by Irina KURGANOVA, Dr. Sc. (Biol.), Valery KUDEYAROV, RAS Corresponding Member, Institute of Physico-Chemical and Biological Problems of Soil Science (Pushchino, Moscow Region)

In the opinion of most experts, global climate warming in the latter half of the 20th century is caused by the higher concentration of CO₂ in the atmosphere. Yet, estimates of the global carbon balance show that the land ecosystems "immobilize" for a time this chemical element sequestering 2.1-2.5 Gt of carbon dioxide per year. Besides, the vast Russian forests, meadows and swamps take the lead in this process. Thereby our country, having a territory above 1/9 of the Earth's surface, accounts for at least 1/5 of the total "budget" of carbon, the basic "life element", which plays a special role in a global biogeochemical cycles.

Organic carbon has changed the Earth's surface mantles and provided stability of its carbon-oxygen system during global evolution. Due to complex processes, this "life element" passes from some "pools" into others. Most of all it is present in the litho-sphere and the World Ocean, 12-10⁶ and 37-39-10³ Gt C (1 Gt = 10¹⁵ g) respectively; its large reserves are concentrated in the organic substance of the soil cover, including peat deposits, and in the land vegetation biomass, 1500-1700 and 500-600 Gt C respectively. Although the two latter figures are too conservative, it is just the land ecosystems that exercise a dominant influence on CO₂ natural fluxes and CO₂ concentration in the atmosphere.

The biogeochemical carbon cycle has not been a closed process--this is one of its distinguishing features. For example, CO₂ drops out of the global carbon cycle and enters different "traps"; at the current stage of biosphere evolution it is in the form of peat accumulation and carbon fixation in stable fractions of the soil organic substance. For hundreds and thousands of years it has been preserved in humus, and with its microbiological decomposition the newly liberated carbon dioxide is conserved in the form of mineral carbonates in the deep layers of soil.

Another significant feature of the global carbon cycle most pronounced today is dynamism conditioned by its high sensitivity to growing anthropogenic stress and expressed in the change of CO₂ concentration in the atmosphere. For the last two centuries this index has been steadily rising if compared to the pre-industrial epoch and, by expert estimate, is a cause of the greenhouse effect and global climate warming. This problem is among the most serious challenges of the 21st century and is discussed in the scientific, social and political spheres.

CARBON PATH IN THE BIOSPHERE

Thus, carbon exchange between the land ecosystems and the atmosphere is a key natural process, and scientists have to assess its intensity in quantitative characteristics in accordance with conditions of the United Nations Framework Convention on Climate Change (Rio de Janeiro, 1992) and the supplemental Kyoto Protocol (Kyoto, 1997) and also the post-Kyoto climate agreements. Let us consider the complex of production* and destruction processes taking place on land and responsible for the organic carbon (C_org) cycle.

In the course of photosynthesis and breathing plants build up their mass, and the so-called net primary production is formed in a certain period of time. This characteristic is included in the carbon global balance formula. The function of the land ecosystems as a source or a "bank" (specialists use the international term "sink") of CO₂ is determined in many cases by the imbalance between the net production and the amount of carbon dioxide liberated at microbial decomposition of the organic substance of soil and the dead soil cover**. To describe this function scientists introduced a special index--the "amount of ecosystems' net production". The third important component in computing the carbon balance of the planet and its particular regions is represented by the anthropogenic (non-soil) emission of CO₂, for carbon dioxide enters the atmosphere through fuel combustion, the burning of peat and agricultural and forest wastes, and during forest fires. Moreover, the amount of C_org, withdrawn for a time from the biogeochemical cycle and kept in the woody parts of plants (from 100 to 200 years), in humus (to 1,000 years) and peat (to several thousand years), is the net biome production of an ecosystem.

* Production process is a set of processes of creation and transformation of a living substance, energy assimilation and flow through trophic (food) levels of ecosystem.--Ed.

** Dead soil cover, fallen leaves, pine needles, twigs, boughs, bark, fruits and other parts of plants involved in the formation of the forest litter and soil.--Ed.

Dynamics of the main items of the global carbon budget (Global Carbon Project, 2011).

