Methane Emission Heterogeneity and Its Temporal Variability on an Abandoned Milled Peatland in the Baltic Region of Russia

Abstract

Methane fluxes in disturbed peatlands can exhibit significant heterogeneity with regard to land cover composition on abandoned peat extraction areas. The temporal and spatial variability of CH₄ fluxes is considered in this paper in the context of a detailed vegetation classification on a typical milled peatland in the Baltic region of Russia (Kaliningrad oblast, Rossyanka Carbon Supersite). The findings are derived from the analysis of 12,000 air samples obtained by the opaque emission chamber method at 10 peatland sites with different environmental characteristics during regular measurement campaigns of 2022–2024. The emission data have been mapped using a multilevel B-spline interpolation procedure. The mean cumulative methane flux was found to be 18.7–28.8 kg ha⁻¹yr⁻¹, which is close to the IPCC conventional value of 32.9 kg ha⁻¹yr⁻¹ estimated for boreal and temperate zones. However, environmental distinctions across the peatland sites result in considerable emission heterogeneity ranging from −0.02 to 11.5 kg ha⁻¹month⁻¹. Temperature is considered a principal factor responsible for the baseline CH₄ emission level in seasonal scale, while hydrology defines emission rate during the warm period of the year and in the inter-annual scales. Five peatland site types have been defined according to a level of methane emissions.

1. Introduction

The measurement of greenhouse gas (GHG) emissions is widely acknowledged as an important objective towards understanding the causes of climate change and verifying the efficacy of mitigation efforts [1,2]. Consequently, a significant number of regional-scale studies have been undertaken globally [1,3]. The extent to which the issue is studied still varies substantially across regions, with some investigations being limited to small temporal scales. The key issues in this regard are long-term GHG monitoring and reliable estimates of the land surface GHG balance at regional and national scales. These activities are in accordance with the contemporary climate agenda and thus provide a robust baseline for further upscaling procedures, taking into account that this is recognised as a fundamental problem in translating climate data to the global scale [2].

The northern peatlands have been shown to constitute an immense global carbon sink [4,5,6]. Recent conventional assessments indicate that peatlands, comprising a mere 3% of the world’s terrestrial landscapes, store approximately 30% of the global terrestrial soil carbon, or 600 ± 100 Gt [4,7,8]. Conversely, they affect the composition of the atmosphere through the exchange of GHGs. In conjunction with other wetland ecosystems (e.g., marshes, lakes), it is assumed that they are responsible for approximately 5 Gt CO₂-eq of global annual GHG emissions [1]. The role of wetlands as the primary source of biogenic methane in the atmosphere was also noted, whereas disturbed peatlands are considered a source of pyrogenic methane due to frequent fires [3,9,10]. An estimated 10–15% of the current peatland area is drained, releasing stored carbon into the atmosphere [11]. Peatland degradation, fires and exploitation may currently be responsible for up to 5% of global anthropogenic GHG emissions [12,13]. Recent calculations estimate the contribution of wetlands to the global methane emissions at 0.16 Gt CH₄ per year (or 0.44 Gt CO₂-eq) for both natural and land-use change-affected ecosystems [3].

These facts have encouraged the launch of a new climate change mitigation strategy targeted for rewetting drained peatlands to prevent increased GHG emissions in the future. Rewetting can significantly reduce or stop net carbon loss and can even lead to new carbon sequestration [14,15]. However, rewetting often may lead to an increase in CH₄ emissions [13]. That is why these activities will require long-term monitoring of a rewetted area, including assessment of GHG exchange at reference sites.

In the Russian Federation, these investigations are carried out on Carbon Measurement Supersites, with the Kaliningrad region involved in this programme in 2021 [16]. In accordance with the programme’s conceptual framework [16,17], its development in the region is envisaged to include regular GHG flux measurement campaigns within a designated test area—the Rossyanka Carbon Supersite [18]. The principal research objectives on carbon measurement supersites include the verification of prediction models, the assessment of indirect approaches used for the estimation of GHG fluxes, and the determination of baseline emissions with a focus on subsequent implementation of sequestration technologies.

Kaliningrad Oblast is situated within the temperate bioclimatic zone of Europe [19], in the westernmost part of Russia, occupying the south-eastern sector of the Baltic Region. In this area, forests and mires are the main terrestrial ecosystems, although they have undergone significant changes over the centuries [20,21]. The disturbed peatlands are widespread in the region, and the measurement GHG fluxes from these areas can provide valuable insights into both carbon research and the development of ‘emission site type’ classifications in the context of upscaling issues.

A comprehensive analysis of numerous methane flux studies e.g., [9,10,22,23,24,25,26] has revealed several knowledge gaps, including the paucity of measurements during the winter period, the insufficient attention paid to land cover structure, microtopography, and vegetation characteristics on disturbed peatlands. The present investigation sought to address these issues, especially in light of the fact that soil–atmosphere CH₄ flux measurements constitute an important part of the ‘bottom-up’ approach, which in turn is considered crucial for the assessment of the regional CH₄ budget [3].

Another significant aspect pertains to the dynamics of methane flux in unmanaged and rewetted peatlands. As the research area of the Carbon Supersite is designated for the rewetting within 2025–2026 [20,27], the obtained data constitute a baseline for the further comparative analysis to examine the impact of peatland restoration.

In this paper, we consider a temporal variability of methane fluxes, regularly measured on 2022–2024 in a typical milled peatland in the Kaliningrad region, in coherence with the spatial heterogeneity of the peatland vegetation cover.

2. Materials and Methods

2.1. Study Area

This research was performed on an abandoned milled peatland known as Vittgirrensky (Figure 1), which constitutes a terrestrial part of the Rossyanka Carbon Supersite [16,18]. This area of 122 ha is located in the central part of Kaliningrad Oblast, in Slavsky District, 80 km east of the city of Kaliningrad (54.799516° N, 21.657558° E). The territory is situated within a flat depression on a local watershed comprising minor streams. The surrounding agricultural landscape is represented by rolling terrains of the glacial ground moraine (20–23 m above sea level) [28], though in geobotanical aspect, the territory is situated on the border of the boreal and nemoral zones [21].

