Introduction

In a peat-forming ecosystem, decay of fresh plant material effectively occurs in the uppermost, predominantly aerobic stratum, acrotelm, while deeper down, in the catotelm, persistently anoxic conditions below the water table promote peat preservation (Ingram 1978). Mesotelm is defined as the intermediate zone, through which C is transferred from acrotelm to catotelm (Clymo and Bryant 2008). In reality, these zones are somewhat arbitrary and their vertical position may change over time by changing hydrological regime and aeration conditions at the site. Fens formed in depressions are fed by groundwater, and thus tend to be influenced by minerogenic nutrients. Fens develop into bogs when the surface grows above the reach of groundwater. The water and nutrient sources of bog vegetation depend on the precipitation and snow, leading to Sphagnum dominance. Sphagnum peat dominated bogs are 13C-enriched compared to fens, where vascular plants dominate (Andersson et al. 2012; Krüger et al. 2016; Nykänen et al. 2018). In addition to the primary atmospheric CO2 source, secondary CO2 sources also affect peat 13C/12C ratios (denoted as δ13C). Reuse of 13C-depleted respired CO2 will deplete the fresh biomass (Jones et al. 2014; Nykänen et al. 2018). The same way, 13C-depleted CO2 derived from methane oxidation by methanotrophic symbionts in Sphagnum mosses leads to the formation of 13C-depleted peat C (Raghoebarsing et al. 2005, Larmola et al. 2010). Furthermore, the Suess effect, anthropogenic 13C-depletion of atmospheric CO2 (Rubino et al. 2013) also affects the δ13C values of the uppermost peat strata (Esmeijer-Liu et al. 2012). Aerobic and anaerobic decay processes affect differently the peat δ13C values. In general, aerobic microbial decomposition increases δ13C in the resulting peat (Ågren et al. 1996), while anaerobic decay preserves or decreases the bulk peat δ13C values (Alewell et al. 2011; Krüger et al. 2014, 2015).

Water source and water table levels are the main factors determining the peatland type and vegetation. Furthermore, water saturation is an important factor in the peatland C balance. Thus, both water source and water levels determine δ13C values of the developing peat deposit. We have studied the effects of drainage-enhanced decomposition on peat δ13C values and especially the 12C and 13C balances, by comparing adjacent, well correlated but contrasting peat profiles, two from undrained and two from the drained side of a partially ditch-drained bog. The peat material we analyzed consists of stored samples collected during an earlier and more extensive study (Pitkänen et al. 2013), which demonstrated the marked effects of ditch drainage on the total peat C balances at this particular bog. Our analyses for δ13C, C% and N% of the two profiles aims to address three research questions: firstly, to determine the effects of drainage-induced surface peat decomposition on C isotope balance; secondly, to determine the long term changes in the stable C isotopes by analysing each of the consecutive 10-cm samples of the complete profiles, and thirdly, by comparing the isotope signatures on sequences of the contrasting profiles, to substantiate their comparability and to assess the reproducibility of the analysis methods (while lack of more replicates prevents any actual statistical analyses of the data).

The Rahesuo bog site provides exceptionally favourable conditions to study the effects of artificial drainage, because the large excentric bog has been partially ditch-drained along the natural water flow direction. As a consequence, the effects of drainage are seen at the border of the undrained and drained parts as surface subsidence and vegetation changes in the latter. Pitkänen et al. (2013) investigated this study setting by analyzing peat bulk C dynamics in seven pairs of matching peat profiles collected from either side of the marginal ditch. In the present study, we have used material stored from two pairs of those profiles.

Materials and methods

Study site

The ombrotrophic Rahesuo peatland is a large raised bog complex (750 ha) situated in Ilomantsi, eastern Finland (62°52′ N, 31°10′ E, Fig. 1a). The annual average temperature at the nearest weather station at Lieksa (distance 76 km) is 2.0 °C and precipitation 711 mm for the period of 1981–2010 (Pirinen et al. 2012). The warmest and wettest month is July (16.0 °C, 92 mm) and the coldest is February (−10.7 °C). There is usually snow cover from November to the end of April. The age of the basal peat layer of the drained site is 10,200 cal. Yrs. B.P. this to form cal yr BP (Mäkilä and Goslar 2008). Details of the study site are shown in Table 1. Rahesuo has been earlier studied by Tolonen (1967) and Geological Survey of Finland (Geological Survey of Finland peatland database and unpublished material in the peat inventory archive of GSF).