Specialists assess the global balance of carbon annually within the Global Carbon Project, which joins together investigations conducted by scientists of most European and North American countries. Their approach is based on determining the difference between the amount of CO₂ liberated into the atmosphere at fuel utilization and tropical forest clearing and the amount absorbed by the atmosphere-ocean system. We have already mentioned the term "carbon sink", meaning elimination of carbon dioxide from the Earth's gaseous mantle and its temporary binding in different reservoirs, not only in oceans but also in the topsoil and plant bio-mass. The dynamics of the above processes for the last 250 years has been rather complex. Alongside the exponential (at the rate of geometrical progression) growth of carbon dioxide emission due to the burning of oil and gas, an equally rapid increase of the atmospheric CO₂ was taking place, while CO₂ emission, caused by changes in land use management, as well as the ocean carbon sink was increasing but gradually in the first two centuries, and after 1970-1980 the directional, steady change gave place to variations of these values.

By expert estimates of foreign scientists, the average emission of CO₂ in 2000 to 2010 made up 7.9±0.5 and 1.0±0.7 Gt C/year (values of standard deviations from the average). The change of the atmospheric CO₂ pool for this period was 4.1±0.2 Gt C/year, and the mean carbon ocean sink computed on modeling data made up 2.3±0.5 Gt C/year. The value of CO₂ absorption by land ecosystems in 2000 to 2010 reached 2.511.0 Gt C/year.

Akihito Ito of Japan has suggested a method of estimating the global carbon balance based on modeling data on the atmosphere-biosphere exchange in land ecosystems; in 2003 he published his results in the Tellus B journal. In his calculations the Japanese scientist relied on climatic factors, changes in the concentration of atmospheric CO₂ and ratios of stable carbon isotopes (δ¹³C) in soil and plants between the years 1953 and 1999. His Sim-CYCLE model allows to assess the C balance by the parameters of two principal heterodirectional fluxes of CO₂, the one caused by CO₂ assimilation by plants in photosynthesis, and the other, by microbial decomposition of organic matter. The difference of values in these two processes, equal to the net production of the global ecosystems, describes the global carbon balance. For a period of 1990-1999 it was equal to 2.1 Gt C/year, which attests to the role of the land ecosystems as an absolute carbon sink.

THE "PLANET'S LUNGS"

The two above-described approaches in calculating the global carbon balance are essentially different but they give close estimates of CO₂ sink into the land ecosystems, 2.1-2.5 Gt C/year, which confirms the fidelity of modern expert ideas about the scope of this process. Such calculations, true, involve statistical and modeling

Distribution of the total carbon sink (Gt C/year) in world forest ecosystems in 1990-2007 (by Pan et al., 2011).

Carbon sink distribution (Mt C/year) in the Northern Hemisphere in 2000 to 2004 in ecosystem types (a) and major industrial countries (b) (by Ciais et al., 2010).

errors; yet proceeding from the available data, Mariya Glazovskaya*, an eminent Russian soil scientist and geochemist, Professor Emeritus of Lomonosov Moscow

* Mariya Glazovskaya is an honorary member of the Russian Geographical Society and the Dokuchaev Pedological Society, author of more than 500 publications on soil geography and landscape geochemistry. In 2011 the National Atlas of Russian Soils came off the press; she contributed greatly to it. In 2012 the world scientific community marked her birth centenary.--Auth.

State University, in her monograph "Pedolithogenesis and Continental Cycles of Carbon" (2009) came to an interesting conclusion that we cannot rule out "the presence of still omitted (not recorded) factors and mechanisms in the conservation of a part of excessive CO₂ which enters annually into the atmosphere". Indeed, so far the origin of carbon accumulation on the Earth is not quite clear, and its nature has no unambigu-

ous explanation. That is why the term "missing sink" comes up in English-language publications.

In our opinion, a composite mechanism of carbon accumulation is most probable that combines the negative feedback with global warming (as expressed in higher CO₂ fixation by vegetation on account of the increased amount of nitrogen available), net accumulation of C_org by soils and the possibility of carbon sinks into deep layers of the pedosphere (soil stratum of the Earth).

How is the so-called "omitted sink" of carbon distributed on our planet? According to estimates published in the Science journal in 2011 by Dr. Yude Pan and his colleagues from the Forestry Service of the US Department of Agriculture, actually all this stock can be concentrated in forest ecosystems. In 1990 to 2007 the total carbon deposition here made up 2.30-2.50 Gt C/year, with about half of it (1.02-1.33 Gt C/year) falling on virgin tropical forests and 1.17-1.28 Gt C/year, on boreal and temperate forests located mainly in the Northern Hemisphere. Some scientists maintain this very region should be considered to be the "planet's lungs" as carbon accumulation here makes up 1.66-1.73 Gt C/year according to model calculations made by an international team of authors headed by Kevin Robert Gurney of Colorado State University (USA, 2003) and Philippe Ciais of Versailles University (France, 2010).

