Abstr.).
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Geoderma j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a
Estimating organic carbon storage in temperate wetland profiles in Northeast China Wen-Ju Zhang a,b, He-Ai Xiao a, Cheng-Li Tong a, Yi-Rong Su a, Wan-sheng Xiang a, Dao-You Huang a, J. Keith Syers c, Jinshui Wu a,⁎ a b c
Hunan Key Laboratory for Agro-ecological Engineering, Institute of Subtropical Agriculture, CAS, Changsha, Hunan 410125, China Institute of Agricultural Resources and Regional Planning, CAAS, Beijing 100081, China School of Science, Mae Fah Luang University, Chiang Rai 57100, Thailand
A R T I C L E
I N F O
Article history: Received 11 March 2008 Received in revised form 31 May 2008 Accepted 4 June 2008 Available online 15 July 2008 Keywords: Temperate wetland Organic carbon distribution Depth of accumulation Organic carbon storage
A B S T R A C T Wetlands contain a large carbon (C) stock and play an important role in global C cycling. To improve knowledge on organic carbon (OC) storage in the temperate wetlands of China, sixteen profiles (0–200 cm) from peat fen (PF, six profiles), humus marsh (HM, five profiles), and marshy meadow (MM, five profiles) from the Sanjiang Plain region of Northeast China were sampled and investigated. The OC content in the upper part of the organic horizon (specific gravity, SG b 1.0) in the profiles varied from 330 to 470 g kg− 1 for PF, from 180 to 450 g kg− 1 for HM, and from 40 to 110 g kg− 1 for MM, respectively. For all of three types of wetlands, the proportion of OC in the light fraction (SG ≤ 1.7), which is considered to be a more decomposable fraction, was significantly correlated with the content of OC in the organic horizon. In the lower illuvial horizon (SG N 1.0), the content of OC was considerably smaller (b 10 g kg− 1) and also little was presented in the light fraction, despite differences in wetland type. OC storage in temperate wetlands is significantly underestimated if only the top 1 m of the profile is considered, because the upper organic horizon could vary considerably in depth. We conclude that considering the organic horizon plus the top 1 m of illuvial horizon gives a more reliable estimate of OC storage in the temperate wetland profiles studied. Using this approach, it was estimated that the mean OC storage was 828 t ha− 1 for PF. The corresponding values were 274 t ha− 1 for HM, and 171 t ha− 1 for MM. © 2008 Published by Elsevier B.V.
1. Introduction Wetland ecosystems on the earth have a total C stock amounting to about 20–25% of the total stock in terrestrial soils, and are considered to play an important role in global C cycling. This directly affects the concentration of atmospheric CO2, with potential implications for global climate change (Gorham, 1995; IPCC, 2000; Lal, 2004). Native and restored wetlands are generally a sink for atmospheric CO2, and therefore make an important contribution to global C sequestration (Inubushi et al., 2003; Moore and Turunen, 2004; Furukawa et al., 2005; Inubushi et al., 2005; Roulet, 2000). However, it has been proposed that about 68% of the total global area of wetlands, and presumably a similar percentage of the C stock, has been lost since the Industrial Revolution because of environmental changes (e.g., a warmer and drier climate) and human activities, such as peat harvesting and converting wetlands to farmland, forestry, and urban areas (Gorham, 1991; Jenkinson et al., 1991; Larson, 1995; IPCC, 2000; Neher et al., 2003). Thus, there are substantial uncertainties concerning the C stock of wetlands at local, national, regional, and global ⁎ Corresponding author. Institute of Subtropical Agriculture, CAS, Changsha, Hunan 410125, China. Tel.: +86 731 4615224; fax: +86 731 4612685. E-mail addresses: [email protected] (J. Wu). 0016-7061/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.geoderma.2008.06.006
