Dissolved organic carbon vertical movement and carbon accumulation in West Siberian peatlands

Dissolved organic carbon is an additional path of carbon cycle but there is a lack of information about its distribution in peatland and rates of downward movement. We dated seven peat cores (separately the dissolved (DOC) and particulate (POC) organic carbon) from Mukhrino peatland (typical zonal oligotrophic bog) in western Siberia to assess the date distribution between those two peat fractions. Our results revealed that the DOC is younger than POC for the surface peatland layers (0-150 cm) and older for the deeper layers. The date differences increases with depth and reaches 2000-3000 years at the bottom layer (430-530 cm). In our hypothesis this date discrepancy caused by more young DOC moving to the deeper and older peat layers. The estimated average value of DOC downward movement was 0.047±0.019 cm yr-1. Th oldest dates found at the lake bottom and ancient riverbed were 10 053 and 10 989 cal yr BP correspondingly. For the whole period of peatland functioning the average peat accumulation rate was estimated as 0.067±0.018 cm yr-1 (0.013–0.332 cm yr-1), the carbon accumulation rate was estimated as 38.56±12.21 g С m-2 yr-1 (28.46–57.91 g С m-2 yr-1).


Introduction
Peatlands unlike the most other ecosystems assimilate carbon and sequestrate it over thousands years as long as net primarily production exceeds the rate of organic matter decomposition. It has been estimated that peatlands occupy about 2.84% (4.23 million sq. km.) of global land area (Xu, 2018) but have accumulated disproportionally huge amount of world soil carbon (~30 %) .
West Siberia is the world's largest wetland where peatlands cover 50-75 % of whole area (Peregon, 2008) that makes this territory one of the most waterlogged place in the world. Peatlands occupy here not only local relief depressions but also vast watershed areas and floodplains (Bleuten and Lapshina, 2001). The West Siberian peatlands are estimated to contain ~20 % of world peat Treeless through flow fens and Sphagnum lawns with hollow vegetation and rare scattered hummocks are developed within limited areas in the lower reaches of the peatland water catchment (Filippov and Lapshina, 2008).
Peatland has a dome shape with the difference in elevation between the edge and central parts in 1.2 meters (Bleuten et al., 2020). Central part is occupied by ridge-hollow complex; at the edge it becomes more inclined and drained letting the ryam and tall-ryam communities grow.

Peatland stratigraphy
Mukhrino peatland has been initiated as a minerotrophic fen with dominance of trees (birch, pine, fir) and herbs (fern, horse-tail and tussock-forming sedges). Remains of these plants form the bottom layer of minerotrophic peat covering whole area of the peatland. Its thickness does not

Peatland hydrology
Mukhrino peatland is fed by the rain and melt snow water. The highest water level is coincided with the snow melt at the end of April -beginning of May. Usually melt water is kept on the peatland surface in local depressions or blocked by ridges until the upper peat layer melt down. In that time all water rapidly seeps into the peatland, dramatically increasing water level. The lowest water level is recorded at the end of summer time (usually in August) then water table rise up because of intense autumn precipitation and low air temperature decreasing an evapotranspiration. Discharge from the streams stops at the mid of October when water freezes. For more information see (Bleuten et al., 2020).

Field sampling
In summer 2016, a total of 7 peat cores (Table 1) were extracted through the entire depth of peatland with a Russian corer (5.0 cm inside diameter, 0.5 m sampling step). Each half-meter part was moved to the plastic cassette, wrapped in stretch-film, transported to the laboratory (Yugra State University) and subdivided into 10-cm subsamples for the further analysis. In the field 68 samples of 1 cm thick were cut from the bottom of each half-meter of 7 peat cores for the AMS radiocarbon analysis. With minimum contact to the environmental to avoid contamination it has been moved to the plastic zip-bags, labeled and sent to the Max-Plank Institute of Biogeochemistry in Jena (Germany). For the description the peatland stratigraphy additionally 34 peat cores has been collected for the period 2010-2016 (Fig. 1).

Plant macrofossils
Plant macrofossil was analyzed in each 10-cm subsample. For that a piece of ~10 cm 3 was sieved through 0.25 mm mesh under flowing water. Plant remains were identified under the binocular microscope (Zeiss Axiostar, 10-40×magnification, Jena, Germany) using both our own experience and the key samples data bank. Peat content was described as abundance of each type of plant remains in percepts, and dominated species remains in a sample determined a peat type.

