Soil organic matter to soil organic carbon ratios of peatland soil substrates

Geoderma 235–236 (2014) 410–417

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Soil organic matter to soil organic carbon ratios of peatland soil substrates C. Klingenfuß ⁎, N. Roßkopf, J. Walter, C. Heller, J. Zeitz Humboldt-Universität zu Berlin, Germany

a r t i c l e

i n f o

Article history: Received 13 September 2013 Received in revised form 23 June 2014 Accepted 1 July 2014 Available online 17 July 2014 Keywords: Conversion factor Peatland soil Peat type Carbon stocks

a b s t r a c t Data of soil organic matter (SOM) content can be used for the assessment of stocks and changes of soil organic carbon (SOC) in peatland soils. Therefore, it is essentially necessary to convert SOM contents into SOC contents by SOM/SOC ratios (“conversion factors”). Various substrates of peatland soils in Northeastern Germany (n = 311) were analyzed in SOM/SOC ratios. Sphagnum peats showed significantly higher SOM/SOC ratios (2.05 ± 0.09) than peats of vascular plants (1.73 ± 0.09) and amorphous peats (1.93 ± 0.29). Amorphous peats and humic sands (2.41 ± 0.46) showed a high variability. The classification using WRB qualifiers featured significant differences (***P b 0.001) between humic, sapric, hemic and fibric substrates, except hemic and fibric peats of vascular plants. Moreover, impacts of drainage on pedogenesis could be proved in different SOM/SOC ratios of drained topsoils and water-saturated subsoils. Due to the high dependency of SOM/SOC ratios on the botanical origin, the implementation of peat type-related conversion factors is most suitable. In contrast, the application of one single conversion factor causes considerable conversion errors. As a consequence, many SOC assessment studies referring to peats should be reviewed. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Peatlands cover only 3% of the world's land area but their peats contain 30% of all the soil organic carbon (Byrne et al., 2004; Joosten and Couwenberg, 2008; Strack et al., 2008). Peatland soils play an important role in the global climate system as a sink or source of greenhouse gases such as carbon dioxide (CO2) and methane (CH4). The assessment of soil organic carbon (SOC) stocks and stock changes becomes increasingly important in the context of climate change and site-adapted land use of peatlands (Lal, 2004; Bellamy et al., 2005; Zauft et al., 2010; Intergovernmental Panel on Climate Change, 2011; Keaney et al., 2013). Much data of SOM contents from peatland soils is available for the assessment of SOC stocks and the changes that affect them. The data derives from the reliable and cost efficient loss-on-ignition (LOI) method (Schmidt and Scheibner, 1988; Schulte and Hopkins, 1996; Bhatti and Bauer, 2002; Konen et al., 2002). SOM/SOC ratios of soil substrates can be used as conversion factors for the calculation of SOC contents. The importance of substrate-specific conversion factors has been pointed out by Baldock and Nelson (2000). However, various studies adopt only one single unspecific conversion factor from literature (1.72 to 2.0) for various soil types and substrates (Table 1). Moreover, there are a low number of investigations on SOM/SOC ratios referring to peats (Table 2). In comparison to peats the average SOM/SOC ratio including organic and mineral soil substrates is 2.2 (Pribyl, 2010).

⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (C. Klingenfuß).

http://dx.doi.org/10.1016/j.geoderma.2014.07.010 0016-7061/© 2014 Elsevier B.V. All rights reserved.