The study area of the Vittgirrensky Peatland has undergone complete drainage, with peat extraction being conducted by the milling method since the late 1970s. In the late 1990s, peat extraction ceased, and the site was abandoned, though occasional peat extraction occurred locally. During the period of peat extraction, the soil profile underwent a substantial transformation in most parts of the Vittgirrensky Peatland [29]. Together with different hydrological regimes and vegetation cover, it defines different mire restoration feasibility of peatland sites and their different GHG emission potential. At present, the peatland area is covered by regrowth communities that have developed during the last few decades [20]. The presence of burnt tree trunks and charcoal residues in the uppermost peat layers indicate that the entire area is subject to occasional fires.

2.2. Climate

The local climate of the area is characterised as temperate maritime or moderately continental, with a very strong influence from the Atlantic Ocean and the Baltic Sea, as indicated by the data from the weather station Sovietsk (WMO ID 26614). The long-term (1991–2020) average air temperature of the coldest month, January, is only −2.19 ± 3.0 °C. Positive temperatures and thaw events are common in December and November. Summers, in contrast, are moderately warm without intense heat. The warmest month, July, has an average temperature of +18.33 ± 1.7 °C. The low annual temperature amplitude (about 20.5 °C between July and January) and the average annual temperature of +7.79 ± 0.78 °C clearly indicate significant moderating maritime influence.

Precipitation is abundant and relatively evenly distributed throughout the year (Figure 2). The annual total is 755.3 ± 121.5 mm. There is no distinct dry season, although summer months receive increased precipitation due to cyclonic activity and thunderstorms. There is a sharp seasonal contrast in sunshine duration. The mean annual sunshine duration (1871.8 ± 162.4 h) corroborates the prevailing cloudiness, particularly during the colder months. The snow cover is characterised by instability, shallow depth, and short duration, with maximum depth occurring in January. In certain years (2019), there is an absence of snow cover during the winter months.

2.3. Selection of Research Sites

According the detailed mapping [20], the vegetation cover of the Vittgirrensky Peatland consists of a complex combination of communities at various successional stages, developing towards tall birch stands in the centre and wet tall shrub or forest around the edges. The present-day vegetation is characterised by a significant coenotical diversity, which is manifested in its division onto 6 vegetation types with the further subdivision onto 22 plant community categories [20]. The area in the central part of the peatland was designated for research purposes owing to its location on a residual peat bed exceeding 40 cm in thickness and its potential for further mire restoration [29]. It comprises 10 plant community categories (land cover sites) that represent the main vegetation cover units (Figure 3 and

Table 1. Research sites for CH₄ flux measurements and their association with vegetation cover units in the Vittgirrensky Peatland (Rossyanka Carbon Supersite).

Research Site	Plant Community Category (Adapted After [20])	Location
1. Bare Peat	Open areas with bare peat	54.798990° N 21.658226° E
2. Juncus	Juncus- and sedge-dominated inundated fen-like communities	54. 802349° N 21. 658880° E
3. Eriophorum	Sparse birch regrowth with Eriophorum	54.798277° N 21.660018° E
4. Post-Fire	Birch stand on post-pyrogenic sites	54.801040° N 21.660440° E
5. Calluna	Dense high birch regrowth with Calluna	54.801853° N 21.656780° E
6. Sphagnum	Wet birch regrowth with Sphagna and Eriophorum	54.797548° N 21. 654183° E
7. Green Moss	Open areas with sparse moss fragments	54.798990° N 21.658226° E
8. Phragmites	Birch stand with Phragmites and Calluna	54.797063° N 21.657030° E
9. Tussock Ditch	Hydrophilic vegetation in drainage ditches with dominance of Eriophorum tussocks	54.798752° N 21.658657° E
10. Sphagnum-Lawn Ditch	Hydrophilic vegetation in ditches with dominance of Sphagnum lawns with Eriophorum	54.801615° N 21.658122° E

). The choice of research sites and measurement plots was based on the following criteria: (a) coverage of vegetation diversity and site area, (b) representation of structural elements, (c) association with areas of different moisture content, (d) peat thickness, and (e) transformation degree of the soil profile.

2.4. CH₄ Flux Measurements

The methane fluxes were studied at the soil–atmosphere interface using the manual opaque static chamber method [30,31]. The measurements were performed at each research site on three different sampling plots within one site to capture the natural intra-ecosystem variability of the methane fluxes. The sampling plots were allocated throughout the research area in a distance of 50–300 m for peat extraction fields and 10–20 m for ditches. We considered this distribution of the plots would be sufficient to provide statistical reliability of the results and to extrapolate the data obtained from three sampling plots to a vegetation cover unit. The plots were relocated annually, or more frequently in cases where vegetation cover changed and became no longer relevant for further investigation.

Plastic chambers 50 × 50 cm and 25 or 50 cm height were used for measurements at sites with moss or grass/shrub ground cover, respectively. At a site with Phragmites australis, custom-made chamber extenders with a height of 100 cm were used (Figure 4). The chambers were covered with opaque heat-insulating material with aluminium foil to prevent overheating of the internal space of the chamber. At each sampling plot, the chamber was placed over a 50 × 50 cm rectangular steel collar set into the peat to a depth of 10–15 cm below the level of the moss cover (Figure 4). Tap water was filled into a groove in the collar to prevent gas leakage from the chamber space. Electric fans, installed inside the chambers and connected to a 12 V portable battery, ensured the air was well mixed within the internal space. The temperature inside the chambers was recorded at one-minute intervals using Elitech RC-51 data loggers. A long plastic tube was connected to the chamber for pressure equalisation during the chamber installation. The wooden footbridges (2 × 0.5 m²) were installed near the observation localities to reduce the load on the peat surface.

The measurement campaigns, each spanning four days, were carried out every month from 2022 to 2024. The measurements were organised at 07:00 and 13:00 local time, with three chambers consequently placed on different sampling plots. The duration of the enclosure was one hour, during which four gas samples, with 0, 20, 40 and 60 min intervals, were taken from each chamber into 20 mL syringes sealed with rubber stoppers. A total of about 12,000 air samples and 3000 methane flux values were obtained in 2022–2024 from all observation sites.