Fig. 1
figure 1

Location of Rahesuo in Finland marked by a star (a) and location of sampling points in aerial map of Rahesuo by white dots (b) (Copyright Maanmittaushallitus)

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Table 1 Major features of the study site
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While the bog complex is largely ditch-drained, there is an undrained section of 30 ha in its western part (Fig. 1b). Determined by land ownership borders, the margin between undrained and drained parts runs parallel to the natural water flow; the slope of the bog surface is about 2.5 m per 1000 m along the marginal ditch. Most of the bog was ditched by 40 m ditch intervals during winter 1971–1972. Thus, there was 37 full growing seasons prior to peat sampling in June 2009. At the drainage border, drying appears to have affected the undrained side only within a few meters from the marginal ditch (Pitkänen et al. 2013). The main indicator species of the bog hollows of the undrained side, such as Sphagnum balticum, Carex limosa and Scheuchzeria palustris are replaced by Mylia anomala and Polytrichum strictum on the drained side. Surface subsidence of the drained side is evident along the entire drainage margin. Surface levelling in the hollow where the sampling was done, indicated a 30 cm subsidence on the drained side. In contrast, only slight sloping of the surface towards the marginal ditch was observed, and only within the nearest 10 m from it. As judged by peat stratigraphy, marked loss of surface peat is evident on the drained side, but deeper down the stratigraphies appear quite similar and occur consistently at the same horizontal levels (e.g. an Eriophorum vaginatum – rich stratum at 52–58 cm in the undrained side and at corresponding depth of 22–28 cm in the drained side profile).

Peat sampling

In Pitkänen et al. (2013), two complete peat profiles from the undrained (UNDR) and drained (DRA) sites were sampled from the same hollow. The distance between the two replicate profiles at both sites was 1 m and continuous samples were taken at 10 cm intervals. The locations of both sites were approximately 7 m from the ditch (Fig. 1b). Samples were collected with a box corer to a depth of 1 m and with a Russian type side-cutting peat corer from the deeper peat layers. Dry BD and ash content were determined (Pitkänen et al. 2013). Drying of the samples took place at 105 °C and homogenized samples were stored in paper bags at room temperature. For the present study, we analyzed samples from two full profiles from undrained and drained sites.

Stratigraphic depths are expressed as depth below the peat surface. The individual 10-cm samples are indicated by their mid-point depth, so DRA225 cm refers to the sample from 220 to 230 cm on the drained side, whereas UNRA5 – UNDR145 cm refers to the uppermost 0–150 cm of the undrained (natural) side. In Pitkänen et al. (2013), the marked downward increase of ignition residue, observed consistently at UNDR245 cm and DRA215 cm were taken as a common base level for profile comparisons and inventory calculations. A synchronous charcoal layer at UNDR165 cm and DRA135 cm was used as an additional marker level for the profiles. The profile matching of Pitkänen et al. (2013) was slightly modified in the present study: 10 cm layers above the Synchronous Charcoal Layer at UNDR155 cm and DRA125 cm (SCLUNDR155/DRA125) was used in the C balance and δ13C calculations. Similarly, the profile base level based on the Synchronous increased Ash content Layer (Pitkänen et al. 2013) was elevated by 10 cm to UNDR235 cm and DRA205 cm (SALUNDR235/DRA205). This change to the synchronous layer positions was done to avoid any over or underestimation of C store because the exact location of the synchronous charcoal layer within each 10 cm sample was unknown.

Analyses of δ13C, C% and N% from peat

In 2014, subsamples (2–4 mg) of the original peat samples were weighted in tin cups and analysed for δ13C, C% and N%. Analyses were done with Elementar Cube (Elementar, Hanau, Germany) coupled with Isoprime 100 (Isoprime Limited, Cheadle Hulme, UK) isotope ratio mass spectrometer. A certified birch leaf standard (Elementar Microanalysis, UK) was used as a reference for C% and N% and also as an internal isotopic standard on Lsvec - LSB-19 scale (Coplen et al. 2006). Peat samples were not acidified based on Nykänen et al. (2018), where acidification did not change the peat δ13C values. The birch leaf standard was analyzed for δ13C, C% and N% after each 5 peat samples to correct for the analyzer drift, while linearity was checked in every run with variable concentration of C and N in standards corresponding to C and N amount of the samples. Standard deviation for the repeated standard runs was: <0.15 for δ13C, <0.3 for C% and < 0.03 for N%.

Stable isotope compositions were expressed in the delta notation as a ‰ deviation of the heavy-to-light isotope abundance ratio in the sample from that of a standard, VPDP for C. The results are reported relative to the standard scale.

$$ {\delta}^{13}C=\left(\frac{\left(\frac{{}^{13}C}{{}^{12}C}\right) sample}{\kern0.50em \left(\frac{{}^{13}C}{{}^{12}C}\right) standard}-1\right)\ast 1000 $$
(1)

The total C mass of each sample was calculated based on the known volume, measured dry BD and C% of peat sample. In order to calculate the masses of 12C and 13C of samples, delta notation (Eq. 1) was converted back to the atom percent (at%) by formula (Fry 2006):

$$ at\%=\frac{\delta^{13}C+1000}{\delta^{13}C+1000+\frac{1000}{0.0118}}\ast 100 $$
(2)

Where: δ13C = δ13C value of the sample, 0.0118 = ratio of VPDB standard and at% = atomic percent.