SOM/SOC ratios depend on the substrate, the pedogenesis and the degree of decomposition (Kluge et al., 2008). Peat in general contains a huge amount of defined structural molecules (lignin, cellulose, hemicellulose) of peat forming plants due to the inhibited decay under watersaturated anaerobic conditions (Bohlin et al., 1989; Bergner et al., 1990; Koppisch, 2001). However, groups of plants differ in the contents of carbon-rich compounds. Sphagnum peats show a high content of comparatively carbon-poor carbohydrates (C 44%; arithmetic mean following to Stevenson, 1994; Paul and Clark, 1996; Pribyl, 2010) and uronic acids (C 37%; arithmetic mean following to Stevenson, 1994; Paul and Clark, 1996) (Van Smeerdijk and Boon, 1987; Bergner et al., 1990; Ringqvist and Öborn, 2002; Kellner, 2003). Woody tissues of vascular plants feature high contents of carbon-rich lignin (C 67%; arithmetic mean following to Ottow, 2011) and aliphatic compounds like cutin, suberin and waxes (C N70%; arithmetic mean following Stevenson, 1994; Paul and Clark, 1996) (Dierssen and Dierssen, 2001; Schellekens et al., 2009; Schellekens and Buurman, 2011). In general, aliphaticity seems to increase with rising peat age and depth whereas carbohydrate content decreases (Pontevedra-Pombal et al., 2001; Buurman et al., 2006). The drainage of peatland soils initializes the aerobic decomposition and mineralization of peats within the aerated topsoil horizons. As a result chemical attributes of peats are strongly altered (Grosse-Brauckmann, 1990; Zeitz and Velty, 2002). Drained peats may contain substantial amounts of secondary microbial metabolites and residues (Paul and Clark, 1996; Kögel-Knabner, 2002; Okruszko and Ilnicki, 2003; Haider and Schäffer, 2009; Schmidt et al., 2011). Continuing decomposition reduces the peat fiber content which can approximately be described by the peat qualifiers fibric (high fiber content), hemic (medium) and sapric (low) (Sprecher, 2001).

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Table 1 Overview of studies in which conversion factors SOM/SOC for the assessment of SOC stocks and stock changes including peatland soils were used. Reference

Reference area

Application

Intention

Used conversion factor SOM/SOC

SOC/SOM

Source

Buringh (1984) Batjes (1996)

World World

All soil types All soil types

SOC stocks SOC stocks

1.72a 1.72a

0.58 0.58

Guo and Gifford (2002) Kuikmann et al. (2003) Bellamy et al. (2005) Jones et al. (2005) Montanarella et al. (2006) Van den Akker et al. (2008)

World The Netherlands England and Wales Europe Europe The Netherlands

All soil types Agricultural soils Soils with approx. SOC N15% All soil types Peat soils Peat soils

1.72a 2.0 2.0 1.72a 1.72a 1.81

0.58 0.5a 0.5a 0.58 0.58 0.55a

State of Berlin (2010)

State of Berlin

All soil types

SOC stocks and land use change SOC stocks SOC losses SOC stocks Peatland map Europe Estimation of emission of CO2 from agricultural peat soils SOC stocks (map)

n.s.b Grewal et al. (1991), Schlesinger (1977) Mann (1986) n.s.b n.s.b n.s.b n.s.b Schothorst (1977)

1.72a

0.58

AG Boden (2005)

a b

Original information. n.s. = non-specified.

The objective of the study was the identification of the level of SOM/ SOC ratios (conversion factors) for different substrates from peatland soils and peat conditions after WRB qualifiers. We expected clear differences between SOM/SOC ratios because of different chemical compositions of peat forming plants and organic substrates. We also analyzed the impacts of drainage on SOM/SOC ratios on soil horizons due to a strong influence of soil water conditions on peat forming and peat decomposition. Furthermore, we discussed possible conversion errors and the requirement of adaptations for future SOC assessments in peatland soils.

2. Methods 2.1. The definition of “peatland soil” According to the German Soil Classification (AG Boden, 2005), peatland soils consist of peat with SOM contents of ≥30% and a thickness of the peat layers ≥30 cm. As a consequence of peat formation, almost all peatland soils, classified after AG Boden (2005), are histosols following the definition of the IUSS Working Group WRB (2007).

2.2. Study area and sampling The 21 test sites are located in Northeastern Germany, where peatland soils developed above sediments from the Late Weichselian glacial (Dickinson, 1964). Except a few coastal sites, peatlands in Northeastern Germany are fens (Succow and Edom, 2001), of which some have formed as Sphagnum mires. The climate is characterized by a transition from the oceanic climate in the North and West to a more continental climate in the Southeastern parts. The annual average temperature is around 8 °C and the annual average of precipitation ranges from 687 mm in coastal areas to 443 mm further inland (Hendl, 1995). In Northeastern Germany considerable areas of agriculturally used peatland soils have been degraded for many decades due to aerobic decomposition and lost their original structural, physical and chemical characteristics (Succow, 2001; Zeitz and Velty, 2002). Humic sands (b30% SOM) from peatland sites have derived from a long-term peat degradation and sand-input by bioturbation or land management (Schleier, 2007). Soil surveys and sampling campaigns were carried out between 2009 and 2012. In total, 311 samples were taken by using a peat chamber drill (Eijkelkamp GmbH). The Sphagnum peats (n = 34) were