scales, and its impacts on global C cycling (Gorham, 1991; Roulet, 2000; Bai et al., 2005). It has been estimated that the global C stock down to 1 m in wetlands is 225 Pg (1 Pg = 1015 g) (IPCC, 2000). This value compares favorably with the estimated range (180–249 Pg) given in earlier work (Post et al., 1982; Armentano and Menges, 1986). Gorham (1991) proposed that the C stock of wetlands was largely underestimated in early work, which used 1 m as the mean depth, as used for terrestrial soil. He estimated that the average depth of the peat layer in boreal and sub-arctic peatlands was 230 cm, and that the total organic C storage down to this depth was 445 Pg, a value much larger than that for global wetlands given by earlier estimates. Other studies suggested that the C storage of wetlands varied considerably for the different climatic zones and was affected by vegetation type and landscape and hydrologic conditions (Gorham, 1991; Tolonen et al., 1992; Roulet, 2000; Bockheim et al., 2003; Moore and Turunen, 2004; Bai et al., 2005). Temperate wetlands are particularly important in this regard as it has been estimated that most of the wetland area and associated C storage is in peatlands in temperate and boreal regions, with roughly 10–30% occurring in the tropics (IPCC, 2000). To reduce the uncertainties in the estimates of the C stock in wetlands, there is an urgent need to obtain more reliable information and this requires that an appropriate sampling depth be used in
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making the necessary calculations for wetlands in different climatic regions. This paper reports on the contents and distribution of OC in profiles of the three major types of temperate wetlands in Northeast China. The aim was to provide factual knowledge as a basis for obtaining a better understanding of the major mechanisms determining C storage in temperate wetlands. 2. Materials and methods 2.1. Sites and sampling The Sanjiang Plain region (129°11′–135°05′N, 43°50′–48°27′E) is a river basin in Northeast China, consisting of typical inland wetlands spreading over hillocks, river terraces, and floodplains. This region has a temperate climate, with annual average temperatures ranging from 1.9 to 3.9 °C and rainfall from 500 to 650 mm. It currently has a total wetland area of about 20, 000 km2; of which 20% is peat fen (PF), 43% is humus marsh (HM), and 37% is marshy meadow (MM) (Wang et al., 2003). Sixteen sites were selected for representative profiles under PF (six profiles), HM (five profiles), and MM (five profiles) in the Sanjiang Plain region, with the general information for each site as described in Table 1. In order to minimize disturbance to the sediment profiles, sampling was done during the frozen period (April) in 2002 and 2003. Sediment profiles were hand-dug down to a depth of 120–200 cm, and triplicate samples were taken from each 5-cm section of the profiles and sealed in plastic bags. After the removal of fresh roots and plant residues by hand picking, samples were dried in an oven at 60 °C and sieved (b1 mm and N0.15 mm) for further analyses. 2.2. Analyses Total organic C, referred to as OC, in the samples was determined using an automated carbon analyzer (VARIO MAX CN, Germany), and dry matter by weighing after 48 h oven-drying (105 °C).
Light fraction organic matter (SG ≤ 1.7) was isolated by a procedure slightly modified from Janzen et al. (1992). A portion (1–5 g, according to litter content) of the sample was weighed into an 80ml centrifuge tube and 40 ml NaI solution (SG 1.7) added. The suspension was dispersed with an ultrasonic vibrator (SB5200, 20–40 kHz, 300 W) for 10 min and then centrifuged at 4200 r min− 1 for 15 min, and the suspended light fraction organic matter was collected on a 420-mesh sieve. The residue was dispersed and centrifuged four more times. The light fraction organic matter collected was washed with 0.01 M CaCl2 and distilled water to completely remove residual NaI, dried at 60 °C for 48 h, weighed, and then ground to pass 100-mesh. Dry matter and OC contents of the light fraction organic matter were determined. 2.3. Estimation of organic C storage in the wetland sediments Because there were no visible air bubbles, the air volume of the frozen sediments can be taken as being negligible. Therefore, the mean density (Di) of a sediment section (referred to as i) can be expressed as follows: Di ¼