Pitkänen et al. (2013) estimated the total C inventories for the undrained and drained Rahesuo profiles above the respective base levels SCLUNDR165 and SCLDRA135. The C mass change was taken as the difference of C inventories between the two sites. In their study, C concentration was estimated to be 50% of the dry mass (Pitkänen et al. 2013). In this study, the total C inventories of the peat profiles were calculated by summing up C masses in each 10-cm sample above the synchronous basal layers by using C%, known sample volume and bulk density to calculate C masses for each sample. We also calculated the C masses of the corresponding 90-cm peat profiles from SCLUNDR155 to SALUNDR235 and from SCLDRA125 to SCLDRA205. The ratio of 13C and 12C masses were converted to δ13C values by eq. 1.

The variance of each profile pair on undrained and drained site C mass and δ13C is expressed as mean ± standard error. Missing or gained C error estimate comes from possible combinations on undrained and drained site (undrained site profiles 1 and 2 and drained site profiles 1 and 2). δ13C values for missing or gained C were determined by calculating the mass weighted means of missing or gained C and δ13C combinations for undrained and drained site and thus error estimate is shown as range of all possible combinations. Mass weighted average with range of individual profiles of δ13C for the UNDR5 – UNDR80 cm and DRA5 – DRA50 cm of the uppermost peat layer were used to compare the current peat upper layer δ13C values affected by drainage to historical ones deeper down in the peat column.

The Long Term Average Rate of Carbon Accumulation (LORCA) was calculated by dividing the total C mass of 250 cm long peat profiles from the undrained site by basal peat age (10,220 yr) from (Mäkilä and Goslar 2008). Similarly, LOR12CA and LOR13CA were calculated to get an estimate for the annual addition of 12C and 13C in the peat profiles.

Statistical analysis

Pearson correlation and regression analyses were done to the peat profiles UNDR5 – UNDR235 cm and DRA5 – DRA205 cm to study the relationships between δ13C and C%, N%, C/N-ratio and dry BD. The effect of drainage (undrained vs. drained) on δ13C, C%, N% and C/N of surface peat was tested using a t-test. Analyses were done with IBM SPSS Statistics version 25.

Results

Peat constitutes and δ13C, C%, N% and C/N-ratio stratigraphy

The peat column inventory done by the Geological Survey of Finland, conducted at a site different from ours, but showing rather similar stratigraphic features, showed that undrained peat surface was composed mostly of Sphagnum and aquatic herbs (Fig. 2g). Deeper in the peat column, the proportion of Eriophorum and sedges increased, and peat material was dominated by sedges from 205 to 245 cm. In basal peat, shrubs also appeared (Fig. 2g). According to the GSF data, the peat humification index (von Post H1–10) rapidly increases below the poorly humified 0–60 cm surface layer (H2–3); maximum value (H8) was found from 150 to 190 cm. Below 190 cm, the von Post values decreased again (Fig. 2g). LORCA was 11.7 ± 0.2 g C m−2 yr−1 (mean ± SE), and it could be divided into 11.56 g m−2 yr−1 of 12C and 0.13 g m−2 yr−1 of 13C.

Fig. 2
figure 2

Rahesuo stratigraphy of undrained site (dotted red line is mean, solid red lines show individual profiles) and drained site (dotted black line is mean, solid black lines show individual profiles). Surface and synchronous levels are marked with vertical lines. δ13C (‰) (a), C% (b), N% (c), C/N-ratio (d), BD (e), amount of C m−2 (f) and peat humification on von Post scale and peat constituenst (g). Surface for undrained site (UNDR = 0 cm) and drained site (DRA = 0 cm). Upper synchronous layer line (SCLUNDR155/DRA125) used in calculations is 10 cm above detected synchronous charcoal layer at UNDR165 cm and DRA135 cm. Lower synchronous layer line (SALUNDR235/DRA205) is elevated 10 cm above ash amount increase determined as transfer layer from peat to mineral subsoil border at depth of UNDR245 cm and DRA215 cm. Peat constituents on six grade scale portions: SH = Scheuchzeria (aquatic herbs), ER = Eriophorum (cotton grass), S = Sphagnum (moss), C = Carex (sedge), N=Nanolignidi (shrubs)

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Original data of δ13C, C%, N%, C/N-ratio and bulk density is in Sup. 1. In the top 10 cm of peat, the δ13C values were statistically significantly (t(2) = −6.73, p = 0.02) higher on the drained side (δ13C −25.9 ± 0.24‰) compared to δ13C −27.6 ± 0.09‰ in the undrained site (Fig. 2a). Mass weighted average of δ13C values for the 50 cm of the current surface peat on drained site were 0.76‰ (0.60–0.91‰) (mean + range) enriched compared to the undrained site 80 cm values.

In both the undrained and drained profile, there is a marked peak of δ13C immediately below the uppermost surface peat layer; peak values of −24.9 ± 0.39‰ and −24.0 ± 0.46‰ were found at UNDR25 cm and DRA15 cm, respectively (Fig. 2a). Deeper down there is a rapid decrease to about −28‰ in both profiles and then a steadier decline rather uniformly in both site profiles, from UNDR75 to UNDR185 cm at the undrained site and from DRA45 to DRA155 cm on the drained site. The most depleted δ13C was found at UNDR185 cm (−29.0 ± 0.15‰) and DRA155 cm (−29.0 ± 0.04‰). Thus, the minimum δ13C values in the UNDR and DRA peat profiles were showing a similar depth difference (30 cm) as the synchronous ash layers between the undrained and drained sites, and conformed to the surface subsidence. δ13C values decreased below UNDR215 and DRA185 cm towards the boundary of the basal peat-mineral soil and increased in the undrained site but not on the drained site (Fig. 2a).