2.5. Meteorological Measurement

In 2022 and January–March 2023, meteorological parameters were estimated from the nearest weather station Sovietsk (WMO ID 26614). Since April 2023, seven automated weather stations, the Atmosphere–Soil Measuring Complexes (APIC), have been installed at the Rossyanka Carbon Supersite (Figure 4). The APIC is designed for the autonomous, long-term, stationary monitoring of atmospheric, soil, and aquatic parameters [32]. Each complex is equipped with sensors that record various environmental parameters, including air temperature and humidity, atmospheric pressure, total liquid precipitation, incoming and reflected solar radiation intensity, groundwater level, soil volumetric moisture and electrical conductivity, and soil temperature (Appendix A). Air temperature and humidity sensors are mounted at a height of 2 m on a 50-cm console extending northward from the mast. These sensors are housed within radiation shields to prevent direct solar heating. Solar radiation sensors are positioned at height 1.5 m on a 2-m console that extends southward from the mast. Soil moisture and conductivity are measured at depths of 5 cm and 20 cm below the surface. Soil temperature sensors are installed at each observation site with triple replication under identical conditions at the following depths: 0, 2, 5, 10, 15, 20, 30, 40, 60, 80, and 120 cm. The measurement interval for all meteorological parameters is one hour.

2.6. Laboratory Analysis of Air Samples

Methane concentrations in air samples were determined using a Crystallux 4000M gas chromatograph (Meta-Chrom Co. Ltd., Yoshkar-Ola, Russia), configured with two flame ionisation detectors and GC columns Hayesep R (60/80 mesh, 0.5 m × 2.0 mm, Tmax = 250 °C). A calibration curve for methane content determination was generated based on a standard gas mixture GSO 10605-2015 [33], which included H₂, O₂, N₂, Ar, CO, CO₂, Kr, Xe, H₂S, CH₄, C₂H₆, and C₃H₈ in a helium diluent. The chromatographic measurement outputs were processed using ‘NetChrom’ software, version 2.1. The CH₄ concentrations in the samples were detected with an accuracy of 0.001 ppm.

A graph was generated for each sampling plot, depicting the changes in gas concentration over time. The values obtained were then recalculated into specific methane fluxes [25]. Concentration values that were identified as erroneous were subsequently eliminated from further consideration. Furthermore, certain specific flux values were also discarded. For instance, negative methane flux values that have been obtained in inundated localities, such as ditches. These values were found to have an insignificant input to the annual methane emission and were therefore excluded from the analysis.

2.7. Flux Calculations and Gap Filling

The CH₄ flux calculation methodology relies on the quantitative assessment of methane accumulation in a closed volume above the soil surface over a certain time interval. The specific CH₄ fluxes were calculated from the measured methane concentration values, using linear regression in time-concentration coordinates over four pairs of values, according to the following formula [25]:

$$F \left[\mathrm{m}\mathrm{g} {\mathrm{m}}^{-2} {\mathrm{h}}^{-1}\right]={\displaystyle \frac{c \times M \times P \times ∆C \times V}{S \times T}}$$

where c = 0.12 [mg mol K kg⁻¹ J⁻¹ ppm⁻¹]; M = 0.01604 [kg mol⁻¹]—molar mass of CH₄; P [Pa]—pressure in chamber; T [K]—temperature in chamber; V [m³]—chamber volume; S [m²]—area of the chamber base, ∆C [ppm h⁻¹]—rate of methane concentration change. The growth rate of the CH₄ concentration in the chamber (ΔC [ppm h⁻¹]) was determined from the slope angle of the linear regression between gas concentration (ppm) and time for 4 samples collected at 20-min intervals.

The processing of methane flux data consisted of multiple averaging steps, undertaken to derive representative values for each study site during the period 2022–2024. The mean CH₄ flux values in mg·m⁻²·h⁻¹ for each month were obtained by averaging the repeated measurements conducted during the corresponding time period for each of the ten land cover sites. Assuming a stable flux throughout the month, the total flux was determined as the product of the mean flux value and the number of hours in the given month. In order to generate a continuous time series despite the presence of irregular measurements, an inter-annual interpolation method was applied. For the months during which no measurements were taken (e.g., January–February 2022), the averaged data from the corresponding months in the other two observation years was used. This methodology is based on the assumption of cyclical seasonal variations in methane fluxes under similar hydrothermal conditions. The resulting monthly values were then aggregated into seasonal and annual averages.

2.8. Emission Mapping

A detailed, GIS-based vegetation map of the Vittgirrensky Peatland [20] was adapted for the research area and used to map the methane emissions at various time intervals. The vegetation map incorporates more than 100 polygonal features, which represent 10 distinct site types (Figure 3). The ‘point-in-polygon’ geoprocessing tool was used for interpolating polygonal vegetation map objects. Each point was then assigned a value corresponding to the methane concentration of the relevant vegetation unit. The multilevel B-spline interpolation technique was then applied to create maps illustrating the spatial distribution of methane emissions. This was achieved using SAGA GIS software, version 9.7.0 [34], after which the data were exported and processed in QGIS 3.34.10-Prizren.

The interpolation results are to be used solely for the purpose of visualisation, and the maps illustrating methane flux distribution are not intended to provide a basis for data interpretation.

3. Results

3.1. Weather Conditions in the Study Period of 2022–2024

The air temperatures at the Rossyanka Carbon Supersite (Vittgirrensky Peatland) ranged from −23.7 °C (1 September 2024) to +35.3 °C (17 August 2023). The most significant microclimatic variations between the base APIC station (‘Bare Peat’ site) and the other were observed from May to October, reaching ±5.8 °C. The mean monthly differences were predominantly positive, ranging from 0.1 to 4.2 °C, indicating that the ‘Bare Peat’ is the coldest observation point, while the ‘Calluna’ and ‘Post-Fire’ sites showed the greatest excess over the ‘Bare Peat’ (0.15 ± 0.11 °C). According to long-term data records, the mean annual air temperature has been higher than the long-term averages (1991–2020) in recent years. The annual temperature was recorded at 0.72 °C, 1.34 °C, and 2.25 °C for the years 2022, 2023, and 2024, respectively. The mean monthly temperature exceeded the norm by 0.12–4.9 °C.