Table 2 Overview of studies carried out on SOM/SOC ratios or conversion factors referring to peatland soils or peat substrates. Reference

Country

Substrates and peat types

N

SOM/SOC ratio (Conversion factor) Low

Average

High

Robinson et al. (1929) Schmidt and Scheibner (1988)

Wales Eastern Germany

n.s.a 54

1.86 1.79

1.88 1.98

1.95 2.18

Succow (1988) Bohlin et al. (1989)b

Northeastern Germany Northern Sweden

Naucke (1990)b Paepke (1992) Kracht and Gleixner (2000)b Bhatti and Bauer (2002)c

Northwestern Germany Northeastern Germany Southwest Germany Continental Western Canada

Kluge et al. (2008)

Northeastern Germany

Peat Peat substrates (Phragmites peat, sedge peat, Cladium peat, brown moss peat, Alnus peat) without degradation by pedogenesis Peat of 70–90 mass-% SOM (“full peat”) Carex–Eriophorum and Carex–Equisetum peat Carex peat Carex-moss peat (brown moss and Sphagnum) Sphagnum-dwarf-shrub peat Sphagnum peat Sphagnum peat Amorphous peat Sphagnum peat Sphagnum peat Woody Sphagnum peat Herbaceous peat Woody herbaceous peat Woody peat Limnic peat Different peat and mud substrates

n.s.a 4 7 7 5 4 n.s.a 17 3 53 55 38 72 14 12 n.s.a

n.s.a 1.59 1.55 1.61 1.63 1.73 1.59 1.92 1.91 n.s.a n.s.a n.s.a n.s.a n.s.a n.s.a 1.64

1.73 n.s.a n.s.a n.s.a n.sa n.s.a n.s.a 1.93 n.s.a 2.0 1.96 1.87 1.87 1.79 1.82 n.s.a

n.s.a 1.64 1.68 1.72 1.76 1.85 2.08 1.94 2.31 n.s.a n.s.a n.s.a n.s.a n.s.a n.s.a 2.84

a b c

n.s. = non-specified. SOM/SOC ratios are derived from SOM carbon contents. SOM/SOC ratios are derived from regression equations.

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sampled from three acid and nutrient-poor kettle hole mires (Diebelsee, Barschpfuhl, Mooskute) and one terrestrialization mire (Möllersches Luch) without any land use, whereas the other peats and substrates were derived from ten minerotrophic and more or less nutrient-rich peatlands which were used as pasture or alder forest (see KML-file). Due to the different drainage depths the sampling depth of drained topsoil horizons varied between the sites and ranged between 0 and 50 cm below surface. Water saturated subsoil horizons were sampled at 20–300 cm depth. We took composite samples (300 g) from tree subsamples per horizon. The samples were sealed in plastic bags and stored at 2 °C awaiting analysis. They were collected in the context of various research projects, carried out at the Division of Soil Science and Site Science of “HumboldtUniversität zu Berlin”.

3. Results 3.1. Substrates and peat types For SOM/SOC ratios of peat types, Alnus peat (1.71 ± 0.1) shows the lowest ratio, whereas the highest ratio was found for Sphagnum peat (Fig. 1, Table 4). No significant difference among peat types could be observed between peats of vascular plants. This same result was found for amorphous peat, brown moss peat and Phragmites peat. In contrast, SOM/SOC ratios of Sphagnum peat and humic sands always showed significant differences (***P b 0.001) compared to other peat types. Moreover, the SOM/SOC ratio of brown moss peat differed from the one of radicell and Alnus peat (**P b 0.01). Amorphous peat showed the highest variation of SOM/SOC ratios among all peat types. The variation within humic sands exceeds the variation of all peats.