At the drained site, there was more variation in the C concentration (Fig. 2b) than on the undrained site. The C% was lower at the drained site to a depth of DRA75 cm, but it increased in the deeper peat layers from DRA105 cm to DRA205 cm (Fig. 2b). On the drained side, C% peaked at depth layers from DRA165 cm to DRA205 cm.

Drainage decreased the uppermost peat N% down to a depth of DRA55 cm (Fig. 2c). However, the N% increased in the deeper peat layers from DRA65 − DRA155 cm, and also in the basal peat layers from DRA175 – DRA245 cm and in the mineral subsoil (Fig. 2c). At both the undrained and drained sites, the C/N ratio was largest in the surface peat layers, peaked similarly as δ13C at UNDR25 cm (~78) and at DRA15 cm (~71) and decreased to 30–40 below UNDRA45 cm and DRA25 (Fig. 2e). C/N ratio was at a minimum (27.3) 10 and 20 cm below the depth where δ13C had minimum value on undrained and drained sites, respectively. Variation in N% was larger compared to C%, and thus the C/N ratio was a mirror image of N% (Fig. 2c, d).

In the uppermost 0–50 cm of both of the profiles, there appeared to be no correlation between the δ13C values with the other measured variables and these samples did not fit with the regressions established between δ13C and C% in the samples UNDR45 to UNDR235 and DRA45 to DRA205 level (Fig. 3). In the peat profile UNDR45 to UNDR235 and DRA45 to DRA205, the δ13C decreased with depth (Fig. 2a). However, a significant negative correlation between δ13C and sampling depth was found only at the undrained site (Table 2). At the undrained site, the δ13C had a negative correlation with the dry BD. The C% correlated positively with sampling depth at the undrained site and with BD at the undrained and drained site. N% correlated positively with the depth at the drained site (Table 2), but not on the undrained site. A significant negative correlation was found between C/N-ratio and peat depth at the drained site (Table 2).

Fig. 3
figure 3

Regression line between δ13C and C%. Undrained site, depths 45–235 cm: δ13C = −0.20 * C% - 18.08, p < .0001, F = 52.4, r2 = 0.58, n = 38; drained side, depths 45–205 cm: δ13C = −0.20 * C% - 17.57, p < .0001, F = 96.7, r2 = 0.75, n = 32

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Table 2 Correlation matrix for undrained (depths 45–235 cm) and drained (depths 45–205 cm) sites of Rahesuo bog
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Carbon deficit

The total C deficit in the drained side profile was 4.2 ± 3.0 kg C m−2 or 114.0 ± 81.5 g C m−2 yr−1, compared with the undrained one (total C inventories above SALUNDR235/DRA205; Table 3). The missing C was 13C-depleted, [δ13C, −31.2 (−4.3 - −31.3) ‰] compared to the corresponding average of the undrained site peat C (δ13C −28.0 ± 0.03‰) (Table 3).

Table 3 Rahesuo carbon stores, δ13C values and δ13C of carbon gained or missing
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For the peat sequences above the SCLUNDR155/DRA125 layer, the C deficit was 5.9 ± 1.1 kg C m−2 due to drainage (Table 3), which when divided by the post-ditching number of years, translates to an average annual C loss of 159.0 ± 1.1 g C m−2 yr−1. The peat profile above SALUNDR155/DRA125 had quite similar δ13C values above the synchronous ash layer both in the undrained (δ13C −27.7 ± 0.04‰) and drained (δ13C -27.5 ± 0.01‰) sites. The missing C [δ13C −30.0 (−29.1‰ − −32.4‰)] was 13C-depleted compared to the corresponding δ13C average of the undrained site peat C (−27.7 ± 0.04‰) (Table 3).

When comparing the deeper parts of the profiles, the C mass was larger in the sequence DRA105 − DRA185 than in the corresponding part in the undrained one (Fig. 2f). Thus, the 90 cm peat section between the DRA125 − DRA205 showed a 3.0 ± 1.9 kg m−2 excess in the C mass at the drained compared to UNDR155 − UNDR235 peat columns (Table 3). The additional C [δ13C −27.8 (−25.4‰ - −31.6‰)] was 13C-enriched compared to the corresponding δ13C average of the undrained site peat C (−28.4 ± 0.01‰) (Table 3).

