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Abstract Soil organic matter largely controls the ecosystem functions of soil and is a source of energy and a nutrient substrate for the soil microbial community as well. The postagrogenic dynamics of organic matter stock (content of organic carbon, Corg; total nitrogen, Ntot; and their ratio, C/N), basic soil properties, and indicators of soil microbial and enzyme activities are analyzed for the chronosequence of Retic Albic Podzol (southern taiga zone, Kostroma oblast, Russia). Different plots of the chronosequence have been withdrawn from agricultural use 11, 16, and 40 years ago. The stratification ratios (SRs) 0–5 : 5–10, 0–10 : 10–20, and 0–10 : 20–30 cm layers for Corg and Ntot contents are also estimated. A statistically significant increase in the contents of Corg and Ntot during the postagrogenic succession is observed in the topsoil layer of the old arable horizon (0–5 and 5–10 cm) as well as an increase in the stratification of their distribution, especially after a closed tree canopy has developed on the former arable land. Postagrogenic succession promotes an increase in the soil basal respiration rate and the content of microbial biomass, the maximum values of which are most often recordable in the soil under a 40-year-old secondary small-leaved forest. During the natural reforestation of the former arable lands, a statistically significant increase in the phosphatase activity is observed within the entire old arable horizon. The peaks of oxidoreductase activity correspond to the stages of succession with the maximum uptake of easily decomposable plant litter. Over the 40 years of postagrogenic succession, all analyzed parameters (except for pHKCl) have increased in a statistically significant manner at all considered depths of the old arable layer, with the highest annual increase in 0–5-cm layer, most enriched for organic matter.

The land use type and agricultural practices significantly affect the aggregate state and many physical properties of soils. In this study, we provide a comparative assessment of the bulk density, water holding capacity of disturbed samples (WHC), and structural state (dry sieving method) of Luvic Greyzemic Chernozems under different land uses (long-used arable land, newly developed arable land, 27-yr-old abandoned land, and virgin steppe) in the Cis-Altai forest-steppe soil province of Western Siberia. At each site, three mixed samples were taken from the layers of 0–5, 5–10, 10–20, and 20–30 cm of the humus horizon. The influence of land use on the bulk density was only seen for the upper 5-cm layer, in which the decompaction after 27 years of abandonment was statistically significant. Differences in the bulk density between the two croplands and between the abandoned and virgin lands were insignificant. Changes in the WHC under the impact of land use were manifested for the upper 20-cm layer. In six years of using the previously abandoned land for growing cereals, the aggregate-size distribution of the soil became almost identical to that in the old-arable soil. Judging the amount of agronomically valuable aggregates and the structural coefficient value, the aggregate state of all the studied soils can be assessed as excellent. Thus, Luvic Greyzemic Chernozems of the forest-steppe zone in Western Siberia are characterized by the high tolerance towards agrogenic impact and retain their natural crumb–granular structure for a long time of their using for crop production.

The succession of natural vegetation on the former arable soils triggers the processes of postagrogenic restoration of soil ecological functions specific of a particular bioclimatic zone. We analyze the postagrogenic dynamics of a set of soil characteristics in the upper (0–5 and 5–10 cm) layers of former arable horizon by the case study of the chronosequence of Retic Albic Podzols (agrosoddy podzols, southern taiga, Kostroma oblast, Russia) withdrawn from agricultural use 15, 20, and 45 years ago. The following soil characteristics are determined: pHKCl; the contents of soil organic carbon (SOC), total nitrogen (TN), and dissolved carbon and nitrogen (DOC and DON); basal respiration (BR) rate; carbon of microbial biomass (Cmic); and relative indicators of the state of microbial community. In addition, absolute (HIX1) and relative (HIX2) humification indices of dissolved organic matter (DOM) are assessed for the soil layer of 0–5 cm in the studied soil chronosequence, as well as the stratification ratio (SR (0–5 : 5–10)). The following changes are observed in the upper layers of the former arable horizon over 45 years of postagrogenic succession: (1) a statistically significant decrease in the pHKCl and humification index of DOM; (2) an increase in the SOC, TN, and Cmic contents and BR rate; and (3) a considerable stratification according to the SOC and TN contents and microbial properties. The highest values of SR for the SOC and TN are observed in the soil of the old-aged forest; for the microbial properties, in the 45-yr-old abandoned soil. Thus, the main characteristics and ecological functions of Retic Albic Podzol gradually restore during the secondary succession on the former arable land in the southern taiga zone.