Bulk density, carbon and ash content
Bulk density, carbon and ash content were determined for each 10 cm subsample using the middle part of 5 cm length (volume 50 cm 3 ). Bulk density (BD: g cm -3 ) was measured by drying the peat at 105° C for 24 hours and later weighting. The dried subsample was grinded and divided into two parts. Ash content was determined from one part by ignition (Nabertherm L9/11/SKM, Lilienthal, Germany) at 525° C for 9 hours, and a carbon content was determined in the other part. In the elemental analyzer (EA-3000, EuroVector, Pavia, Italy) sample combusted in the oxygen and helium flow on the Ni/Cu catalysts and separates on the chromatography column. Elements are identified on the thermal conductivity detector using Atropine (C=70.56 %, N=4.84 %, H=8.01 %, O=16.59 %) as a standard.

Accumulation rates and dissolved organic carbon downward velocity calculations
Peat accumulation rate (A) was calculated for each 50 cm part (or more in case of a dating lack) using the next Eq. (1): where A -peat accumulation rate, cm y -1 , dl -lower dated depth, du -upper dated depth, al -the date of lower depth, au -the date of upper depth.
Carbon accumulation rate (CAR) was calculated for each 10 cm part using Eq.
where CAR -carbon accumulation rate, g C m -2 y -1 , V -volume of peat 10000, cm 3 (i.e. 1 m 2 of 10 cm depth), BD -bulk density, g cm -3 , LOI -loss on ignition, %, CC -carbon content, %, 10/Aiyears to grow 10 cm of peat, years. Long term rate of carbon accumulation (LORCA) was using Eq. (3): where LORCA -long term rate of carbon accumulation, g C m -2 yr -1 , CARcumulative -cumulative carbon storage in a square meter in the core, g C m -2 , Abottom -the bottom age of the core, years.
DOC downward movement velocity was calculated The next Eq. (4) has been used: where v -DOC movement velocity, cm yr -1 , di -depth of current DOC age, cm, ddoc_i -POC depth of the same age as a current DOC, cm, apoc -age of POC on the current depth, years, adoc -age of DOC on the current depth, years.

Separation of DOC and POC
Using the approach from (Schulze, et al., 2015) it is possible to separate the DOC from the POC by dispersing the peat sample in distilled water. First the frozen peat sample was thrown carefully, weighed, dissolved in distilled water and shaked for 2 hours. The solution was wet-sieved with a 63 and 36 μm mesh sieve. The residues from the sieve were freeze dried (Piatkowski, Munich, Germany). The sieved solution (< 36 μm) was adjusted to pH 9 by adding NaOH, shaked another 20 Minutes and centrifuged at 2900 g for 30 min (Megafuge 3.0, Heraeus, Hanau, Germany). The obtained supernatant was filtered with a vacuum flask using a 1.6 μm glass fiber filter (Sartorius) which has been baked at 500° C beforehand. The filtered matter < 1.6 μm was freeze dried and defined as dissolved organic matter (DOC).
The filter residues > 1.6 and the pellet remaining from the centrifugation < 36 µm were merged and freeze-dried. This fraction (> 1.6, < 36 µm) was defined as particulate organic matter (POC).

AMS 14 C analysis
The peat core DOC and POC samples were analyzed with the Accelerator Mass Spectrometer (AMS) radiocarbon ( 14 C) method (Steinhof, 2016(Steinhof, , 2017. For one measurement 0.7 mgC is needed. The Samples pass a chemical preparation whereby the sample was combusted and the emerged CO2 trapped and catalytically reduced to graphite under presence of Fe 2+ powder and H2. The resulting graphite was pressed into targets and finally measured in the AMS system. The graphite was ionized in the AMS system (negative charge) and accelerated within an electric field to a final energy of 400 keV. The 14 C isotope ratios was been  . Table A1) with the IntCal20 (Reimer et al., 2020) and NH1 post-bomb (Hua et al., 2013) atmospheric curves using the package 'clam' (Blaauw, 2020). The age-depth model was developed using the Bayesian-based package 'rbacon'  with 95 % confidence intervals.

Peat stratigraphy profile, ages and accumulation rate
Average peatland depth based on 35 cores is 340 cm ranging from 85 till 530 cm ( Fig. 2. western point is a depression of primary lake (Core 18; 480 cm) partly covered by gyttja (100 cm) and peat (380 cm); the point on the east (Core 2; 530 cm) is associated with an ancient streambed rush-peat at the bottom. Other cores have minerotrophic grass-woody peat at the bottom (40-60 cm) and have depth till 350-400 cm. The oldest peat is 10 989 cal yr BP, the oldest gyttja is 10 053 cal yr BP. Peat growth started via terrestrialisation when the lake sediments filled the lake basin between 7 000 and 6 000 cal yr BP.
The average age of peatland initiation based on 8 dates is 10 265 cal yr BP (Appendix . Table A1).