2.3. Laboratory analyses 3.2. Substrate and peat conditions For the analyses of SOM we applied the LOI method at 550 °C for 4 h (Schulte and Hopkins, 1996). Prior to the LOI treatment living plant remnants were removed and soil samples were dried at 60 °C. Organic matter contents were calculated as the difference between the initial sample weight without living roots and the final sample weight after drying. The SOC was measured with the “vario max C” element analyzer (Elementar GmbH, Hanau), which works on the basis of catalytic dry combustion at 600 °C. All data was presented on a dry mass basis (oven-dried at 105 °C) and performed as duplicates (DIN ISO, 10694, 1994). 2.4. Data analyses First, we separated the class of humic sands, which only represents drained topsoils of peatlands that contain b 30% SOM. Secondly, we identified two peat type groups by their botanical origin. These vascular plant peats and moss peats were further subdivided into peat types (AG Boden, 2005) (Table 3). Mixed peat types like brown moss-radicell peat were separated and not evaluated due to unclear proportions of peat types. In order to evaluate peat conditions, we classified the dataset after WRB qualifiers (IUSS Working Group WRB 2010) describing the degree of decomposition and fiber contents (Sprecher, 2001). Hemic and fibric peats were divided into classes of moss peats and vascular plant peats on the basis of the differences in their botanical origin. Sapric peats were always amorphous peats of unknown botanical origin. The impacts of drainage were considered by grouping drained topsoil horizons and water-saturated subsoil horizons (Table 3). All plots and statistical analyses were made using the free available software package R (R Development Core Team, 2008). Differences were analyzed by boxplots, which visualize the variation within classes. Quantitative differences between all the classes were evaluated by performing the non-parametric Wilcoxon test accounting for the nonnormality of same substrate classes.

The dataset was arranged following the qualifiers of the WRB (IUSS Working Group WRB, 2007) (Fig. 2). The analyses of SOM/SOC ratios revealed no significant differences between the fibric (1.73 ± 0.08) and hemic peats (1.75 ± 0.12) of vascular plants. In contrast, fibric moss peat (2.01 ± 0.08) and sapric peat (1.93 ± 0.28) showed significant differences (***P b 0.001) compared to other substrate and peat condition classes. 3.3. Impacts of drainage depth on soil horizons The level of SOM/SOC ratios of water-saturated horizons was determined by the attributes of peat types and peat conditions. Subsoils from vascular plant peats are significantly different (***P b 0.001) compared to drained topsoil horizons of amorphous peat (Fig. 3). Furthermore, the results for drained topsoil horizons varied more than those of watersaturated subsoil horizons (Table 4). Humic sands from drained topsoil horizons showed the highest variation. 4. Discussion 4.1. Level of SOM/SOC ratios The used SOC data found in the literature mainly derived from Walkley and Black's oxidation method (Walkley and Black 1934) with K2Cr2O7 and seems to underestimate SOC amounts (e.g. De Vos et al., 2007). Therefore, the reviewed SOM/SOC ratios may sometimes be too high. Despite the application of different methods for SOC analysis our findings for peats (SOM/SOC ratios: 1.71–2.05) were consistent with average SOM/SOC ratios between 1.73 and 2.0 reported in eight studies focusing on peat substrates (Table 2). Mean SOM/SOC ratios of all investigated peat types from Northeastern Germany were lower than the empirical mean (2.2) found by Pribyl (2010) for all soil substrates.

Table 3 Substrate classes for data analyses of SOM/SOC ratios on peatland soils in Northeastern Germany (n = 311). Substrate classification after SOM content

Peat type groups

Peat types

Substrates and peat conditions

Soil horizons

Peat (SOM ≥ 30%)

Peat of vascular plants (n = 86)

Radicell peat (n = 64)a Phragmites peat (n = 9) Alnus peat (n = 13) Sphagnum peat (n = 34)

Hemic (n = 54)

Fibric (n = 32)

Water-saturated subsoil (n = 86)

Fibric (n = 34)

Brown moss peat (n = 10)

Hemic (n = 2)b Sapric (n = 106) Humic (n = 73)

Water-saturated topsoil and subsoil (n = 34) Water-saturated subsoil (n = 10) Drained topsoil (n = 106) Drained topsoil (n = 73)

Moss peat (n = 44)

Humic sand (n = 73) (SOM b 30%) a b c

Amorphous peat (n = 106)c –

–

Radicell peat contains roots of sedges (Carex sp.) and other herbaceous plants. No statistical analyses. Peat without determinable peat forming plants.