The bark of coniferous trees (BCT) is an essential component of the litter in boreal forests. The effects of temperature and mineral additives (N and P) on the rate of BCT decomposition (DecR), its constant (k), total loss of C–CO2, and the changes in BCT chemical composition are assessed in a long-term (12 months) laboratory experiment with soil–bark substrates (SBSs) using three contrasting temperatures (2, 12, and 22°C) and sufficient moisture. The temperature coefficient (Q10) for the mean DecR during the experiment varies from 1.1 to 2.5 depending on temperature range and SBS composition. The effect of temperature was the most pronounced during the first and second months of the experiment, explaining 47% of the DecR variation. At later stages of the experiment, the SBS composition affected by the addition of mineral N and P compounds is the key factor influencing the BCT decomposition; it explains 18–63% of the DecR variance. The maximum losses of C–CO2 (158–187 g С/(kg bark), or 34–41% of the initial С content) are observed at 22°C. Irrespectively of the temperature, the most significant loss of ethanol-soluble compounds (56–64%) is recorded in the variant with the mineral N additive, whereas the cellulose content most significantly decreases (by 64–69%) in the variant with combined application of N and P and is almost independent of temperature. The loss of lignin was insignificant, amounting to only 3–12% of its initial content. The addition of mineral N and P is a key factor stimulating the BCT decomposition, considerably shortening the decomposition time and improving the quality of the resulting substrates.

Modeling the carbon cycle in forests is often restricted by modeling the main components, including emissions (respiration of soils and debris) and production (deposition of carbon in plants and soils). In this study we examine the applicability of various versions of the T&P model to estimate monthly, seasonal, and annual fluxes of CO2 from the sod-podzolic soil in the mixed forest of the Prioksko-Terrasny Nature Reserve, Moscow oblast. The model is parameterized and verified, and the accuracy is tested using a database of 20 years of monitoring CO2 emissions from soils and independent weather variables, including mean monthly air temperature and monthly precipitation. Numeric experiments show that all versions of the T&P model (both initial and parameterized by training sets at different temporal intervals) satisfactorily describe the long-term dynamics of mean monthly respiration of the sod-podzolic soil under forest cover (SRm). Parameterization of the T&P model with experimental data as a training set practically does not improve the quality of modeling in any of the test intervals. The use of weather data averaged over 20 years for the calculation of SRmod-mean and estimates of seasonal and annual soil fluxes of CO2 on their basis (SeSRmod-mean) in most cases overestimates the corresponding experimentally obtained values (SeSRexp). SeSRmod-mean for annual, summer, and winter soil CO2 fluxes are on average 4.5–6.7% higher than SeSRexp, and SeSRmod-mean for the warm season shows an overestimation of approximately 3%. The largest discrepancy of calculated estimates to experimental data is found for the spring period: the overestimation amounts to ~22%. Thus, the use of weather data averaged over 20 years has shown the applicability of an ensemble of versions of the T&P model for estimating seasonal and annual fluxes of CO2 from soil in a humid continental climate.

Long-term (2009–2019) field studies of the CO2 fluxes over the Valday upland in north-western European Russia were performed in an old-growth spruce-dominated forest subject to the combined effects of climate change, bark beetle attacks and windfall events. The annual carbon uptake within the study area decreased from − 300 g C m−2 yr−1 in 2010–2011 to − 95 g C m−2 yr−1 in 2018 as a result of progressive tree mortality. However, mortality did not cause a significant reduction in specific ecosystem respiration. The respiration of the damaged forest was formed by efflux from soil (64.8%), living vegetation (15.5%), hotspots under dry standing trees (12.1%) and decomposition of woody debris (7.6%). High correlations between net ecosystem exchange and dry standing spruce stocks and average soil CO2 efflux were found. The decrease in the carbon sink was followed by a decline in evapotranspiration from 0.0142 ± 0.0003 g H2O m−2 s−1 for May–October 2010 to 0.0116 ± 0.0002 g H2O m−2 s−1 in 2018. We assumed that the decrease in carbon uptake was due to both the reduction in primary tree production and the decrease in the area of the unaffected stands. Our estimates show that an increase in tree mortality up to 27% of a stand area could turn an old-growth spruce forest into a net source of CO2. This should be taken into account when considering human-induced and climate-related effects on boreal forests.