Analysis of the water levels across the study sites has enabled their subdivision into three groups: ‘dry’, ‘wet’ and ‘moderate wet’. The dry group is represented by the ‘Post-Fire’ site, where the water level varies from 47 to 140 cm below the surface. The wet group, which includes ‘Sphagnum’, ‘Juncus’, and ditches, is characterised by prolonged periods of inundation. The moderate wet group includes ‘Bare Peat’, ‘Eriophorum’, ‘Calluna’, and ‘Phragmites’ sites, where water levels respond most rapidly to liquid precipitation and fall to 60–90 cm in autumn, but can rise to 5–20 cm in summer and winter.

The soil surface exhibits significant seasonal contrasts, with temperatures rising to a maximum of +51 °C during the summer months, and occasionally dropping to below −15 °C in the winter. The most pronounced summer heating was observed at the ‘Bare Peat’, which can be explained by the combination of low surface albedo and the absence of vegetation absorbing the solar energy. However, the situation changes during the winter months, when this site becomes the coldest due to increased radiative cooling of the open surface. The ‘Post-Fire’ site warms less than other sites in summer and shows the least cooling in winter. The temperature patterns persist within the soil profile, although the absolute amplitude of differences between sites decreases with depth. At the 20 cm level, the annual temperature range is from +0.6 °C to +22.5 °C. At a depth of 120 cm, the temperature regime becomes more stable and inert.

Total annual precipitation was 78.3 mm and 71.9 mm above the long-term averages in 2022 and 2023, respectively, and by 26.8 mm below the average in 2024. The annual anomalies resulted from complex redistribution of precipitation within the years. Excess precipitation (from 15 to 55 mm) was observed during the winter months in 2022–2024; in May–June 2022; March, August, October 2023; and April, July, November 2024. Precipitation sums were below average in the other months.

3.2. Methane Fluxes at the Soil–Atmosphere Interface

The present estimates and earlier published data [27] demonstrate that current methane emissions at the Vittgirrensky Peatland are more than two orders of magnitude lower than CO₂ fluxes. However, the methane flux in the peatland remains relatively stable throughout the year (Figure 5), ranging from 18.7 to 28.8 kg ha⁻¹yr⁻¹, with an average monthly variation of 1.1–3.5 kg ha⁻¹month⁻¹ (Table 1). The flux levels increase from May to September and then decrease until December. The most significant variations in the data are observed between June and October. In this context, the annual average methane emission value of 1.9 kg ha⁻¹month⁻¹ should be considered a small but stable source of greenhouse gas with a high warming potential.

The cumulative annual methane fluxes, calculated from all records, are shown in

Table 2. Averaged cumulative annual and inter-annual CH₄ fluxes on research sites representing different land cover classes (vegetation categories) in the Vittgirrensky Peatland (Rossyanka Carbon Supersite).

Research Site	Cumulative Annual/Inter-Annual Methane Fluxes
	2022 kg (CH4) ha−1yr−1	2023 kg (CH4) ha−1yr−1	2024 kg (CH4) ha−1yr−1	2022–2024
				kg (CH4) ha−1yr−1	kg (CH4) ha−1month−1
Sphagnum-Lawn Ditch	87.3	113.8	213.2	138.1	11.5
Tussock Ditch	50.3	81.1	113.4	81.6	6.8
Juncus	46.3	65.4	119.0	76.9	6.4
Sphagnum	44.0	53.6	89.2	62.3	5.2
Phragmites	38.6	25.6	63.1	42.4	3.5
Eriophorum	20.2	18.6	22.3	20.4	1.7
Calluna	7.8	6.8	8.7	7.8	0.6
Post-Fire	2.5	1.7	2.5	2.2	0.2
Bare Peat	0.5	−0.3	−0.1	0.02	0.002
Green Moss	0.1	−0.4	−0.3	−0.2	−0.02
Total (from 1 ha of the peatland area)	19.4	18.7	28.8	22.3	1.9

. The negative monthly fluxes have only been identified for the ‘Green Moss’ and ‘Bare Peat’ research sites (Figure 6 and Figure 7). The presence of negative flux values suggests that methane is being absorbed by the peat soil in this area. The minimum flux value of −0.15 kg ha⁻¹month⁻¹ was recorded at the ‘Green Moss’ research site in August 2022.

Significant methane emissions (exceeding 60 kg ha⁻¹yr⁻¹) were recorded at the hydrophilic research sites (‘Juncus’ and ‘Sphagnum’) and in drainage ditches, with a maximum yearly emission value of 213.2 kg ha⁻¹yr⁻¹. Medium methane fluxes (40–60 kg ha⁻¹yr⁻¹) were registered at the ‘Phragmites’ and ‘Eriophorum’ research sites. Relatively dry sites (‘Calluna’ and ‘Post-Fire’) exhibited a lower methane flux intensity, with average annual values below 10 kg ha⁻¹yr⁻¹.

3.3. Seasonal Dynamics of Methane Fluxes on Land Cover Sites

Table 3. Seasonal cumulative CH₄ fluxes on research sites representing different land cover classes (vegetation categories) in the Vittgirrensky Peatland (Rossyanka Carbon Supersite).