Discussion

Using peat δ13C values as an indicator of changed environmental conditions

Andersson et al. (2012) and Esmeijer-Liu et al. (2012) studied the δ13C stratigraphy of natural peatlands connected to different climatic periods or peatland development stages. The effect of water table lowering on the δ13C profiles has been studied on peatlands where the water table was lowered due to peat uplifting caused by permafrost formation (Krüger et al. 2014), drainage for agriculture (Krüger et al. 2015) and forestry (Krüger et al. 2016; Nykänen et al. 2018). A new approach in this study was the calculation of the absolute amounts of 13C and 12C by the same methods used earlier in C balance calculations (Minkkinen et al. 1999; Pitkänen et al. 2013; Krüger et al. 2016).

The calibrated age of the basal peat of Rahesuo bog was 10,220 yr B.P. (Mäkilä and Goslar 2008). Thus, the studied peat profiles have stored the history of the whole Holocene climatic periods on peat C and also the effects of natural diagenetic processes on δ13C values. Also, the natural succession of vegetation due to peat surface elevation in relation to the mineral water source is stored in the peat profiles. Furthermore, water-table level change induced by drainage has altered the balance between C uptake and release on the drained side during the last several decades. Thus, this study demonstrates that the analysis of δ13C values in depth profiles is a useful tool to identify the effect of water table lowering on δ13C values as well as comparing it to the δ13C values of natural state peatland in similar climatic conditions. Furthermore, synchronous layers in the nearby peat columns offered a possibility to quantify the carbon stocks and to calculate actual 12C and 13C amounts and show changes in δ13C values quantitatively.

The changes (above) reflected in peat δ13C values are mainly connected to changes in hydrology, while 13C depletion (13C-Suess effect) of atmospheric CO2 and increase of CO2 concentration are affecting C supply and its δ13C imprint. In general, most of the annual C exchange and formation of the initial 12C/13C ratios in peat occurs in the surface layer facing the atmosphere. However, the bulk δ13C values are measured from the stored peat of different ages. As an example, on a boreal ombrotrophic Sphagnum fuscum bog hollow, 10 km from Rahesuo bog, 188 g C m-2 yr−1 was captured in photosynthesis and 198 g C m−2 was respired during an exceptionally dry summer, while 30 g C m−2 was lost through respiration during the winter, with a total loss of 62 g C m−2 yr−1 when also leaching (loss 7 g C m−2 yr−1) and CH4 flux (loss 15 g C m−2 yr−1) were included (Alm et al. 1999). Compared to these carbon flows, the recent apparent rate of C accumulation during the 100 years for Rahesuo (26.3 g C m−2 yr−1) (Mäkilä and Goslar 2008) is also relatively small compared to the mean LORCA100 values of bogs in Finland (Turunen et al. 2002). In addition to this, the LORCA for the whole peat column in Rahesuo is smaller (11.7 g C m−2 yr−1) than that of the Larsson et al. (2017) average (13.7 ± 5.5 (SD) g m−2 yr−1) for a Swedish oligotrophic fen. Furthermore, the LORCA for Rahesuo was only half of the average of 127 northern peatland sites (23 ± 2 (SE) g C m−2 yr−1) as given by Loisel et al. (2014). In any case, δ13C values in peat profile are always measured from highly processed peat, which is only a fraction from the originally captured C.

Before distinctive changes in bulk peat δ13C values can occur, there must be environmental changes in active processes that lead to a 12C/13C ratio change. In general, if both C uptake and release will decrease by the same amount, there will be no change in 13C-values stored in peat. However, if C uptake remained the same but respiration increased, it will lead to a peat 13C-enrichment. e.g. when water-table decreases due to increasing hummock height. Furthermore, the amounts of stored 12C and 13C in peat profiles depend on many processes, which can have opposite effects on the final bulk δ13C values. Also, diagenetic processes decrease the original δ13C values of peat, since anaerobic microbes preferentially use isotopically heavier compounds instead of more recalcitrant lignin that has a lower δ13C value (Benner et al. 1987). Thus, processes leading to an increase in lignin fraction in bulk peat have a major role in depleting the stored peat δ13C values.

Peat δ13C values of the undrained site

In this study, the basal peat layer, consisting of shrubs, sedges and Sphagnum mosses was relatively 13C-enriched compared to the peat layer above it and also had a low humification index, comparable to the peat profile study from East European Russian Arctic (Andersson et al. 2012).

The most 13C-depleted peat layer was found at UNDR185 and DRA155 cm, i.e. exactly at the same level taking into account the surface subsidence at the drained site. The same depth layer also had the maximum C%. Maximum nitrogen content and minimum C/N ratio was 10 and 20 cm below the most depleted layer on undrained and drained sites, respectively. Bulk density was highest at the same depth as the δ13C minimum on undrained side and 10 cm over the δ13C minimum at drained site. Furthermore, humification index and the volumetric amount of C was largest at these same depth layers. Similarly, Kaislahti Tillman et al. (2010) found that peat with higher C% and lower C/N ratio, as indications of degradation, was also most depleted in 13C.