A combined analysis of the current climate trends and the dynamics of soil respiration under soddy–weakly podzolic sandy-loamy soil (Entic Podzol (Arenic)) in the forest and meadow cenoses in the Prioksko-Terrasny Biosphere Reserve (southern part of the Moscow oblast) is performed. During the observation period (1998–2018), distinct trends of an increase in the average annual air temperature (Tair) and aridity of the summer season as well as a decrease in the duration of stable snow cover are observed. A general trend of a decrease in the total CO2 fluxes from the Entic Podzol (Arenic) under a forest and a meadow for all seasons of the year and throughout the year is also observable on this background. The linear trends of a decrease in the annual and fall seasonal CO2 fluxes from the soils in both cenoses are statistically significant (P < 0.05). Over the 21-year observation period, the monthly average CO2 fluxes from soils have tightly correlated with the average monthly Tair (r = 0.78–0.84, P < 0.001). In dry years, the temperature sensitivity of soils expressed via the Q10 coefficient is by 10–12% lower, than in the years with normal moistening. The average long-term values of annual soil respiration (AnSR) in the forest and meadow cenoses amount to 581 ± 54 and 727 ± 71 g C/(m2 year), respectively, with an interannual variation of 20–22%. The AnSR and summer hydrothermal coefficient are the most closely related explaining 51–56% of the variance in annual CO2 fluxes from soils. The warm season (May–October), which coincides with the period of active plant vegetation, accounts for 73–77% of the AnSR. However, the share of the cold season (November–April) in the annual CO2 flux from soils may reach 38–39% in individual years. Expansion of the network of stationary long-term year-round monitoring of soil respiration is a necessary condition for obtaining both more realistic estimates of the CO2 fluxes from soils and predictions of the ecosystem responses to the current and future climate changes.

The aim of this study was to validate the ROMUL and Romul_Hum simulation models based on the data of long-term measurements of soil respiration performed by the Institute of Physicochemical and Biological Problems in Soil Sciences of the Russian Academy of Sciences in the south of Moscow oblast (54°50′ N, 37°34′ E). We estimated the dynamics of carbon stocks and soil CO2 emission for three scenarios of plant litterfall compiled with the account of experimental data on the composition and mass of litterfall entering the gray forest soil (Albic Luvisol) under secondary deciduous forest. The calculations of long-term series of temperature and moisture content in the forest litter and upper organo-mineral soil horizons in simulation experiments are based on the real data on air temperature and precipitation for 1973–2016. The correspondence of simulation results to the data of field measurements was better for the Romul_Hum model, which comprises a description of the effects related to the vital activity of soil fauna, leading to the formation of different fractions of soil organic matter. The best correspondence of simulated values of CO2 emission, carbon stocks, and the C : N ratios in gray forest soil horizons to field data was for the litter scenarios, which excluded the fraction of small branches. The revealed differences between the simulated and real values of soil parameters are explained by specific features of the input and transformation of different litterfall fractions, which are not always taken into account during soil sampling. The results of the study attract the attention to the uncertainties of estimates of carbon budget in forest ecosystems due to the difficulties of accounting both large wood residues (dead tree and coarse branches debris) and smaller wood fractions in the litterfall.

Validating methodology for quantifying of carbon (C) loss from dead wood is critical for understanding C turnover in forest ecosystems. We compared estimates of CO2 fluxes with a closed dynamic chamber method using an infrared gas analyzer with: 1) small chambers installed on the downed woody debris (DWD) surface and 2) DWD segments placed in large chambers. The fluxes were measured from 6 intermediately decayed DWD fragments, wood and bark of Betula pubescens and Populus tremula and calculated on the surface area, volume and mass bases. For most DWD fragments, the tendency with an underestimation of CO2 flux in the first approach up to 90% was observed due to a barrier effect of a thick bark layer and possible gas leakages. Our results demonstrate the importance of taking into account contribution of bark into total DWD respiration rates especially in upscaling the piece-level results into ecosystem level.

The postagrogenic dynamics of organic carbon (Corg), total nitrogen (Ntot), and density fractions of organic matter (OM) in the dark gray soil (Haplic Phaeozem, Belgorod oblast, Les na Vorskle (Forest on Vorskla) Reserve) and migrational–mycellary Chernozem (Haplic Chernozem, Kursk oblast, Streletskaya Steppe Reserve) were examined. The most significant postagrogenic changes in all the studied soil properties were observed in the top 0–5 cm layer. The contents of Corg and Ntot in this layer of the Phaeozem chronosequence increased by 1.2–1.3 times over 45 years after cessation of farming. The contents of Corg and Ntot in the top 0–5 cm layer of Chernozem chronosequence increased by 1.9–2.0 times over 60 years of the postagrogenic development. The accumulation of Corg took place in all density fractions. In both chronosequences, the free fraction of organic matter (density