Ciais studied how the global carbon sink was distributed in 2000 to 2004 according to the types of ecosystems and proved that out of the total equal to 1.66 Gt C/year approximately 60 percent, or 1.0 Gt C/year, fell on forests of the Northern Hemisphere, which agreed well

Carbon sink distribution (Mt C/year) in Russia in 2000 to 2004 according to ecosystem types (by Ciais et al., 2010).

with the estimate 1.02-1.33 Gt C/year made by Jude Pan. Moreover, our domestic forests account for 56.3 percent of this amount. In other words, the role of the Russian land ecosystems in the global carbon biogeo-chemical cycle is quite substantial.*

In recent years this country's carbon balance has been drawing increasing attention. The first attempts were made in the early 1990s within a special state program under the guidance of Acad. Nikolai Laverov, Acad. Georgi Zavarzin in charge of its biological chapter. Research findings on the main components of the carbon budget in Russian land ecosystems relied on the experimental database relative to soil respiration and on information on the net primary productivity of biocenoses.

A few years ago the authors of this article published in Russian and foreign publications their findings and proved that the value of the total soil runoff of CO₂ in Russia was close to the total productivity of land ecosystems. However, these first estimates were not quite correct as the total carbon dioxide runoff from soil was considered instead of only its part supplied by microorganisms through decomposition of organic matter. Besides, the anthropogenic carbon emission in Russia was not considered. Thereafter these shortcomings were removed owing to new experimental data and, foremost, thanks to new approaches, model constructions and geoinformational technologies.

In line with the current concepts (data of Acad. Adolf Mokronosov published in 1999 and updated by his followers in 2005), the primary productivity of the flora in the territory of Russia makes up 4.34-4.73 Gt C/year, or 6.8-7.8 percent of the global photosynthetic sink. These estimates are based on the so-called chlorophyll

* See: A. Syrin, L. Rysin, A. Gulbe, "Our Changing Forests", Science in Russia, No. 4, 2012.--Ed.

index (average amount of chlorophyll per unit of leaf surface area) with account of areas under a particular type of plants. The second component of the CO₂ balance in the Russian territory, i.e. the microbial component of the overall respiration of soils, was calculated at our Institute and the International Institute of Applied Systems Analysis (IIASA, Austria) in 2000. This characteristic varies from 2.6 to 3.2 Gt C/year, which corresponds to 4.2-5.2 percent of the carbon dioxide recovery from organic matter decomposed by microorganisms. It should be noted that appropriate research in the Kyoto Protocol signatory nations started only in 1990, and the above-cited figures were obtained from literary data, most part of which dated from the 1950s to 1990s, and from maps (pedological, land use and vegetation maps) dated from 1988 to 1990. The third component of the balance calculation, i.e. the anthropogenic emission of CO₂ in the territory of our country in 2002 to 2006, was estimated by us at 0.8 Gt C/year (from official statistics).

The follow-up estimates of the carbon balance in the Russian territory made at our Institute showed that our land ecosystems served as an absolute accumulator of atmospheric CO₂ to the extent of 0.81-1.10 Gt C/year. This agrees with model constructions carried out in 2010 by Professor Ciais, his estimate is 0.83 Gt C/year. Professor Anatoly Shvidenko and his coauthors (IIASA, Austria) suggested for the first decade of the 21st century (2000-2010) their calculations which also testify to a prevalence of the carbon sink equal to 0.5-0.7 GT C/year over its losses. In this instance the newly recovered carbon happened to be confined to particular ecosystems as shown by mathematical modeling data; in 2000 to 2004 about 70 percent of carbon was found to be accumulated in forests, and from 4 to 10 percent, in the other biotopes (tundra+bushes, plains+semideserts, rivers, intact peatlands).

Current estimates of the main components of the biogeochemical carbon cycle in land ecosystems of the world and Russia. Given in brackets is the share of Russia's territory in carbon global fluxes and reservoirs on the planet.