Research Site	Winter			Spring			Summer			Autumn
	2022	2023	2024	2022	2023	2024	2022	2023	2024	2022	2023	2024
Sphagnum-Lawn Ditch	2.9	4.5	5.7	5.5	5.1	12.8	17.2	12.5	41.8	3.5	15.9	10.7
Tussock Ditch	1.1	2.9	4.6	6.0	7.6	7.5	6.4	6.8	17.3	3.3	9.6	8.4
Juncus	2.5	3.2	3.7	2.5	3.2	6.3	6.8	4.9	13.5	2.8	6.6	6.2
Sphagnum	1.3	2.5	2.7	2.5	2.7	5.4	9.9	7.3	26.4	1.7	9.4	5.1
Phragmites	1.5	1.6	1.5	2.5	1.2	6.4	6.7	3.2	10.8	2.2	2.5	2.4
Eriophorum	1.5	1.6	1.7	0.9	1.5	1.3	2.3	1.2	1.7	1.9	1.9	2.7
Calluna	0.7	0.7	0.7	0.5	0.5	0.6	0.8	0.4	0.6	0.6	0.7	1.0
Post-Fire	0.3	0.2	0.2	0.1	0.1	0.1	0.3	0.04	0.2	0.2	0.2	0.3
Bare Peat	−0.01	−0.02	0.01	0.1	0.02	−0.01	0.01	−0.1	−0.01	0.01	−0.02	−0.02
Green Moss	−0.01	0.1	0.1	−0.1	−0.1	−0.1	0.1	−0.1	−0.1	0.01	−0.1	−0.04

demonstrates the seasonal values of methane fluxes on different land cover sites in the Vittgirrensky Peatland (Rossyanka Carbon Supersite).

The ‘Green Moss’ research site is located in the same part of the peatland where the bare peat surface is partially covered by the dense carpets of green mosses (Figure 6) such as Polytrichum strictum and/or an alien species Campylopus introflexus. Although the moss fragments represent only a small area within the bare peat fields, similar elements are also found in the communities of the other vegetation units. The residual peat bed thickness is 140–150 cm. As demonstrated in Figure 6, the moss carpets are the only peatland sites that indicate methane sequestration throughout the year and in the long term (−0.2 kg ha⁻¹yr⁻¹). It has been observed that sequestration may have been interrupted in cold winters (but not mild ones) and in very wet summer months. In other conditions, presumably favourable to methanotrophic microflora, these sites switch to active CH₄ sequestration, which is 2–5 times more in magnitude than the mean annual level (−0.02 kg ha⁻¹month⁻¹), with minor inter-annual variations.

The ‘Bare Peat’ research site is located on slightly elevated areas of the peat extraction fields along the central trunk drainage channel, where the peat bed is not covered by vegetation, being exposed on the surface (Figure 7). The thickness of the residual peat layer is 140–150 cm. Across the peatland sites, the patches of bare peat demonstrated the lowest average methane emission rates, with values approaching zero, and annual fluctuations ranging from −0.3 to 0.5 kg ha⁻¹yr⁻¹ (Table 1). The clear seasonal dynamics is not evident (Figure 7); the cumulative seasonal flux values could vary from −0.1 to 0.1 kg ha⁻¹month⁻¹ in every season. It can be inferred that, in the long term, the ‘Bare Peat’ site appears to be balanced between negligible methane emissions and negligible sink. This could be attributed to the contrasting temperature fluctuations in the upper peat horizon throughout the year. Negative flux values indicate the methane-oxidising microbial activity, which can occur under specific weather conditions.

The ‘Post-Fire’ research site represents vegetation at a particular stage of the pyrogenic succession. This is a dry scattered birch coppice where the ground layer consists of alternating large fragments of solid burnt peat crust, moss carpets, lichen synusia, Eriophorum vaginatum tussocks and dwarf shrubs of Calluna vulgaris (Figure 8). The site is characterised as the driest in the abandoned milled peatland, with the greatest water table fluctuations and strong pyrogenic degradation in the upper peat horizon [29]. The residual peat bed thickness is 80–90 cm. Communities of this type constitute large entire areas, but are also found as separate patches in other parts of the peatland [20]. The site is characterised by very low methane emissions, close to the mean annual value of 0.2 kg ha⁻¹month⁻¹ (Figure 8). Seasonal and inter-annual variations in methane fluxes are negligible, possibly due to the thermal dynamics of the surface peat layer, which demonstrates the least warming in summer and the least cooling in winter compared to the other peatland sites. Methane untake has been primarily observed on patches covered by mosses and lichens.

The ‘Calluna’ research site represents communities of a dense and high birch stand with the field layer dominated by the dwarf shrub Calluna vulgaris (Figure 9). The presence of pyrogenically modified elements, such as moss and lichen synusia, has been observed on burnt peat, scattered among the dominant heather bushes. The residual peat bed is 100–120 cm. The environments are similar to those in the ‘Post-Fire’ site, which is probably reflected in similar methane flux dynamics. The site demonstrated a consistent and low level of CH₄ emissions, with an annual mean value of 0.6 kg ha⁻¹month⁻¹ (Figure 9). This is three times higher than the level observed at the ‘Post-Fire’ site; however, seasonal and inter-annual variations in are also negligible. Methane uptake has been detected in certain instances, predominantly in moss and lichen synusia, and less frequently in some plots with Calluna vulgaris.

The ‘Eriophorum’ research site was established in a community of a low sparse birch regrowth with numerous tussocks of cotton-grass (Eriophorum vaginatum) present in the field layer (Figure 10). This type of community is widespread in the central part of the Vittgirrensky Peatland, where it covers the largest area of 22 ha [20]. The thicker residual peat bed (140–150 cm) results in higher levels of moisture and reduced drainage compared to the birch coppice dominated by heather. This is a typical stage of vegetation recovery on disturbed peatlands in the region. The site exhibited stable, positive methane fluxes, which approached the mean annual level of 1.7 kg ha⁻¹month⁻¹ throughout the year, though with a slight increase in autumn. Consequently, the birch coppice with Eriophorum demonstrates methane flux dynamics similar to those observed in drier birch coppice sites (‘Post-Fire’, ‘Calluna’), but with fluxes being 3–8 times higher in magnitude. This may be associated with the wetter conditions and the occurrence of additional methane bypass through the aerenchyma in the dense tussocks of Eriophorum vaginatum [22]. Negative CH₄ flux values were not recorded at the ‘Eriophorum’ site in 2023 and 2024, but were observed on separate research plots in 2022 in the late spring.