The decomposition and mineralization of various plant residues (oak and aspen leaves, pine needles, small branches and thin roots of trees, aboveground biomass and roots of meadow grasses, aboveground biomass and roots of clover, and straw and roots of barley) were investigated in the laboratory experiments by quantitative measurement of produced C–CO2. The plant residues were mixed with vermiculite or gray forest soil (Greyzemic Phaeozems Albic) and incubated under constant temperature and moisture conditions. After a year of incubation, 25–67% of Corg in plant residues were mineralized. Oak leaves, aboveground mass of meadow grasses, and aboveground mass and roots of clover were characterized by a three-pool structure of organic matter with moderate (0.1 > k1 > 0.01 day–1), slow (0.01 > k2 > 0.001 day–1), and very slow (k3 < 0.001 day–1) mineralization rates, while the other types of plant residues had only a two-pool structure with slow and very slow mineralization rates. An opposite relationship between the decomposition rate and the C : N ratio in the plant residues was found. Poorly decomposable types of plant residues were the main source for particulate organic matter (CPOM) in the soil, while highly decomposable types were the main source for microbial biomass (Cmic). The content of potentially mineralizable organic matter in the soil with plant residues correlated positively with CPOM and with Cmic.

This chronosequential study focuses on the vegetation succession, Luvisol pedogenesis, and long-term carbon dynamics during post-agricultural restoration in the temperate broad-leaved forest of European Russia. The study comprises three chronosequences of 37 years (North), 120 years (Middle), and 42 years (South) to cover climatic differences within the study area. The sites of each catena were mostly comparable in terms of climate, soil texture, and land history, but the duration of agricultural abandonment differed between sites. Plant succession developed via a birch forest or a pear-ash woodland to a birch-oak forest with Aegopodium podagraria after 120 years, which indicates the plant association is near to a climax oak forest. Former homogenous plow horizons formed a new morphological stratification similar to natural Luvisols, due to the formation of a well rooted Ah horizon and an O horizon of mull; a change in soil structure from coarse blocky to granular; and decreasing bulk densities. However, plow features were identified even after 120 years of Luvisols restoration. During restoration, the former plow horizons showed minor changes of pH 5.4–5.6 (CaCl2). However, a slightly increased pH value within the newly developed Ah and O horizons induced a new pH stratification of the former plow horizon, approaching the natural soils. Compared to the former plow horizons below, soil organic carbon (SOC) and plant available nutrients (P, K) enrichment was observed within the newly developed Ah horizons. The SOC stocks of the 5 cm mineral topsoil increased from 0.8 to 1.7 kg m−2 (North), from 1.3 to 1.7 kg m−2 (Middle), and from 1.4 to 2.3 kg m−2 (South) during restoration. The surface organic layers showed SOC stocks of 0.1–0.2 kg m−2. However, the levels of the near-natural soils were not reached after 37 years (North) and 42 years (South) of restoration. The model predicted a recovery time of about 150 years to achieve a comparatively steady state. A negative effect of mowing was observed for the modelled SOC accumulation. Despite all these alterations, the study showed no full restoration for many parameters within the chronosequential timescale of 120 years.

We have studied the microbiological activity of postagrogenic soddy–podzolic, gray, and dark gray forest soils representing succession stages of natural reforestation on former agricultural lands in various forest zones. The chosen succession chronoseries of postagrogenic soils are uniform and include arable soil, abandoned lands of various ages, and forest cenoses. The content of organic carbon (Corg) and total nitrogen (N), pH, water holding capacity (WHC), basal respiration (Vbasal), microbial biomass carbon (Cmic), and ecophysiological parameters of the status of microbial communities (metabolic coefficient qCO2; the Cmic: Corg ratio, and specific rate of basal respiration calculated as the Vbasal: Corg ratio) are determined in mixed soil samples taken from 0–10 and 10–20 cm layers. It has been revealed that the transformation of arable soils into abandoned lands constantly occupied by meadow or forest vegetation usually results in the progressive accumulation of organic carbon in the 0–10-cm layer. This causes more active soil respiration and a significant increase in the pool of microbial carbon. Parallel to this, the processes of podzol formation upon the development of forest vegetation result in a pronounced increase in acidity in the 10–20 cm layer, which causes a decrease in Vbasal and Cmic in soils of forest cenosis. For all the studied chronoseries of postagrogenic soils, the correlation between microbiological parameters (Vbasal and Cmic) and the general soil properties (Corg, N, and WHC) is the closest. The following factors (in decreasing order) exert effect on the dynamics of all the studied properties at postagrogenic evolution: forest zone/soil type > age of abandoned land ˜ depth in the arable layer.