Bulk density and ash content
Bulk density (BD) values increase linearly with depth from 0.016 till 0.348 g cm -1 . Mostly it is caused by changes of peat stratigraphy: oligotrophic Sphagnum moss peat types have the lowest BD values because they consist only of the Sphagnum remains which mostly resistant to the decomposition processes and thus keep their initial volume (Pologova, Lapshina, 2001). Mukhrino peatland many well-preserved Sphagnum fuscum peats (decomposition rate 5-10 %) have been found on the depth below 200 cm. Minerotrophic peat contains a lot of woody and sedge remains which over time lost volume structure and create dense peat layers, also it additionally may mix with mineral sediments at the bottom (Loisel, 2014).
Ash content changes irregular with the local maximum (5-8%) at the 100 cm depth, with the next decline (~2.5%) and slow increase till the mineral bottom (5-7%). These variations are related to the plant remain composition of peat -oligotrophic Sphagnum peat types have the lowest ash content while herbs and wood peat have increased concentration of ash (Loisel, 2014).

Peat accumulation rate and carbon content
LORCA ranges between 24.80-28.92 g С m -2 yr -1 (average value is 26.93±1.76 g С m -2 yr -1 ). The carbon accumulation rate was estimated also on a base of averaging data for each 10 cm layer. It has an average value 38.56±12.21 g С m -2 yr -1 (28.46-57.91 g С m -2 yr -1 ). This value exceeds the LORCA because of irregularity of a carbon accumulation process.
The average C content for the 10 cm layer is 6.16±1.46 kg С m -2 . The total amount of carbon stored in 4.3 m depth (average depth of all cores used for calculation) is 264.9±62.8 kg C m -2 .

POC & DOC
Relation between the DOC and POC dates is well-correlated (r 2 =0.98, slope 0.93) till the breaking point at ~6000 cal year BP (Fig. 2). Older dates are with the less slope (0.7) and more distributed (r 2 =0.75). Differences between DOC and POC ages from the same depth are linear (r 2 equal 0.55, slope is 0.14) and range from 80 yr at 50 cm untill 2000-3000 yr at the bottom layers (430-530 cm).
The linear (cores 5, 5-5, 18, 19, 27 and 31) and exponential (core 2) models have been used for calculations ddoc_i (the depth POC came from). The average velocity of DOC downward movement is 0.047±0.019 cm yr -1 (Fig. 2). The minimal -0.52 cm yr -1 and maximal 17.7 cm yr -1 values are found for the upper 50 cm and probably related to the dating uncertainty of modern dates. Excluding those values the rest of calculated DOC movement rates are in range from -0.24 to 0.97 cm yr -1 .
Negative values mean an upward DOC movement which found for 10 samples (~15 %).