1 ðWw =Dw þ Wm =Dm þ Wo =Do Þ
where Ww, Wm, and Wo (all in g g− 1) indicate the contents of water, mineral material, and organic matter in the section, respectively; and Dw, Dm, and Do (all in g cm− 3) indicate the densities for water (frozen), mineral, and organic components, respectively. The bulk density of dry matter (Dm, excluding water) in the section is calculated as: Dm ¼ Di ð1−Ww Þ
Profile Type of no. wetlandsa
Landscapeb
Predominant vegetation
Sampling depth (cm)
1
PF
River floodplain (P)
200
2
PF
River floodplain (P)
3
PF
River floodplain (P)
4 5
PF PF
River floodplain (P) River floodplain (P)
6
PF
River floodplain (P)
7 8 9
HM HM HM
River floodplain (P) Hillock (P) River floodplain (P)
10
HM
River floodplain (P)
11
HM
River floodplain (P)
12 13 14
MM MM MM
Terrace of river Terrace of river Terrace of river
15
MM
Terrace of river
16
MM
Hillock (S)
Carex pseudocuraica and Deyeuxia angustifolia Carex pseudocuraica and Deyeuxia angustifolia Carex lasiocarpa and Carex pseudocuraica Deyeuxia angustifolia Carex pseudocuraica and Deyeuxia angustifolia Carex pseudocuraica and Deyeuxia angustifolia Carex meyeriana Carex lasiocarpa Carex meyeriana and Deyeuxia angustifolia Carex meyeriana and Carex pseudocuraica Phragmites australis and Deyeuxia angustifolia Deyeuxia angustifolia Deyeuxia angustifolia Salix brachyboda and Deyeuxia angustifolia Betula platy phyla and Deyeuxia angustifolia Deyeuxia angustifolia
a
200 200 200 200 200 160 160 200 150 160 200 200 120 120 150
PF refers to peat fen (or fen in the European classification), HM to humus marsh (or marsh in the European classification), and MM to marshy meadow. b Letter P in the parentheses refers to permanently waterlogged and S to seasonally waterlogged.
ð2Þ
Taking Di from Eq. (1), then: Dm ¼
Table 1 Description of sampling sites for the wetland profiles in the Sanjiang Plain region
ð1Þ
ð1−Ww Þ ðWw =Dw þ Wm =Dm þ Wo =Do Þ
ð3Þ
Density of OC (Ci, g cm− 3) for the section is calculated as: Ci ¼ Dm Wc
ð4Þ
where Wc indicates OC content of dry matter (g g− 1). Thus, OC storage (TC, t ha− 1) of the sediment profile from the top down to the j section can be calculated as: j
TC ¼ ∑ Ci Hi
ð5Þ
i¼1
where Hi indicates the thickness (5 cm) of the i layer. For the calculation of TC, the densities of the water (frozen) and mineral components were assumed to be 0.9 and 2.6 g cm− 3, respectively (Boyd, 1995). For the organic horizon where specific gravity was smaller than 1.0 g cm− 3, the mean density of the organic matter was assumed to be 1.0 g cm− 3. For the illuvial horizon (SG N 1.0, as proposed in this study), the mean density of the organic matter was assumed to be 1.25 g cm− 3, as proposed in previous studies for mineral soils (Boyd, 1995; Avnimelech et al., 2001). 3. Results 3.1. Bulk density For each of the six profiles from PF, bulk density (excluding water) was small (0.1–0.3 g cm− 1) in the top 50–75 cm (Fig. 1). In
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313
ing 3 profiles from PF, OC content declined smoothly from 70 cm down to 120–150 cm. In the 5 sediment profiles from HM, OC content increased substantially from values of 180–290 g kg− 1 in the top section (0–5 cm) to 310–450 g kg− 1 at a depth of 15–25 cm (Fig. 2). In 4 of these 5 profiles, OC content declined sharply to a level smaller than 5 g kg− 1 by 25–45 cm, with only one exception in which OC content declined gradually between 15 and 75 cm. For the MM profiles, OC content declined sharply from a value within the range of 40–110 g kg− 1 in the top to levels below 5 g kg− 1 at 15–50 cm (Fig. 2). 3.3. Light fraction organic carbon contents In PF, light fraction OC varied from 1.4 to 90.3% of total OC (with a mean value of 48.7 ± 36.4%) in the organic horizon (Fig. 3). In the top 60 cm of this wetland type, light fraction OC accounted for about 70% of total OC. For HM, light fraction OC in the upper 0–20 cm was
Fig. 1. Bulk density of wetland profiles from the Sanjiang Plain of Northeast China. PF, HM, and MM refer to peat fen, humus marsh, and marshy meadow, respectively; the numbers refer to the profiles as listed in Table 1.