The ‘Phragmites’ research site is located on a peat extraction field with a sparse birch regrowth, the tall-herb layer dominated by Phragmites australis, and the field layer dominated by Calluna vulgaris (Figure 11). In many locations, the ground layer demonstrates a substantial presence of the tall hummocks of Polytrichum commune. A reduced thickness of the peat layer (100–110 cm) provides enhanced drainage and facilitates the growth of heather, while the thin residual layer of the raised bog peat (30 cm) allows for better water supply and eutrophic conditions more conductive to the spread of common reed.

The site demonstrates discernible seasonal fluctuations in methane fluxes, with a decline below the mean annual level of 3.5 kg ha⁻¹month⁻¹ (Table 1) during the autumn and winter months and an increase (of two to three times higher) recorded in summer. This fact can be attributed to the active growth of the common reed plants during the warm period. The process is likely to be coherent with weather conditions, as essential inter-annual variations in CH₄ fluxes (1.2–10.8 kg ha⁻¹month⁻¹) were recorded just in summer and spring, while they are negligible in the cold period, when the reed biomass with aerenchyma is absent from the site. Negative CH₄ flux values were not recorded at the ‘Phragmites’ site.

The ‘Sphagnum’ research site is located on wet peat extraction fields, where sparse birch regrowth is combined with small tussocks of Eriophorum vaginatum, clumps of green mosses with Carex rostrata, and Sphagnum carpets (Figure 12). These communities are characterised by the abundance and diversity of Sphagnum species. They are found in the edge zone of the peatland (ca. 2.5 ha), exhibiting features of a transition mire with its combination of typical bog and fen species. The residual peat bed is estimated to be 100–110 cm.

This site also demonstrates high cumulative methane emission (62.3 kg ha⁻¹yr⁻¹) with substantial flux fluctuations compared to the mean annual value of 5.2 kg ha⁻¹month⁻¹ (Figure 12), which may have been related to the water table dynamics. The fluxes are below the mean annual level at high water table in winter and spring during the period of the active surface runoff, whereas they increase significantly in summer months, when the water level drops (apparently, up to the depths optimal for methanogenic microbial activity). As the water level rises during the autumn months, there is a decline in methane fluxes, which stabilise at annual mean values. Negative CH₄ flux values were never recorded at the ‘Sphagnum’ site.

The ‘Juncus’ research site represents a recovery succession of mire vegetation, which develops towards poor fen formation on fields that have undergone extensive peat extraction. This resulted in the peatland surface being located closer to the groundwater level. The residual peat layer is very thin, rarely exceeding 30–40 cm. The long-term inundation of the peat substrate has been demonstrated to facilitate the development of hydrophilic communities with Juncus effusus and Sphagna (Figure 13).

Fen-like communities demonstrate similar seasonal and inter-annual CH₄ flux dynamics to those of the ‘Sphagnum’ site, but they exhibit a higher total emission level (76.9 kg ha⁻¹yr⁻¹) and greater magnitude differences between seasons with high (winter-spring) and low (summer-autumn) water tables. During the period of inundation, the emission values decline two or three times below the mean annual flux level of 6.4 kg ha⁻¹month⁻¹ (Figure 13), while can exceed it more than four times in the summer, when water subsides below the surface of the Sphagnum carpet. Furthermore, considerable inter-annual fluctuations in summer and autumn flux have also been observed. Negative CH₄ flux values were never recorded at the ‘Juncus’ site.

The plots of the ‘Tussock Ditch’ research site were installed in a narrow (1.5 m wide) and shallow (0.7–1 m) drainage ditch between peat extraction fields. The major space of the ditch is occupied by large tussocks of Eriophorum vaginatum (Figure 14), between which small loose mats of Sphagnum cuspidatum and patches of waterlogged peat without vegetation are scattered as separate fragments in a ratio of ca. 70%:25%:5%.

As demonstrated in Table 1 and Table 2, the total methane emission in the ‘Tussock Ditch’ was found to be remarkably high, with a recorded value of 81.6 kg ha⁻¹yr⁻¹. Most of the year, when the water table is close to the bottom, the fluxes are relatively stable, exhibiting a slight increase above the mean annual level of 6.8 kg ha⁻¹month⁻¹ (Figure 14). The emission levels decline to more than two times lower than the baseline only during the winter months, when the canals are flooded. Inter-annual variations have been recorded in all seasons, but most notably in summer and autumn, presumably due to substantial fluctuations in precipitation levels. Negative CH₄ flux values were recorded in certain cases on the plots installed on Sphagnum carpets in the ditch, but never on Eriophorum tussocks.

The ‘Sphagnum-Lawn Ditch’ research site was established in a shallow (0.7 m) but broad (3.5–4 m wide) drainage ditch between peat extraction fields, which is completely filled with a dense mat of Sphagnum cuspidatum and small tussocks of Eriophorum vaginatum distributed in the moss carpet in a ratio of 60% to 40% (Figure 15). This site was identified as having the highest level of total methane emission, with a recorded emission rate of over 138 kg ha⁻¹yr⁻¹, which corresponds to a mean annual level of 11.5 kg ha⁻¹month⁻¹ (Table 1). The maximum emission is observed to occur during the summer months, when it can reach increased peak values (up to 41.8 kg ha⁻¹month⁻¹). In autumn, the mean level of emission approximates the mean annual value. However, it should be noted that absolute values demonstrate significant inter-annual fluctuations, similar to those observed in the summer fluxes. During the winter and spring period, methane emissions exhibited a decrease in magnitude more than two times lower than the baseline level. Nevertheless, this emission rate (ca. 3–6 kg ha⁻¹month⁻¹) is regarded as significant for the cold period in comparison with the other peatland sites. In contrast to the tussock ditch, negative CH₄ flux values were not recorded at the ditch with Sphagnum lawns.