Discussion
The main physic-chemical peat properties such as carbon and ash contents, bulk density along the peat profile has been discovered in the current study. It is shown that ~2/3 of a peat body consists the sphagnum oligotrophic peat with low ash-content and low bulk density what nicely match to the existing data for the West Siberian lowland (Bleuten and Lapshina, 2001). These properties are High ash content values at the upper peatland layers (50-100 cm) has been found in all peat cores in this and other studies at the Mukhrino peatland (Lamentowicz et al., 2015;Tsyganov, et al., 2021). It might be explained by extremely high flooding 1-2 kiloyears ago with alluvial material sedimentation or by a probable fire occupied a peatland. But the fire history of Mukhrino FS over the last 1300 years have not discovered any significant amounts of charcoal (Lamentowicz et al., 2015).
The average peat accumulation rate (A) is 0.067±0.02 cm yr -1 . The lowest average value (0.04±0.02 cm yr -1 ) found for the hollow (core 31) and the highest average value (0.10±0.08 cm yr -1 ) found for the ryam (core 2). The shape of the age-depth model was close to linear for the cores 19a and 31, sshape for the cores 5a, 27 and 5-5, convex for the core 2 and broken for the core 18. Any features combining all cores into these groups have not been found. The majority of peat age-depth published models have concave shapes, meaning that a decay process is ongoing in the catotelmlower anaerobic peat layers (Yu, 2001). Absence of this shape model at the Mukhrino peatland may be caused by dominance of peat-moss (oligotrophic Sphagnum) remains (till 90 % of core) which are the most resistant to decomposition (Thormann et al., 1999).
The A was the highest for oligotrophic peat (0.080±0.038 cm yr -1 ), less for minerotrophic peat (0.062±0.033 cm yr -1 ) and the lowest for transitional peat types (0.061±0.027 cm yr -1 ). Oligotrophic peats mostly consist the remains of Sphagnum mosses, which are mostly resistant to the decomposition process. (Pologova and Lapshina, 2001) showed similar values for Great Vasyugan mire, where oligotrophic Sphagnum peat has higher A (0.115 cm yr -1 ) then minerotrophic peat (0.059 cm yr -1 ). Probably, these higher values explained by the location of Great Vasyugan mire in Southern taiga, the most favorable meteorological condition zone for peatlands development (Ivanov and Novikov, 1976), and the local plant biodiversity. In (Lapshina, 2011) the average A for middle taiga zone is 0.056 cm yr -1 , when in Southern Taiga zone this value is 0.074-0.08 cm yr -1 , that supports the concept of the different external conditions of peat accumulation.
In case of eutrophic phase of peatland development, the peat accumulation process is determined by the mineral soil proximity and hence favorable geochemical conditions together with faster peat accumulation (Frolking et al., 2001) due to higher litter input (Thormann et al., 1999). Also the fen vegetation is less sensitive to climate conditions thus has more stable A (Frolking et al., 2001).
Nonetheless, initial mass loss rate for fen plant species and older age, i.e. longer priod of decomposition, causes lower A value.
Altogether this means that the type of water supply (rain or ground water) and hence a way of nutrient income is one of the main limiting factors in peat accumulation process.
The average rate of carbon accumulation (CAR) is 37.99±11.4 gC m -2 yr -1 (median value is 26.17 gC m -2 yr -1 ). This value inconsistency is caused by the skewed data to the high values mostly at the upper layers due to high A (0.15-0.33 cm yr -1 ) and at the bottom layers due to higher bulk density water by DOC which concentration systematically increases with depth and may prevent its further active penetration. Limited number of papers covers this topic (Chasar et al., 2000;Cole et al., 2002, Clymo andBryant, 2008) and report concentrations ~2 mmol dm -3 at the surface and 6-22 mmol dm -3 at the bottom. There was not found any information for the West Siberian peatlands.
The negative values of DOC velocity mean an upward flux which might be caused by water table movement at the surface layers. A rising water table may catch a part of DOC produced in the lower layers and move it up to the surface thereby making converse between DOC and POC ages (Schulze et al., 2015). Several negative DOC velocities were found on the deeper layers (200-300 cm). This caused by the methodological flaw in the value calculations when the s-shape (cores 5a and 27) agedepth model is approximated by the linear regression.
There are few publications estimating DOC vertical velocity values. (Charman et al., 1999) used a vertical hydraulic conductivity equal to 31.5 cm yr -1 to estimate DOC vertical transport in the UK.
This value exceeds our results in ~600 times because based on a potential water movement which significantly ranges under conditions of saturation and peat physical properties (Chabson and Siegel, 1986). This value might be used as a potential rate of DOC downward movement but has to be considered as a maximum possible velocity, i.e. a limiting factor.
The difference between DOC and POC ages in our study increases with depth (from 9 till 3 044 yr, excluding three negative differences found for the upper 50 cm) that supported only by the concept of DOC dowanward motion. Another reasons of date differences might be largely excluded: -sedge and Scheuchzeria roots growing down the peat till the 2 meters (Glaser et al. 2012) have not been found in any dating sample ( and discharges to the streams (Fig. 3). The mechanism of DOC downward movement is described in number of publications (Aravena et al., 1993;Charman et al., 1993Charman et al., , 1994Charman et al., , 1999Chanton et al., 1995) showing that the gas 14 C ages are significantly younger in DOC than the particulate peat. The possible reason may be described by conversion of young DOC transported from the upper peat layers to CO2 and CH4 by microbial activity. In our previous study in Western Siberia the maximal age difference 6 500 years has been recorded (Schulze et al., 2015). The results from Southwest England show that DOC is 830-1260yr younger than the peat it was extracted from (Charman et al., 1999). Clymo and Bryant (2008)  published perfect results about a 7 m deep peatland where differences between the peat and DOC ages increase with depth since 80 till 1 835 yr.
In (Kraev et al., 2017) shown a possible way of methane displacement to the deeper soil horizons due to freezing of thick strata of epigenetic permafrost. The same mechanism potentially might be found for the peatlands, because high peat porosity is a favorable substrate for vertical water movement. The surface Mukhrino peatland layer freezes at the end of September -beginning of November. The water discharge completely stops at that time, thus a peatland becomes a huge reservoir filled by the high porosity substrate and water, which completely confined by ice pack