STORAGE RESERVOIR FOR "UNRECORDED CARBON"

The above discrepancies in carbon budget calculations in the territory of our country are due to differences in estimates made by scientists concerning the microbial component of soil respiration, the scope of CO₂ emission in the agrarian sector (in cattle breeding above all) and values of the fire and postfire emission of CO₂. Besides, in most cases no changes were taken into account in domestic land use early in the 1990s. At that time about a fourth part of our ploughlands was abandoned, and, according to data furnished by the RAS Institute of Geography, this was equivalent to 45.5 mln hectares and could not but affect the "credit and debit items" of the carbon budget. Our latest research findings have shown: the fast transition of cultivated lands to fallow tracts (i.e. natural ecosystems), as it occurred in 1990 to 2005, results in an additional sink of CO₂ at the rate of 0.11±0.03 Gt C/year, which can compensate for about 25 percent of carbon dioxide emission from fuel combustion in the Russian Federation.

Our analysis allows the following conclusion: the current carbon balance in Russian territory can be estimated within 0.5-1.0 Gt C/year, i.e. our country comes out as an absolute accumulator of atmospheric carbon dioxide. Its land ecosystems account for no less than a fifth of the global sink of atmospheric CO₂, mainly in woodlands and idle lands.

Of no less importance is the role of land ecosystems in the formation of net biome* production removed for a time from the carbon biogeochemical cycle and accumulated in soil in the form of organic compounds. This carbon stock, or pool, important for the biosphere, is very sensitive to any type of anthropogenic pressing (deforestation, tilling of wild lands, removal of peat beds or disuse of cultivated lands) and natural disasters (forest and peat fires, pests). According to data furnished in 1995 by the Moscow University Department of Soil Science headed by Dr. Dmitry Orlov and by the International Institute of Applied Systems Analysis (Dr. Vyacheslav Rozhkov), the soil carbon reserves (including peat beds) in the 0-100 cm soil layer made up 298-342 Gt, which corresponds to 18-23 percent of the world carbon reserves in pedosphere.

In the opinion of Mariya Glazovskaya the capacity of present (and buried) soils as a carbon reservoir is largely understated. This is because of unrecorded processes of carbon accumulation in soils in the form of stable

* Biome, a totality of ecosystems of one natural-climatic zone.--Ed.

organic compounds deposited in deep layers of auto-morphous* soil types and formation of secondary carbonates in soils of subarid and arid regions. Studies of carbon content in lower soil horizons of atmospheric humidification in the temperate zone carried out by the Chair of Soil Geography and Landscape Geochemistry of Moscow University's Department of Geography showed that a 100-200 cm layer contained from 15 percent to 25 percent of the total carbon reserves. Thus, the carbon pool concentrated in the territory of our country is higher than is commonly believed, on the average by 64 Gt (up to 384 Gt), and the share of "domestic" carbon in the world pool makes up to 25 percent.

Besides, research conducted at our Institute demonstrated clearly that upon the withdrawal of cultivated lands in 1990 to 2010 the carbon reserves in their profile,

* Soils whose water status depends solely on atmospheric humidification.-- Ed.

Plowland dynamics in Russia in 1913 to 2010.

Age-dependent changes in carbon reserves in the cultivated layer of different soils.

as a rule, increased, and its additional accumulation due to the changed land use over an area of 45.5 mln hectares must have reached 512 Mt. The value of carbon "sequester" determined by us and averaging 26±3 Mt per year is quite tangible and thus taken into account in annual Russian reports to the Secretariat of the United Nations Framework Convention on Climate Change.

Thus, we have shown that Russian land ecosystems account on the average for not less than 20 percent of the global carbon accumulation within the net production of ecosystems and the soil reservoir. But the ratio of these carbon sink components is different in various natural-climatic zones, and their quantitative evaluation in the current century should remain among top-priority tasks of ecology and soil sciences.

The dynamic industrialization begun in many countries more than 200 years ago upset the global carbon cycle as manifested in an unprecedented increase of carbon dioxide concentration in the atmosphere. Luckily, the evolutionary process provided for a defense mechanism, i.e. continuous active accumulation of carbon within forest and swamp ecosystems of the planet. Mankind should spare no effort in protecting them for the sake of the health and well-being of future generations. We should always remember how slender the balance of carbon fluxes is on our planet, how closely it is connected with the present climate changes and how much we are responsible for that.

The above-cited original data have been obtained with the support of the Russian Fund of Basic Research, Program of the RAS Presidium No. 4 and presidential decision No. 6620.2012.4.