4. Discussion

4.1. Potential Factors Affecting Seasonal Variability of Methane Emission in the Study Peatland

The methane flux within natural ecosystems is determined by a multifaceted interaction of factors [35,36]. The predominant drivers are the depth of the water table, the temperature within the anaerobic zone of the peat, and the availability of organic substrates [37]. The water table depth controls the thickness of the aerobic surface layer, where microbial methane oxidation occurs. Peat temperature within oxygen-depleted layers directly regulates the metabolic activity of methanogenic archaea responsible for CH₄ production. The relationship between temperature and CH₄ flux is exponential; a 10 °C increase in anaerobic peat temperature can enhance CH₄ production rates by 2–4 times due to accelerated microbial metabolism. Simultaneously, a high water table position near the peat surface drastically reduces the depth of the overlying aerobic zone. This layer normally acts as a biofilter, oxidising 50–90% of upward-diffusing CH₄ before it reaches the atmosphere. The presence of shallow water tables therefore suppresses this oxidative capacity, resulting in substantially higher net emission levels, regardless of production rates. These factors can explain the methane emission distribution pattern recorded in the Vittgirrensky Peatland during different seasons of 2022–2024 and illustrated in Figure 16.

During the winter season, as demonstrated by the maps (Figure 16), the gradient of flux values across the sites is minimal, indicating a low level of methane emission in the studied peatland. The absolute values of the fluxes are less than the mean annual values on the majority of sites. It is apparent that low temperatures, the presence of snow cover, and the aggregation of wet moss plants and wet peat particles into a solid ice shell have a negative effect on the viability of methane-oxidising microorganisms. This effect is particularly evident in wet sites, where the development of ice and increased snowfall plays a significant role in reducing methane flux. Therefore, despite the occurrence of precipitation during mild winters, frequent thaws and maximal water tables, the general cold environments of peat and air result in methane flux values from the entire peatland being relatively uniform.

In spring, the fluctuations in flux intensity are more pronounced across the various sites, which may suggest that the depth of the water table becomes a more significant factor as the temperature rises. It can be concluded that the interaction of both these factors appears to define more distinct annual and inter-annual differences in CH₄ emission on peatland sites.

The summer absolute flux values are higher than the mean annual ones on most sites, and often reach their peak level, which reflects the optimal combination and equal significance of both factors affecting microbial methane production, either temperature and water table depth. Furthermore, the most substantial annual and inter-annual variations in methane emission values are observed at all sites.

The autumn values of the fluxes demonstrate the closest approach to the mean annual level, thus suggesting that this can be attributed to the values of soil temperature and humidity. Inter-annual variations are clearly discernible in gradient maps.

4.2. Annual and Inter-Annual Variability of Methane Emission in the Study Peatland

The variations in yearly fluxes are illustrated in Figure 17 and Figure 18. These maps indicate clearly distinctions between dry, moderate dry and wet sites in annual scale. The latter are characterised by increased methane emissions, which could be primarily related to the influence of hydrological factor. At these sites, the water table is closer to the surface than at any of the other sites, which distinctly affects the decrease in the thickness of the aerobic layer and the increase in methane emissions from the surface.

On the other hand, the gradient distinctions on maps are differently pronounced from year to year, exhibiting substantially varying rates of CH₄ fluxes, especially on wet sites with sometimes enormous emission level. Therefore, the impact of hydrological factor should be considered in coherence with the other meteorological parameters.

The observed increase in CH₄ fluxes during 2024, despite below-average annual precipitation (26.8 mm below the long-term norm), represents an apparent paradox resolved by examining specific climatic events and system dynamics. The record-breaking mean annual air temperature anomaly, which exceeded the 1991–2020 baseline by 2.25 °C, provided fundamental thermal forcing that accelerated methanogenesis throughout the growing season. It is important to note that this thermal effect peaked during July 2024, coinciding with an extreme precipitation event. Despite the annual precipitation being deficient, July alone received 205 mm of rainfall, which is 238% of the monthly norm (86.1 mm). This intense, concentrated downpour triggered a rapid rise in the water table precisely during the warmest month. The convergence of peak soil temperatures (optimising microbial activity) and a suddenly shallow water table (minimising the aerobic oxidation zone) created optimal conditions for maximising CH₄ flux. Furthermore, the hydrological legacy from significantly wetter preceding years (precipitation exceeding norms by 78.3 mm in 2022 and 71.9 mm in 2023) preconditioned the peatland’s moisture status. The sustained saturation meant that despite the 2024 annual deficit, the system retained sufficient moisture, amplifying the impact of the July deluge on water levels. This hydrological memory effect, combined with the July heat-rainfall extreme, overrode the influence of the annual precipitation deficit, driving higher fluxes in 2024 compared to 2022–2023. Secondary contributors were likely to have included enhanced substrate availability resulted from the accelerated peat mineralisation under warmer conditions.

A similar trend of increased methane emissions was identified in spring 2024. In this case, the primary driving factor was the anomalously high water table level following snowmelt and spring rains. Despite the low annual total, precipitation during the early spring was sufficient to saturate the peat profile. This resulted in the anaerobic conditions that are essential for methanogenesis.

Thess sequences highlight how short-term weather extremes and previous hydrological conditions can have a greater influence on annual methane budgets than cumulative annual climate averages.

Nevertheless, it could be stated that, on annual and inter-annual scale, the hydrological differences between land cover sites generally determine the rates of methane emissions in the disturbed peatland in the temperate climatic zone. This can also be seen on a ‘resulting’ map showing inter-annual cumulative emissions from the studied peatland (Figure 18).

4.3. Inter-Regional Comparisons

The results obtained during the study (Table 2) are relevant to the average methane fluxes from disturbed peatlands in the boreal and temperate zones [13], which are currently estimated at 32.9 kg ha⁻¹yr⁻¹ (versus our value of 22.3 kg ha⁻¹yr⁻¹). To date, many regional studies have focused on methane emissions from intact and disturbed peatlands, though few have considered the land cover heterogeneity that develops on peat extraction sites after abandonment. A further issue that arises during the comparison of the land cover units used for emission measurements is the question of ‘unification’. In

Table 4. Inter-regional comparisons of CH₄ emissions from land cover units of a similar structure.