In the Rahesuo bog, the mean peat growth was 0.25 mm yr−1 (Mäkilä and Goslar 2008) and the most negative δ13C values of the peat profile originated approximately from 7400 to 7600 years BP. Thus, these peat layers were probably formed during the warm and moist period of the Mid-Holocene 7950–6750 years B.P. (Seppä and Birks 2001). One possible explanation for the 13C-depleted peat may be the increased amount of more 13C-depleted vascular plant material instead of Sphagnum mosses during the warm climatic period. In the GSF peat inventory, remains of sedges in depth layers between 155 and 245 cm were abundant. In addition to vascular plants 13C-depletion, Sphagnum peat can become 13C-depleted during a warm and moist climate due to increased methanogenesis (Jones et al. 2010). Furthermore, recycling of C during the wet fen stage can lead to vegetation 13C-depletion (Jones et al. 2014).

In addition to the initial formation of 13C-depleted wet fen stage peat, decay also leads to further peat 13C-depletion. July temperatures were ~3 °C warmer during Mid-Holocene compared to the present day (Seppä and Birks 2001), thus promoting a higher decay rate. Vascular plants are mainly composed of lignocellulose, whereas Sphagnum mosses are primarily composed of cellulose (Benner et al. 1987; Kracht and Gleixner 2000). Both aerobic and anaerobic decay of peat containing lignin is known to cause 13C-depletion (Fernandez et al. 2003). In this study, the most 13C-depleted peat and the highest concentrations of C and N were found at the same depths. This was likely due to a faster decay rate of organic matter of vascular plants compared to bryophytes, leading to higher C%, N%, and smaller C/N ratio (Andersson et al. 2012). Furthermore, active microbes throughout the peat profile enable continuing slow decay through the whole peat profile. Putkinen et al. (2009) found active archaeal communities in deep peat layers, while Mpamah et al. (2017) showed that there were considerable amounts of viable microbes throughout the peat profile, mainly in the surface and basal layers but also in the middle section of the peat profile.

In the Rahesuo bog, the anaerobic catotelm layer had a negative correlation between δ13C and C%, and the corresponding slope in our equation: δ13C (peat) = −0.20 * C% (peat) – 18.1, was between those of Larsson et al. (2017) for Sphagnum spp. and Eriophorum spp. peat. Thus, diagenetic processes in peat containing Sphagnum spp. and vascular plants decreased the peat δ13C (above) and, possibly also amplified the effect of already 13C-depleted vegetation cover effect on peat δ13C.

Above the most 13C-depleted peat layer, peat 13C-enrichment begins, possibly due to the cooling and drying of the climate leading gradually to an increased proportion of Eriophorum vaginatum and to a decrease in N% and humification index. These changes possibly indicate a major decrease in water flow thus ending the fen stage. In an otherwise 13C-enriching peat profile, there is a 0.4‰ shift to more negative δ13C values at a depth of 135 cm on undrained site, and also a small increase in N%. The climatic period or change in hydrology leading to this shift is unknown. Overall, the peat δ13C profile of the undrained side followed the main vegetation assemblages comparable to the results of Andersson et al. (2012). Changes in vegetation type were a consequence of past climatic periods and possibly from a change in hydrology when the peat surface grew above the minerogenic water sources.

Effect of artificially lowered water table on peat 13C/12C-ratios

Most of the 15 Mha of peatlands drained for forestry, are found in the boreal and temperate zones of Russia and Fennoscandia (Joosten and Clarke 2002). Globally, the peatland area drained for forestry, cropland or grassland is ca. 50.9 Mha (Leifeld and Menichetti 2018).

To compare the effects of lowering water table on carbon balance and peat 13C/12C-ratios, the studied sites have to be originally similar, with a long lasting water table lowering and hydrology affected so that the water table drop due to ditching is affecting only the drained site. In the original study (Pitkänen et al. 2013), Rahesuo was chosen since the border ditch runs along the natural water flow direction of the bog. Since the natural upper margin of the bog is intact, the natural water flow regime is minimally affected on the undrained side, whereas effects of drainage are seen right up to the marginal ditch. Thus, comparable profiles for the undrained and drained conditions were available close to each other from either side of the marginal ditch from the same original hollow.

Drainage of Rahesuo bog for forestry 37 years before sampling, resulted in a lowered water table level, increased dry BD, subsided peat surface, changed vegetation composition and led to C loss, but did not markedly enhance tree growth on the drained side (Pitkänen et al. 2013). In this study, the average C loss based on mass difference inventories from the whole peat column was clearly larger, 114 g C m−2 yr−1 compared to the contemporary net ecosystem C balance of Alm et al. (1999), whose study was conducted during an exceptionally dry summer at a nearby natural bog. It is not clear which CO2 flux component, CO2 uptake or respiration most affected the C balance at our site. In a natural bog, an early and prolonged drought period decreased carbon uptake, while a short drought period increased respiration (Lund et al. 2012). In any case, respiration increases in drained peatlands (Silvola et al. 1996). In general, isotopic fractionation during decomposition and respiration increases the δ13C values of remaining C (Ågren et al. 1996). The upper parts of peat profiles of both the undrained and drained sites of Rahesuo were the most 13C enriched sections of the studied peat columns and drying further increased the δ13C of the surface peat. While respiration leading to preferred release of 12C may be the most important factor in peat 13C-enrichment, CH4 production and methanotrophy are known to decrease when the water table drops (Urbanová et al. 2013), which can also cause 13C-enrichment of aerobic peat layers.