Fibric (n = 8)

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Fig. 1. SOM/SOC ratios of different substrates and peat types (botanical origin) from peatland sites of Northeastern Germany.

plant peat) containing specific carbon contents. Therefore, they show a relatively low natural variation of SOM/SOC ratios, compared to high-decomposed, amorphous peats (Table 2). The main driver for high carbon contents within SOM are carbonrich molecules such as lignin. A particularly high lignin percentage in peats (Bergner et al., 1990; Koppisch, 2001; Heller and Zeitz, 2012) is supposed to cause low SOM/SOC ratios in less decomposed peats of vascular plants and woody peats, as we found in radicell peat (1.73) and Alnus peat (1.71). High contents of alder (Alnus glutinosa) woods within Alnus peats involve high lignin contents and therefore low SOM/SOC ratios. Wood contains high portions of lignin up to 30% (Bridgham and Lamberti, 2009; Haider and Schäffer, 2009; Crawford et al., 1982). This is in line with the results of Bhatti and Bauer (2002), who also showed the lowest SOM/SOC ratios (1.79) for woody peats among six peat types from continental Western Canada. In addition, high contents of carbon-rich aliphatic compounds in vascular plants (Dierssen and Dierssen, 2001; Schellekens and Buurman, 2011) are likely to be responsible for comparatively low SOM/SOC ratios. During the aging of peat, those aliphatic compounds are slowly accumulating while carbohydrate content is decreasing (Pontevedra-Pombal et al., 2001). Compared to Alnus and radicell peats, SOM/SOC ratios of investigated Phragmites peats showed a skew distribution. These samples

The results indicate differences of the SOM composition of peat in comparison to the humus of mineral substrates (b30% SOM). Further, humic sands showed significantly higher SOM/SOC ratios and greater variations compared to all peats. Their mean SOM/SOC ratio is close to the theoretical limit of 2.5, which applies to “a soil consisting entirely of fractions having the lowest OC/OM ratios of about 40% carbon” Pribyl (2010). The high ratio suggests relatively low amounts of carbon-rich compounds (e.g. lignin, lipids, fats, waxes and resins). Assumably, enhanced mineralization because of drainage and intensive agricultural use results in strong qualitative changes of SOM from highly-organic to degraded sandy soils of former peatlands (Zeitz and Velty, 2002; Mueller et al., 2007; Heller and Zeitz, 2012). Microbial metabolites (e.g. carbohydrates) from aerobic peat decomposition can form a substantial part of SOM (Tate, 2000) and have to be considered also as a possible explanation for the relatively low carbon contents of SOM in humic sands. 4.2. Peat types — botanical origin Peat types are determined by peat forming plants. Fibric peats mostly consist of undecomposed and well-defined structural plant compounds like cellulose, uronic acid (Sphagnum peat) or lignin (vascular

Table 4 Characteristics of investigated peatland soil substrates, peat types and peatland soil horizons in Northeastern Germany. Substrates

Soil horizons

Study results

Moss peat Sphagnum peat

Brown moss peat

≥30% SOM

Vascular plant peat (radicell peat; Alnus peat; Phragmites peat) ≥30% SOM

≥30% SOM

≥30% SOM

Humic

Sapric

Hemic and fibric

Hemic and fibric

Hemic and fibric

Drained Topsoil Secondary soil development and higher sand content 12.8 ± 8.7 5.8 ± 4.4 2.41 ± 0.46 0.43 ± 0.07 73

Drained Topsoil Secondary soil development

Water-saturated Subsoil No secondary soil development

Water-saturated Topsoil and subsoil No secondary soil development

Water-saturated Subsoil No secondary soil development

64.2 34.2 1.93 0.53 106

84.9 49.1 1.73 0.58 86

95.4 46.7 2.05 0.49 34

91.7 50.1 1.83 0.55 10

Substrates and peat type (groups)

Humic sand

Defined SOM content range (AG Boden, 2005) Substrate conditions (IUSS Working Group WRB, 2007) Soil water conditions Soil horizon position Pedogenesis

b30% SOM

SOM content (%) SOC content (%) SOM/SOC ratio SOC/SOM ratio Number of observations (n)

Amorphous peat

± ± ± ±

18.0 11.3 0.29 0.07

± ± ± ±

7.6 4.8 0.09 0.03

± ± ± ±

3.1 1.9 0.09 0.02

± ± ± ±

5.2 1.8 0.08 0.03

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Fig. 2. SOM/SOC ratios of different substrates and peat decompositions as defined by WRB from Northeastern Germany.

