1. Introduction

2. Materials and Methods

3. Results

3.1. GHG Fluxes from Ecosystems of the Russian Federation

Natural ecosystems area included in the calculation by [1] do not always coincide with statistical data (Table 1). Thus, the recorded wetlands appear to partially overlap with forest land and tundra. At the same time, estimates of GHG fluxes from peatlands on forest lands according to the methodology used do not lead to double counting, but complement each other. Whereas in the case of tundra, there is a possible risk of double counting. This issue requires further study.

In the inventory, the category “managed grasslands” includes hayfields and pastures as well as overgrown cropland. However, in the study by [1], abandoned arable lands were fully accounted for in the secondary steppe category, which raises some doubts. Apparently, it is a consequence of overestimation of these areas in the steppe zone. As a result, overestimation of carbon accumulation in secondary steppe soils occurs. The sum of hayfields, pastures and steppes turned out to be higher than the total area of grasslands in the state statistics, which may also indicate overestimation of the areas of natural steppes outside forage lands. A recent paper by the authors of this assessment obtained roughly the same results [28]. In this study, the areas of grasslands and fodder lands are harmonized with the data of the Russian Land Cadastre [17]. Natural steppes are mostly used as pastures [46], which was taken into account during redistribution and in order to eliminate double counting of areas. In addition, clarification of the share of steppe fallow lands allowed to correctly identify fallow lands in other regions and natural zones of the country (almost 20 thousand hectares). Natural steppe areas were also redistributed between managed and unmanaged grasslands.

The analytics of the obtained results on GHG fluxes of natural ecosystems by gas type and by administrative division of Russia according to [1] is shown in Figure 1. Maximum net absorption of CO₂ is a characteristic of the most forested regions, including those with extensive wetlands. They are in the Northern regions of the European part of the Russian Federation, Western and Eastern Siberia, and the Far East (Figure 1a). Methane emissions are also associated with wetlands and tundra ecosystems, and nitrous oxide emissions are associated with tundra ecosystems only. Accordingly, we see the distribution of methane and nitrous oxide emission intensity by regions of Russia (Figure 1b,c). Maximum fluxes of each GHG are typical for Krasnoyarsk Territory, characterized by natural ecosystems of all climatic zones, largely compensating each other. Net GHG flux of Krasnoyarsk Territory refers to a small net source (9 million tons of CO₂-eq.).

Net absorbers in terms of the sum of all GHGs are forest regions in the North-Western Federal District of Russia, Central, Siberian and Far Eastern Federal Districts. At the same time, significant wetland areas in these counties determine relatively high methane emissions, which offset part of the carbon sequestration in forest biomass. Therefore, in the Asian part of the country, maximum net absorption is characteristic of regions rich in forests but with a relatively low percentage of wetlands (southern subjects of the Asian part of the Russian Federation). Southern regions in the European part of the country, the Southern, North Caucasus and Volga Federal Districts, with smaller areas of forests and larger areas of pasture lands and freshwater bodies are net sources. In general, the territory of natural ecosystems of our country has a negative balance for all types of GHGs and belongs to net absorbers.

The data for forest land obtained by [1] require updating for the value of net CO₂ uptake (-804 million tons of CO₂). These estimates were obtained based on initial data on timber reserves from the State Forest Register (SFR). SFR information, according to expert estimates, is 15–30 years old. In addition, due to the existing peculiarities of accounting, SFR include information only about the first tier of the stand having a marketable value, while SFI sample plots collect information on all trees, which is more correct in terms of estimating carbon balance. Therefore, it is likely that the resulting estimates of the forest carbon budget may be underestimated by 30–40% [47,48]. According to modeling estimates based on the comparison of 16 dynamic global vegetation models used in [6], net uptake in managed forests of the Russian Federation exceeds the national inventory data (-0.68 billion t CO₂) and amounts to about -1.1 billion t CO₂ per year [49]. However, it should be noted that the range of estimates of carbon sequestration by Russian forests available in the literature is quite wide and ranges from -220 to -2500 million tons of CO₂ [50].

The results of the refined assessment for forest lands based on updated information on timber reserves within the SFI and updated estimates for fallow lands, fodder lands, natural and secondary steppes within this study are shown in Table 2 in comparison with the data of [1].

Table 2. GHG net fluxes according to [1] and in the present work, mln t CO₂-eq. (negative values are absorption, positive values are emissions).

Table 2. GHG net fluxes according to [1] and in the present work, mln t CO₂-eq. (negative values are absorption, positive values are emissions).

Types of land	In the work by [1]	In this study
Forest lands	-775.2	-895.6*
Grasslands	-370.8	-330.7
out of which: - natural steppes, total	-107.0	-107.0
- unmanaged grasslands outside the steppe zone	-	-62.1
- fodder lands (hayfields and pastures) outside the steppe zone	49.5	-21.7
- fallow lands, including:	-	-139.9
- secondary steppes	-313.3	-111.5
Wetlands	32.5	32.5
out of which: - swamps	-119.2	-119.2
- areas under water	151.7	151.7
Tundra	64.7	64.7
Total	-1048.6	-1129.1

* mean value for 2017–2021. Out of which CO₂ is -937.5 million tons CO₂-eq; СН₄ is 25.5 million tons CO₂-eq, and N₂O is 16.4 million tons CO₂-eq.