three of the PF profiles, bulk density increased up to 1.0 g cm− 1 at a depth of about 80 cm, but for the other three profiles not until a depth of between 115 and 155 cm. For profiles from HM, bulk density reached this value at a depth varying from 25 to 75 cm (Fig. 1). In MM profiles, bulk density increased sharply and reached a value of 1.0 g cm− 1 in the top 15–40 cm (Fig. 1). As the depth increased by a further 10–15 cm, bulk density for all profiles from the three types of wetlands increased to 1.3–1.8 g cm− 1 and then remained relatively constant. 3.2. Organic carbon contents and distribution OC contents in the top 50–70 cm of the PF profiles varied from 330 to 470 g kg− 1 (Fig. 2). For 3 PF profiles, OC content declined sharply from 60 or 80 cm to an amount smaller than 5 g kg− 1, and below this depth it remained essentially constant. For the remain-
Fig. 2. Organic C content of wetland profiles from the Sanjiang Plain. See Fig. 1 for the description of abbreviations and numbers.
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Fig. 3. Percentage of light fraction organic C to the total organic C in selected wetland profiles of peat fen (○, profile no. 1), humus marsh (●, profile no. 9), and marsh meadow (△, profile no. 15) from the Sanjiang Plain.
more than 70% of total OC. From 20 to 40 cm, this percentage decreased to 1%. Light fraction OC varied from 1 to 86% (mean of 46.0 ± 35.3%) in the organic horizon of the HM. However, in MM, light fraction OC accounted for only 4 to 18% (with a mean value of 13.2 ± 5.2%) of total OC in the organic horizon. Hardly any light fraction organic matter could be separated by this fractionation method from the illuvial horizon. There was a highly significant correlation ( R 2 = 0.96 ⁎⁎ ) between light fraction OC and total OC for the three types of wetlands (Fig. 4). This relationship also showed that the content of light fraction OC changed only slightly when total OC was low (less than 200 g kg− 1). However, when total OC was more than 200 g kg− 1, light fraction OC increased considerably, according to the content of total OC. 3.4. Organic carbon density The distribution of OC density in the PF profiles showed a dual pattern in that the upper layer had high but variable OC density, and the lower layer had a low but more uniform OC density (Fig. 5). Also, the thickness of the upper, high OC density layer (80–155 cm) was almost the same as that of the layer with a low bulk density (Fig. 1) and high OC content (Fig. 2). For the six PF profiles, OC density of the upper layer varied from 36.7 to 164.8 kg m− 3, with a mean of 71.5 kg m− 3 and a coefficient of variation of about 21% (Fig. 5). OC density of the upper layer varied in the range of 24 to 74 kg m− 3 (with a mean of 47.0 kg m− 3), and 29 to 45 kg m− 3 (with a mean of 38.7 kg m− 3) for the HM and MM profiles, respectively. For all the
Fig. 5. Organic C density of wetland profiles from the Sanjiang Plain. See Fig. 1 for the description of abbreviations and numbers.
PF, HM, and MM profiles, OC density in the lower layer varied from 2.4 to 48.8 kg m− 3, with a mean of 8.3 kg m− 3. 4. Discussion 4.1. Distribution of organic C in the wetland profiles
Fig. 4. Correlation between light fraction organic C and total organic C at selected wetland profiles (no. 1, 9, 15, Table 1) from the Sanjiang Plain.