Fig. 3. SOM/SOC ratios of different peatland soil horizons from Northeastern Germany affected by drainage depth.

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originated from two peatland sites formed by terrestrialization of an open water body (Joosten and Clarke, 2002). Therefore, some peat samples contained notable amounts of highly decomposed organic mud, including great portions of microbial derived sugars and proteins which might diminish total carbon contents of some samples. Sphagnum peat had significant higher SOM/SOC ratios than peat from vascular plants (Alnus peat, radicell peat). The findings of Bhatti and Bauer (2002) and Bohlin et al. (1989) are consistent with our results. In addition, data of Kracht and Gleixner (2000) indicated SOM/ SOC ratios about 2.0 for Sphagnum peat. Obviously, the considerable difference is caused by higher amounts of relative carbon-poor compounds (carbohydrates, uronic acids) in Sphagnum peat in comparison to vascular plant peat. Besides, higher amounts of relative carbon-rich compounds (lignin and aliphatic molecules) of vascular plants are responsible for this (Schellekens et al., 2009). SOM/SOC ratios of brown moss peat differ significantly from both Sphagnum peat and vascular plant peat and ranged between them. The lack of lignin (Weberling and Schwantes, 2008; Maksimova et al., 2013) and low contents of uronic acid (Hämäläinen, 1991) may be the reason for the intermediate level of brown moss peat SOM/SOC ratios. The proportion of carbon-rich “lignin-like” compounds (e.g. polyphenols) in Bryales (Glime, 2007) seems to be too low to affect SOM/SOC ratios conspicuously. The mean SOM/SOC ratio of amorphous peat lies within the range of the other peat types, whereas the high variation may be a sign of manifold and strong SOM transformations within the peat composition by aerobic decay. 4.3. Peat decomposition as defined by WRB Peat conditions are characterized by peat qualifiers after WRB which account for the degree of decomposition and visible parts of fiber and wood IUSS Working Group WRB (2007). The significant difference of SOM/SOC ratios between fibric Sphagnum and fibric vascular plant peats reflects their specific chemical attributes, as stated above. In contrast, we found no significant differences on SOM/SOC ratios of fibric and hemic peats from vascular plants. Hence, we cannot consider notable carbon losses or enrichment within SOM during the transformation from fibric to hemic peat. The fiber content is not an adequate parameter to characterize differences in SOM/SOC ratios of hemic and fibric peats. The high variety in processes of peat decomposition and biochemical transformations of large molecules into smaller parts as studied e.g. by Buurman et al. (2006) or Schellekens and Buurman (2011) may cause high variations in SOM/SOC ratios of sapric peats compared to hemic and fibric peats. The use of SOM/SOC ratios of peat after WRB qualifiers (fibric, hemic, sapric) as conversion factors is not sufficient because of the substantial variation in chemical compositions of peat types. 4.4. Impacts of drainage depth The drainage of peatland soils leads to a secondary soil development, which is characterized by aerobic processes of mineralization and humification (Byrne et al., 2004; Kalisz et al., 2010). Hence, only sapric and amorphous peats were found in drained topsoils which had relatively high SOM/SOC ratios caused by relatively lower SOC contents. This could be explained by a higher microbial activity due to aerobic decomposition in aerated topsoils. In contrast, water-saturated peats were always hemic or fibric, in accordance with Heller and Zeitz (2012), who described increasing fiber contents with increasing soil depths and water saturation for two drained fens in Northeastern Germany. The SOM/SOC ratios of water-saturated horizons differed significantly between moss peats and vascular plant peats. Obviously, peat type attributes determined properties of the soil horizons. Moss peat data showed that water-saturated peat layers appear in both subsoil and topsoil horizons. Besides the study results, buried peat layers may exist in the subsoil representing former aerobic episodes. Therefore, a