Region	Land Cover Site and Flux Value, kg(CH4)ha−1month−1
Kaliningrad (milled peatland)	Green moss −0.02	Bare peat 0.002	Post-fire 0.2	Calluna 0.6	Eriophorum 1.7	Pragmites 3.5	Sphagnum 5.2	Juncus 6.4	Tussock ditch 6.8	Sphagnum-lawn ditch 11.5
Belarus (milled peatland) [38]	Polytrichum 1.8	Bare peat 1.8		Calluna negligible	Eriophorum 9.1		Sphagnum hummock 21.3			Sphagnum lawn 157.9
Canada (milled peatland) [24]	Moss 0.01	Peat −0.2		Shrub 0.2	Herbaceous −0.1				Ditch 29.1
Denmark (grazed pasture on peat) [39]								Juncus tussock 10.7
NW Germany (drained bog grassland) [40]									Ditch with floating Sphagnum mats 16.6
Moscow oblast (milled peatland) [9]									Ditch 277.6
Finland (forestry-drained peatland) [26]									Vascular ditch 7.3	Sphagnum + vascular ditch 3.2
UK (communities on a bog) [22]									Sphagnum + Eriophorum 21.9

, we compared methane emissions in different regions from land cover units of a similar structure, as described in the papers [9,22,24,26,38,39,40].

Table 4 illustrates a wide range of data, including both similar and significantly different results to our findings. It may be concluded that local environmental factors and the structure of disturbed peatlands can play an important role in affecting methane production. In this regard, it should be noted that a detailed land cover unit classification is required for GHG inventories, as well as for regional calibration when applying the results.

4.4. Methane Emission Site Types on an Abandoned Peatland

Following the analysis of the data obtained during the investigation, five site types were identified on the studied abandoned peatland according to the level of methane emission recorded (Figure 19).

• Sites emitting 100 kg ha⁻¹yr⁻¹ or more (ditches with a thick Sphagnum carpet, exhibiting an average monthly CH₄ production of more than 10 kg ha⁻¹month⁻¹, with a decrease during periods of ice development).
• Sites emitting 50–100 kg ha⁻¹yr⁻¹, i.e., 5–7 kg ha⁻¹month⁻¹ (areas with a well-developed carpet of Sphagnum mosses, which experience considerable waterlogging throughout the year: ‘Juncus’, ‘Sphagnum’, ‘Tussock ditch’).
• Sites emitting 20–50 kg ha⁻¹yr⁻¹, i.e., 2–4 kg ha⁻¹month⁻¹ (moderately dry abandoned peat extraction fields, which are dominated by aerenchymatous plants: ‘Phragmites’ and ‘Eriophorum’).
• Sites emitting 1–10 kg ha⁻¹yr⁻¹, i.e., 0.2–0.6 kg ha⁻¹month⁻¹ (dry peat extraction fields, dominated by dwarf shrubs, often with regenerating post-pyrogenic patches: ‘Calluna’ and ‘Post-Fire’).
• Weakly emitting or weakly methane-sequestering sites—locations capable of emitting and sequestering methane in negligible quantities, no more than 1 kg ha⁻¹yr⁻¹, in average from −0.1 to 0.1 kg ha⁻¹month⁻¹ (‘Bare Peat’ and ‘Green Moss’).

5. Conclusions

Despite the extensive amount of data available on methane emissions from disturbed peatlands, such fluxes can be highly heterogeneous within an ecosystem. This heterogeneity is primarily influenced by the highly heterogeneous land cover composition in the studied peatland, which is contingent on the specifics of peat extraction and the following development stages. The heterogeneity of the Vittgirrensky Peatland (Rossyanka Carbon Supersite, Kaliningrad Region) is determined by a combination of ten environmentally distinct land cover sites, with clear distinctions in vegetation structure, water level dynamics, microclimatic differences, and properties of the residual peat bed.

Distinctions in the environment of peatland sites can impact local methane production, resulting in variations in cumulative CH₄ emissions values across different sites throughout the year. These variations can span more than two orders of magnitude, ranging from −0.02 to 11.5 kg ha⁻¹month⁻¹. In general, methane fluxes remain consistent throughout the year in the entire Vittgirrensky Peatland, reaching a cumulative level of 18.7–28.8 kg ha⁻¹yr⁻¹. This value is in accordance with a conventional value of 32.9 kg ha⁻¹yr⁻¹ assessed by the IPCC as a sustained CH₄ emission factor for the peat extraction areas in the boreal and temperate zones [13].

Temperature is a principal factor responsible for the baseline CH₄ emission level in a seasonal scale, while water table depth has a stronger effect as warmer environments become more pronounced in the disturbed peatland. The hydrological factor is thus a determining factor in the general methane emission rate during the warm period of the year, as well as in the annual and inter-annual scales.

Methane flux values were for the first time identified in the Kaliningrad Region of Russia. Inter-regional comparisons demonstrate both similarities and marked differences in methane budgets, which may be attributable to inconsistencies among authors in the treatment and volume of land cover units. Similar studies require a vegetation unit unification system and regional calibration of results.

Based on data obtained during our investigation, we have defined five site types according to methane emission levels: (1) ditches with thick Sphagnum lawn, emitting > 100 kg ha⁻¹yr⁻¹; (2) inundated and wet sites, emitting 50–100 kg ha⁻¹yr⁻¹; (3) moderately moist sites with aerenchymatous plants, emitting 20–50 kg ha⁻¹yr⁻¹; (4) dry sites with pyrogenic modifications, emitting 1–10 kg ha⁻¹yr⁻¹; and (5) open sites with bare peat and green moss fragments, performing slight CH₄ sequestration or exhibiting negligible emissions from −0.1 to 0.1 kg ha⁻¹yr⁻¹.

While the observed spatial heterogeneity in CH₄ fluxes across the different land cover sites is visually evident and considerable (ranging over two orders of magnitude), it is important to note that this study focuses on the net emission budgets and their mapping. Formal statistical analyses to quantify the significance of these spatial differences will be presented in a subsequent paper, which will provide a detailed statistical characterisation of the flux time series and their drivers.