In the permanently waterlogged PF and HM profiles, OC content varied to a much larger extent than that in seasonally waterlogged MM profiles. The hydrological condition is one of the main factors that cause the large amount of OC to be accumulated in PF and HM profiles (Table 1). Permanent waterlogging induces anaerobic conditions, under which OC in litter decomposes only slowly (Esteves et al., 2001). However, with seasonal change in the hydrologic condition in MM, the decomposition environment is aerobic for most of the time and there is no similar restriction. Brinson et al. (1981) suggested that decomposition was optimized where cycles of wetting and drying prevail. It has also been reported that brief flooding, followed by moist but well-aerated
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conditions, causes maximum OC loss (Lockaby and Wailbridge, 1998). Therefore, it is expected that the content of OC in MM will be substantially lower than that in PF and HM and this was found to be the case. The order for OC content is similar to that reported by Ma et al. (1996) for the Sanjiang Plain. However, the OC contents in the three types of wetland are much higher than those reported for river marginal floodplains in China (Bai et al., 2005). The low and rather constant OC content in the illuvial horizon resulted from illuviation of mineral matter under waterlogged conditions. This is usually a characteristic of OC in mineral subsoil below peat (Moore and Turunen, 2004). Wetland profiles usually develop an organic horizon in the upper layer and an illuvial horizon (Neustadt,1984; Gorham,1991; Frolking et al., 2001). However, there has been no clear basis for separating these two layers and this becomes particularly important in establishing the profile depth to be considered in estimating C storage. Here, we propose that the layer of the profile that has a bulk density smaller that 1.0 g cm− 1 should be taken as the organic horizon in the wetland profiles studied in our work. A careful analysis of the raw data for bulk density with increasing depth (Fig. 1) indicates that, in almost all cases, this corresponds very closely with the pattern for OC content (Fig. 2) and OC density (Fig. 5). Thus, there is no advantage in taking a bulk density different from 1.0 g cm− 1 for deciding the depth of the organic horizon. It is worth noting that the thickness of this horizon could possibly change with seasonal fluctuation in water level of the wetlands studied in this work. As indicated by the data presented in Fig. 1, the thickness of the organic horizon differed substantially for the three different types of wetlands in the Sanjiang Plain. Even for the same type of wetland, the thickness of this horizon also varied quite widely, as in the range of 80 to 155 cm for PF, 25–75 cm for HM, and 15–40 cm for HM (Fig. 1). The mean thickness of the organic horizon in PF was 100 cm, which is comparable to that (110 cm) in wetlands in temperate Fennoscandia (Lappaleinen, 1980) but substantially smaller than that (230–250 cm) for peatlands in the Frigid Zone. Also, the thickness of the peat layer determines the amount of OC stored in wetlands. For example, Neustadt (1984) estimated that the mean thickness of the peat layer was 250 cm for peatlands in Russia. Gorham (1991) proposed a mean thickness of 230 cm for the peat layer of boreal and sub-arctic peatlands but in the temperate zone it is clearly smaller. The mean thickness of the organic horizon of HM and MM profiles in the Sanjiang Plain is much thinner than that of the PF profiles. However, the thickness of the organic horizon of HM and MM was in the range of that in drained thaw-lake basins in Arctic Alaska (Bockheim et al., 2004). The developmental history of wetland ecosystems is expected to contribute mainly to these differences, because there is usually a significant linear relationship between the thickness of the organic horizon and 14C age (Bockheim et al., 2004). Data presented in Fig. 5 show that OC density in the organic horizon varied substantially among the three types of wetlands, being much higher in PF than that in agricultural or forest soils in the same general climatic area (Wang et al., 2002a). For HM and MM, OC in the organic horizon was equal to or a little higher than that in paddy topsoils in China (Pan et al., 2003). Although OC contents in the illuvial horizon were quite small for all the three types of wetlands, OC density was reasonably high, due to the high bulk density of this horizon. Moore and Turunen (2004) also suggested that mineral soil beneath peat could accumulate substantial amounts of OC and it was estimated that the subsoil OC pool could be 10 Pg globally, this being derived primarily from the sorption of dissolved OC in the subsoil and preserved under anoxic condition (Turunen et al., 1999). 4.2. Organic C storage in temperate wetlands For the three types of wetlands, OC storage was estimated for the depth of the organic horizon, for a total depth of 1 m (which includes the organic horizon), and for the organic horizon + 1 m of the mineral illuvial horizon (Table 2).