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classification of SOM/SOC ratios according to the absolute sampling depth (topsoil/subsoil) seems to be inappropriate for the deduction of conversion factors without a consideration of soil water conditions. 4.5. Implementation of conversion factors Due to clear differences and a relatively low variation of SOM/SOC ratios, the implementation of conversion factors derived from peat types seems to be appropriate and accurate for the assessment of SOC. Furthermore, the system can be simplified by summarizing peat types of vascular plants. In our study vascular plant peats (Alnus glutinosa, Phragmites australis, Carex sp.) can be considered as a homogenous class in respect to SOM/SOC ratios. Compared to mosses they exhibit woody supporting tissues in roots, stems, leaves and fruits (Weberling and Schwantes, 2008). Presumably, the chemical composition of woody supporting tissues determines similar SOM/SOC ratios among a broad range of plant taxa and ecotypes. The vascular plants mentioned commonly appear in peatlands of the boreal and temperate zone in the northern hemisphere. Therefore, we suppose that our results can be transferred to other geographic and climatic regions. However, further investigations on SOM/SOC ratios of peat types, especially from the southern hemisphere (Kleinebecker, 2007), are required to prove our assumptions. For example, peat of Astelia pumila and Donatia fascicularis from the Andes/Chile (Kleinebecker, 2007) or Raphia sp. from South Africa seem to be widely spread peat types. Sphagnum peat reveals a unique chemical composition in contrast to peats from other Bryales genera and vascular plants (Hämäläinen, 1991; Bohlin et al., 1989). According to that and the low variation in our study, we consider Sphagnum peat SOM/SOC ratio being valid as a conversion factor for global use. Our findings on brown moss peat SOM/SOC ratios must be proven due to the low number of samples. With regard to the high variation of amorphous peats we suppose that the implementation of the mean SOM/SOC ratio is probably more appropriate than commonly used conversion factors. The importance of organic soils within the carbon cycle was described by Moore and Bellamy (1974) and others. Since then there have been many attempts to estimate the carbon stocks of organic soils on international, national and regional scales. Therefore, different studies on SOC inventories and SOC losses used SOM/SOC conversion factors, ranging from 1.72 to 2.0 (Table 1). However, simplified assumptions cause considerable conversion errors. The widely used conversion factor of 1.724, for example, generally overestimates the SOC stocks (Jones et al., 2005; Montanarella et al., 2006). Using an incorrect conversion factor of 1.73 (vascular plant peat) instead of 2.05 (Sphagnum peat) the conversion error exceeds 18%. Assuming that the data of comprehensive studies (Table 1) certainly contains different peat types a mean conversion error of about 10% could be possible. A review of SOC assessment studies by using more specific conversion factors is required. The study of Farmer et al. (2014), who identified a site adapted conversion factor for tropical peats (1.88) under oil palm plantations, can be regarded as an exception. Unfortunately, the description of peat types was not available. A simple regression approach to derive a cogent conversion factor for the relationship of organic matter and SOC is probably not a useful way because of the differences in the characteristics of organic matter (Howard and Howard, 1990). In addition, limits of histic horizons (18% SOC/30% SOM and 12% SOC/20% SOM) defined in the WRB classification (IUSS Working Group WRB, 2007) need to be adapted. The conversion factor of 1.66 is not in accordance with these studies' results of 1.71 to 2.05 for different peat types. If the WRB classification retains a single conversion factor it should follow the definite higher levels of peat SOM/SOC ratios. 5. Conclusions Our results indicate the importance of implementing specific conversion factors for substrates of peatland soils due to considerable