According to the data in Table 2, the recalculations made for forest lands (increase in net absorption by 120 million tons of CO₂-eq.) and clarification of the distribution of grassland areas between steppe and non-steppe zones (decrease in net absorption by 50 million tons of CO₂-eq.) did not lead to a significant difference in the aggregate estimate of the net balance of GHG from the country’s natural ecosystems. The difference amounted to 80.3 million tons of CO₂-eq., which is within the area of uncertainty of the aggregate estimate.

Table 2 data for this study are summarized in Figure 2 by GHG and ecosystem types. Vertical lines show uncertainty ranges (95% confidence interval). Two almost equal but differently directed fluxes of CO₂ and CH₄ in wetland ecosystems determined a high uncertainty of the resulting value: ± 1580%. Thus, CO₂ uptake was calculated with an uncertainty of ± 200% and CH₄ was calculated with an uncertainty of ± 100%. Figure 2 shows that the key fluxes in Russia include carbon sequestration by forest, wetland and tundra ecosystems, and methane emissions by wetlands and tundra. Humid areas, where anaerobic conditions are often created in soils, although they store carbon, are characterized by relatively high methane emissions into the atmosphere and may be net emitters of GHGs, given that the global warming potential of methane is 25–30 times higher than that of CO₂. High methane emissions from wetlands in the boreal zone, as well as their increase during 2007–2021 in relation to the level of 2000–2006 are noted by [51].

Obtained GHG fluxes estimates (Figure 2), according to the authors, require further study on clarification of the following directions:

- specification of data on carbon stock changes in litter and soil pools in forest ecosystems. It is reasonable to consider stock changes in the 0–100 cm soil layer, given that boreal forests are characterised by high soil carbon stocks. The change in these stocks in forests occurs, among other things, at depths below the first 30 cm. Apparently, the specification of data on these pools can cause an increase in total net carbon sequestration in forests by -30...-40 million tonnes of CO₂.

- considering that estimates for forest lands are based on state statistics data, this study does not include forests on overgrown agricultural lands, which may lead to underestimation of up to -90 million tonnes of CO₂ uptake in stand biomass alone (expert assessment by [52]). Carbon storage by soils of overgrown lands is accounted for in the inventory under the land-use change category;

- relatively high values of N₂O emission in tundras. A much smaller estimate of N₂O emission in this zone is found in the scientific literature. Hence, according to [53], on average for permafrost soils the specific global emission of N₂O for 1993–2019 is 2 orders of magnitude lower. In terms of comparable units, it is about 0.57 g C-CO₂ m^-2 per year lower than the 21.7 g C-CO₂ m^-2 per year that was obtained by [1]. At the same time, the overall assessment of the tundra zone as a weak net source or territory with zero GHG balance corresponds to the IPCC assessment obtained later [54,55]. It seems then that carbon uptake [1] may also be somewhat overestimated for tundra ecosystems, which together with compensatory estimates of N₂O emission shows consistent results with those of other authors;

- the areas of small and temporary freshwater bodies are likely to be underestimated on the basis of state statistics and, consequently, GHG emissions from them as well. Deeper thawing of permafrost soils during the warm period of the year may be accompanied by the formation of temporary shallow reservoirs, which are characterised by high methane emissions. It is necessary to conduct further studies to estimate the areas of small lakes, streams and ponds on the tundra territory using remote sensing data for more correct estimation of annual emission of methane and CO₂ into the atmosphere.

It is advisable to continue working on clarifying the GHG balance in managed and unmanaged grassland ecosystems.

Figure 3 shows the GHG flux balance of natural ecosystems on the territory of Russia. It can be seen from Figure 3 that CO₂ net flux is characterised by a negative value, i.e., the absorption of carbon dioxide prevails over its emissions and corresponds to -2.7 ± 1.7 billion tonnes of CO₂ per year. This estimate corresponds to the data of other authors [4,5,14,56,57]. In accordance with the Biomass Carbon Monitor global service data on changes in aboveground biomass carbon stocks, Russia absorbed about -2.14 billion tonnes of CO₂ in 2016, which also matches the obtained data on carbon dioxide in the study by [1]. In the study by [58], the mean value of CO₂ net uptake by ecosystems in the Russian Federation according to inversion modelling data is estimated on average as a smaller value equal to -1.6 billion tonnes of CO₂ with a range of estimates by different models from -3.0 to -0.7 billion tonnes of CO₂.

Methane and nitrous oxide have net emissions to the atmosphere totalling 1.6 ± 0.8 billion tonnes of CO₂-eq. per year, which offset more than 60% of CO₂ uptake. There is not much data in the literature on emission estimates of non-CO₂ gases, and their variation is relatively large. Therefore, according to [59], natural methane flux in the Russian Federation is estimated to be almost 5 times lower than according to [1]. Methane emissions to the atmosphere from wetlands obtained in our study are almost 1.4 times higher than the maximum estimate from GOSAT satellite monitoring data [60]. It should also be noted that information on methane emission from tundra is apparently absent in GOSAT and other satellite data.

Total GHG flux balance of all studied ecosystems in Russia corresponds to a net uptake of -1.1 ± 1.7 (±161%) billion tonnes of CO₂-eq. High uncertainty is primarily determined by the uncertainty of flows in wetlands and tundra.

According to the data available in the publications, as a result of the expected increase in methane emissions from permafrost soils, the growth of methane emissions from the shelf, as well as the gradual decrease in the productivity of forest and other ecosystems as a result of further temperature changes and redistribution of precipitation, it is likely that GHG fluxes to the atmosphere from the territory of Russia will increase and the country’s natural ecosystems will become a net source of GHGs in the second half of the 21st century [59].

4. Discussion

5. Conclusions

References

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