315
Table 2 Storage of organic C estimated for different depths in the three types of wetlands in the Sanjiang Plain region Wetland Numbers type of profile
PF HM MM
6 5 5
Thickness of OH Organic C storagea (t ha− 1) (cm) In OH In 1 m 100 ± 29.8 39 ± 18.8 20 ± 9.4
OH + 1 m IH
715 ± 240 (86.3) 645 ± 86 (77.9) 828 ± 223 179 ± 88 (65.3) 252 ± 93 (92.0) 274 ± 106 79 ± 32 (46.2) 160 ± 23 (93.6) 171 ± 27
a Values in parentheses indicated the percentage of organic C storage in the organic horizon and in 1 m depth to the total organic C storage in the organic horizon plus 1 m illuvial horizon combined; OH and IH denoted the organic horizon and the illuvial horizon, respectively.
The mean OC storage per unit area in the organic horizon for PF was 715 ± 240 t ha− 1, which was about four and nine times larger than that in HM and MM, respectively (Table 2). For a total depth of 1 m, it was 252 ± 93 and 160 ± 23 t ha− 1 for HM and MM, respectively. These values amounted to 92 and 94% of those in the organic horizon + 1 m of the illuvial horizon. However, for PF, OC storage per unit area in 1 m was 645 ± 86 t ha− 1, which was only 78% of that in the organic horizon plus 1 m of the illuvial horizon. The fact that OC storage varied in the different wetland profile types and also with the depth used for calculation (organic horizon, total depth of 1 m, or organic horizon + 1 m of the illuvial soil), shows that the depth used for estimation is particularly important for calculating OC storage in different types of wetlands. Traditionally, OC storage has been reported for the upper 1 m, which is helpful for comparing the results of different researchers (Post et al., 1982). However, the thickness of the organic horizon in PF is sometimes more than 1 m and OC storage per unit area is greatly underestimated if only 1 m is taken as the depth in making the estimate. The data obtained suggest that the organic horizon plus 1 m of the illuvial horizon provides a more reasonable basis for obtaining reliable OC storage data for the temperate wetland profiles studied here. Light fraction OC is mainly composed of plant residues which are incompletely decomposed. The proportion of light fraction OC in wetlands is considered to be sensitive to management and changes in microenvironment (Barrios et al., 1997; Six et al., 2000). The high ratio for light fraction OC to total C in the present study implies that the OC accumulated in the PF and the HM is dominated by the light fraction and that this could be decomposed easily when hydrological conditions changed. The fractionation result for MM clearly shows that seasonal flooding accelerates the decomposition of light fraction OC and this may be the reason why OC content decreases so rapidly as a wetland is drained. In fact, much of the MM peatland on the Sanjiang Plain has been cultivated in recent years, with the area occupied by this wetland type decreasing from 7309 km2 in 1980 to a very low figure in 2000 (Wang et al., 2002b). In the same time period, PF showed no change whereas for HM the corresponding areas had decreased from 8300 to 5228 km2. 5. Conclusion By the use of a bulk density value of 1.0 g cm− 1 it was possible to make a more reliable assessment of the depth of the organic horizon in the wetland profiles in the Sanjiang Plain in temperate Northeast China. The mean thickness of the organic horizon varied from 100 to 39 to 20 cm for the PF, HM, and MM profiles, respectively, which is of particular significance in selecting an appropriate depth for assessing OC storage. For each profile, a total depth of the thickness of the organic horizon plus 1 m of the underlying mineral soil was used. Approximately 828 ± 223, 274 ± 106, and 171 ± 27 t ha− 1 OC have accumulated in profiles of PF, HM, and MM, respectively. Light fraction OC was the main OC fraction accumulated in the organic horizons and
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