The C mass in the topmost 10 cm of the Rahesuo bog was 1163 g C m−2 in the undrained site and 2468 g C m−2 in the drained site. Based on the actual rate of C accumulation during the last 100 years (ARCA100), dry BD and C% for the surface peat, approximately 8.4 cm of new peat has accumulated on the undrained site surface during the last 37 years. Thus, only the top 10 cm of the surface peat got most of the newly formed 13C-depleted plant C material. At the drained site, surface peat δ13C value increased ~1.7 ‰ in the top 10 cm. This indicates that the peat surface C balance has changed: 13C-enrichment due to respiration has been clearly larger than the C uptake in photosynthesis counteracting 13C enrichment. Opposite to the drained Rahesuo bog site, the top 9 cm of the drained peat surface of the Sphagnum fuscum pine bog at the Lakkasuo peatland was ~2.0 ‰ depleted (Krüger et al. 2016). This 13C depletion probably followed from the C addition to the surface layer from new vegetation and also from the litter falling from the trees, C input being there jointly larger than the amount of released C from decaying peat. The different response in the 13C/12C-ratio to the water level lowering between these two drained and natural bog pairs was mostly due to differences in their development after drainage. Primarily, in Lakkasuo the original water flow was perpendicular to the ditches, and thus drainage possibly affected the undrained site hydrology, whereas in Rahesuo, water flow was parallel to the ditches. This difference is linked to their C balance estimate measured by the C mass comparison method: in this study and in the original study (Pitkänen et al. 2013), the Rahesuo bog acted as a C source (114.0 ± 81.5 g C m−2 yr−1), while the drained bog at Lakkasuo was a C sink of 70 g C m−2 yr−1 in an earlier study (Minkkinen et al. 1999) and even a bigger sink in recent study (179 ± 83 g C m−2 yr−1) (Krüger et al. 2016) when compared to its undrained pair.

Another factor leading to differences between Lakkasuo and Rahesuo surface δ13C values is the addition of vascular plants containing lignin (Fernandez et al. 2003), being more distinct in Lakkasuo (Krüger et al. 2016) compared to the Rahesuo bog (Pitkänen et al. 2013). In Rahesuo, δ13C of the missing C from the layer above the synchronous ash layer was lower (δ13C, −30‰) than peat δ13C on the undrained site and clearly lower than the atmospheric CO2 (δ13C, ~ −8 ‰, Rubino et al. 2013). Thus, when respiration increases, the amount of 13C-depleted CO2 released from decaying peat increases in photosynthesis compared to atmospheric CO2 (δ13C ~ − 8‰), leading to further 13C-depletion of vegetation and surface peat (Nykänen et al. 2018). However, this effect was not visible in Rahesuo.

Clear 13C-enrichment at the peat surface “turning point” was found both in the undrained and drained sites of the Rahesuo bog. Both on the undrained and drained sites of the Rahesuo peat profiles, C% and N% had their minimum values and biggest C/N ratio at the same layers (UNDR25, DRA15) where δ13C values peaked. Thus, it would be logical to locate the δ13C turning point to the original mesotelm. The mesotelm is defined as a metabolically active interface between the oxic acrotelm and anoxic catotelm. However, the mesotelm is also a horizon that is mainly anoxic but periodically oxic (Clymo and Bryant 2008). Thus, microbial processes can change in the mesotelm from aerobic to anaerobic due to changes in water saturation. Furthermore, both respiration and leaching are active at this depth.

Similar enrichment in δ13C values near the surface was initially found from the uplifted palsa peat profiles formed due to frost upheave ca. 2500 cal yr BP (Andersson et al. 2012) or palsa uplifting leading to drying of formerly wet peat 155–671 years earlier (Alewell et al. 2011). However, the turning point was not found in peatlands drained for agriculture (Krüger et al. 2015) or forestry (Krüger et al. 2016) even though these practices lead to a drastic decrease in the water table levels and to increased C loss. Turning point δ13C enrichment was 0.8‰ larger and C% 0.5% smaller on the drained site, thus indicating increased C loss from the peat surface compared to the undrained site. Due to subsidence the maximum δ13C value was closer to the current peat surface on the drained site than on the undrained site, thus indicating that this δ13C maximum was there before artificial drainage.