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differences of SOM/SOC ratios. The classification of SOM/SOC ratios after peat types is most suitable for deriving reliable conversion factors. Our results for Sphagnum peat and vascular plant peats are adaptable to other geographic and climate regions. Because of the high variation we do not recommend the application of our amorphous peat and humic sand conversion factors in other regions. SOM/SOC ratios of brown moss peat need to be proved through further investigations. Another consequence should be investigation of structural processes during peat formation and decomposition. Acknowledgments We thank the Landesamt für Bergbau, Geologie und Rohstoffe Brandenburg for providing data of Sphagnum peat from kettle hole mires in Brandenburg/Germany. Further, we thank Phelim Burgess and Martin Klingenfuß for the language correction. Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.geoderma.2014.07.010. These data include Google map of the most important areas described in this article. References AG Boden, 2005. Bodenkundliche Kartieranleitung (Soil survey instruction book), Hannover. Baldock, J.A., Nelson, P.N., 2000. Soil organic matter. In: Sumner, M.E. (Ed.), Handbook of soil science. CRC Press, Boca Raton, pp. 36–42. Batjes, N.H., 1996. Total carbon and nitrogen in the soils of the world. Eur. J. Soil Sci. 47 (2), 151–163. Bellamy, P.H., Loveland, P.J., Bradley, R.I., Lark, M.R., Kirk, G.J.D., 2005. Carbon losses from all soils across England and Wales 1978–2003. Nature 437 (7056), 245–248. Bergner, K., Bohlin, E., Albano, A., 1990. Vad innehaller torv? – En sammanstallning av botaniska, fysikaliska och kemiska data. A compilation of botanical, physical and chemical data of peatCentre for Peat Research, Box 4097, 90203 Umea, Sweden (in Swedish). Bhatti, J.S., Bauer, I.E., 2002. Comparing loss-on-ignition with dry combustion as a method for determining carbon content in upland and lowland forest ecosystems. Commun. Soil Sci. Plant Anal. 33 (15–18), 3419–3430 (New York). Bohlin, E., Hämäläinen, M., Sundén, T., 1989. Botanical and chemical characterization of peat using multivariate methods. Soil Sci. 147. Bridgham, S.D., Lamberti, G.A., 2009. Ecological dynamics III. Decomposition in wetlands. In: Maltby, E., Barker, T. (Eds.), The wetlands handbook. John Wiley & Sons, Oxford, pp. 326–340. Buringh, P., 1984. Organic carbon in soils of the world. The role of terrestrial vegetation in the global carbon cycle. Measurement by remote sensing, Vol. SCOPE, 23. Buurman, P., Nierop, K.G.J., Pontevedra-Pombal, X., Martínez-Cortizas, A., 2006. Molecular chemistry by pyrolysis–GC/MS of selected samples of the Penido Vello peat deposit, NW Spain. In: Martini, I.P., Martínez-Cortizas, A., Chesworth, W. (Eds.), Peatlands. Evolution and records of environmental and climate changes. Elsevier, Amsterdam, pp. 217–240. Byrne, K.A., Chojnicki, B., Christensen, T.R., Drösler, M., Freibauer, A., Frolking, S., Lindroth, A., Mailhammer, J., Malmer, N., Selin, P., Turunen, J., Valentini, R., Zetterberg, L., 2004. EU peatlands: current carbon stocks and trace gas fluxes. CarboEurope-GHG, concerted action synthesis of the European greenhouse gas budgetDepartment of Forest Science and Environment, Viterbo, Italy, pp. 5–30. Crawford, D.L., Barder, M.J., Pometto III, A.L., Crawford, R.L., 1982. Chemistry of softwood lignin degradation by Streptomyces viridosporus. Arch. Microbiol. 131 (2), 140–145. De Vos, B., Lettens, S., Muys, B., Deckers, J.A., 2007. Walkley–Black analysis of forest soil organic carbon: recovery, limitations and uncertainty. Soil Use Manag. 23 (3), 221–229. Dickinson, R.E., 1964. Germany: a general and regional geography. p. 700. Dierssen, K., Dierssen, R., 2001. Moore. Ökosysteme Mitteleuropas aus geobotanischer Sicht (Peatlands. Ecosystems from a geobotanical point of view). Ulmer, Stuttgart, p. 69. DIN ISO 10694, 1994. Bodenbeschaffenheit: Bestimmung des organischen Kohlenstoffgehaltes und des Gesamtkohlenstoffgehaltes nach trockener Verbrennung (Elementaranalyse). Farmer, J., Matthews, R., Smith, P., Langan, C., Hergoualc'h, K., Verchot, L., Smith, J.U., 2014. Comparison of methods for quantifying soil carbon in tropical peats. Geoderma 214, 177–183. Glime, J.M., 2007. Bryophyte ecology. Physiological ecology, vol. 1. EBook sponsored by Michigan Technological University and the International Association of Bryologists (Available from http://www.bryoecol.mtu.edu/). Grewal, K.F., Buchan, G.D., Sherlock, R.R., 1991. A comparison of three methods of organic carbon determination in some New Zealand soils. J. Soil Sci. 42 (2), 251–257.

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