δ13C Suess effect and peatland carbon exchange

δ13C of atmospheric CO2 has decreased 1.6‰ from 1850 to 2002, to the current ~ –8‰ (Rubino et al. 2013). At the same time, the concentration of CO2 in the atmosphere has increased from preindustrial values of ~280 ppm to ~390 ppm (Rubino et al. 2013). In the Rahesuo bog, 13C-depleted atmospheric CO2 has been used by the surface vegetation since 1850 and thus the surface peat to a depth of ~25 cm has used the progressively depleting CO2 based on ARCA100, peat dry BD and C concentration. In the Rahesuo bog, 13C depletion of the surface peat due to 13C-Suess effect was not clear or different compared to the deeper peat profile, as it was in a 30 cm long surface Sphagnum peat column studied in northern Finland (Esmeijer-Liu et al. 2012). In this study, 13C depletion was not detected, since respiration, CO2 reuse, CH4 production and flux, and methanotrophy have all affected the peat δ13C values both on the undrained and drained sites in the same depth layers as the 13C-depleted atmospheric CO2. In addition to this, the increase of CO2 concentration is not affecting peat δ13C values, since 13C/12C fractionation in plants is independent of natural CO2 concentration variation (Kohn 2010).

Carbon (12C and 13C) storage change

In this study, the actual masses of 12C and 13C in whole peat profiles were calculated. The splitting of C to 12C and 13C masses was possible, since gravimetric and volumetric C masses were known with their δ13C values. To our knowledge, this was the first time that this kind of calculation was done. The close proximity of the sites, vegetation pattern, composition of peat and similarity of C%, N% and δ13C profiles strongly support the presumption that at least the upper half of the undrained and drained site peat profiles were similar before drainage, and thus the differences of δ13C values in peat were due to differences in C flows as affected by drainage. The calculated C loss was 5.9 ± 1.1 kg m−2 for the upper peat profile, and for the whole peat profile 4.2 ± 3.0 kg m−2. In other Finnish undrained and drained bog pairs studied by similar methods, a clear increase in C amount following drainage has been detected (eg. Minkkinen et al. 1999; Krüger et al. 2016). However, further discussion of reasons leading to these fundamental differences are beyond this MS, but probably related to factors that explain differences in 13C-depletion in Lakkasuo and 13C-enrichment in Rahesuo bog surface peat.

Drainage affects mostly the upper peat layers, thus the smaller C loss for the whole profile C balance calculation was due to a larger C mass below the synchronous ash layer at the drained site. The 90 cm peat section below the synchronous ash layer had 3.0 ± 1.9 kg m2 more C on the drained site. One 10 cm slice of peat contains 4–7 kg C m−2, thus increased peat compaction cannot explain this addition, since marker layers remained at the same levels on undrained and drained sites. Leaching of DOC is one possible explanation for C addition since unlike CO2 and CH4, DOC is retained and captured in the peat samples used for C mass and δ13C analyses. In a 2 m deep pristine bog, 14C analyses showed that part of the DOC in the whole peat column originated from the newly formed surface peat (Wilson et al. 2016). However, an increase of downward DOC leaching is not effective enough to move 3 kg C m−2 in 37 years from the upper peat profile to the deeper peat profile. Thus, it is also possible that the larger amount of C was due to original differences in the deep peat layers between the sites.

Drainage as an analog of past dry periods

In general, the positive shift in the peat δ13C values indicates drier conditions, even though Lakkasuo bog δ13C data shows that vegetation development can lead to decreased δ13C values in the upper peat profile (Krüger et al. 2016). The upper peat profile on the undrained and on the drained site was also the most 13C-enriched section of the whole peat column. Drainage led to a peat C loss, dry bulk density increase, and to ~1.9‰ 13C-enrichment of the surface peat. However, the increase was only 0.76‰ when δ13C of the corresponding surface peat layers (UNDR80 cm to DRA50 cm) were compared. Drained peatlands have been used as analogs to climate change induced drying and concomitant changes in vegetation and C flows (Laine et al. 1996), even though factors leading to natural permanent water table decrease oblige changes also in precipitation and air temperature. According to the results of this study, the effects of artificial lowering of the water table on δ13C could be comparable to the effects of the earlier severe dry periods values in peat columns. In general, δ13C values of surface peat layers decrease from their original values via diagenetic processes. In this study, the decrease in δ13C values was 0.20‰ per one % increase of C% (Fig. 3 equation). Therefore, earlier positive (or negative) shifts in δ13C values of ancient surface peat are noticeable in peat profiles as changes in δ13C value trends, but the change is probably smaller than the difference in δ13C values between undrained and drained sites in this study.

Conclusions

On both the undrained and drained sites, the most depleted δ13C values were at the same depths as the maximum peat dry BD, C%, N% and the minimum C/N ratio. This indicates the existence of a wet fen stage during the moist and warm period and diagenetic processes further accelerating 13C-depletion in the peat layer. Drainage and 37 years of water table decrease led to the loss of 13C depleted C and to increased δ13C values in the drained Rahesuo bog surface. The drained site was a source of 13C depleted CO2 to the atmosphere. The increased value of δ13C in the peat column due to an artificial lowering of the water table can be compared to earlier severe dry periods as a positive shift in δ13C values in the peat column accompanied by an increase in BD and C%. Analysis of δ13C can be used as an additional tool to follow current and past C flows in the peat columns. The method used here to calculate 13C and 12C balances in a known peat profile may be useful when studying the environmental changes in peat or lake sediment profiles when accurate 14C-datings and depth levels are available.