The effect of contrasting moisture regimes on the CO2 emission from the gray forest soils (Haplic Luvisols (Loamic, Cutanic, Humic)) under a grass vegetation and bare fallow was studied in a field simulation experiment in June–September, 2015 (Moscow region). Two short soil droughts (53 and 34 days) and a long one (94 days) were simulated on plots isolated from precipitation. A variant with regular irrigation, where the soil moisture was maintained 60–70% of their water holding capacity, was used as a control. Over the whole observation period, the CO2 emissions from the soils studied decreased by a factor of 1.8 compared to the control only in the variant with the grass vegetation under prolonged drought. During the first hours after irrigation of the dry plots, the soil respiration intensified due to the “Birch effect”. The magnitude of this effect was 84–104% in the soils under the grass vegetation and 114–133% in the fallow areas. Owing to this phenomenon, the total CO2 emission from the soils subjected to two short droughts was equal to the CO2 flux under regular moistening for the grass plots and exceeded it by almost 1.3 times for the fallow plots as compared to the control. However, the share of extra CO2 flux induced by moistening of the dry soils did not exceed 8–10% of the total CO2 emission over the whole observation period.

Abandoned lands formed in the place of former arable lands occupy considerable areas in the steppe zone and are a basic reserve for the restoration of the steppe biome in Russia. Taking into account the secondary steppe ecosystems developed in the place of the abandoned lands, the total carbon dioxide sink in the steppe zone of Russia can reach 92-121 Mt С per year. This is comparable to the СО2 sink in the managed forests of the Russian Federation (96 Mt/yr) and accounts for 10-20% of the total sink of СО2 in the terrestrial ecosystems of Russia. To increase the sink potential on the territory of the Russian Federation, the natural and restored steppe ecosystems should be preserved.

The postagrogenic dynamics of acidity and some parameters of humus status have been studied in relation to the restoration of zonal vegetation in southern taiga (podzolic and soddy-podzolic soils (Retisols)), coniferous-broadleaved (subtaiga) forest (gray forest soil (Luvic Phaeozem)), and forest-steppe (gray forest soil (Haplic Phaeozem)) subzones. The most significant transformation of the studied properties of soils under changing vegetation has been revealed for poor sandy soils of southern taiga. The degree of changes in the content and stocks of organic carbon, the enrichment of humus in nitrogen, and acidity in the 0- to 20-cm soil layer during the postagrogenic evolution decreases from north to south. The adequate reflection of soil physicochemical properties in changes of plant cover is determined by the climatic zone and the land use pattern. A correlation between the changes in the soil acidity and the portion of acidophilic species in the plant cover is revealed for the southern taiga subzone. A positive relationship is found between the content of organic carbon and the share of species preferring humus-rich soils in the forest-steppe zone.

This study estimates carbon balance in a mixed mature forest on sod-podzolic sandy-loamy soil (Albeluvisols sandy, the Prioksko-Terrasny Nature Reserve) and in a secondary deciduous forest at the Experimental Field Station of the Institute of Physicochemical and Biological Problems of Soil Sciences, Russian Academy of Sciences (gray forest loamy soil, Luvisols loamy). CO2 emissions from soils have been continuously measured every week since 1998. Net primary production was estimated in 2000–2014 by remote sensing using the normalized difference vegetation index. Long-term weather monitoring has revealed a distinctive trend toward increasing aridity of climate in the southern part of Moscow region in the observation period (1998–2014). Based on long-term ground-based and satellite monitoring data, this study shows that in the growing season, mixed and deciduous forests of the southern part of Moscow region are the sink of carbon with a mean flux of 41–112 g C m–2, depending on the contribution of root respiration. Taking into account the CO2 emissions from soils during the cold season, the forests are very likely to function as sources of atmospheric carbon at an amount of 30–100 g C m–2 yr–1, sometimes reaching very significant values of C flux (170–300 g C m–2 yr–1). In mature forest ecosystems, a significant influence on the quantitative estimate of the C balance is hampered by the CO2 emission activity from coarse woody debris, which can reach up to 14% of the total losses of C during the decomposition of soil organic matter in the mixed forest, which turns it into a persistent source of CO2 to the atmosphere. It is shown that the sink function of the forest ecosystems was more pronounced in dry years, whereas the excessive wetness diminishes their sink potential, turning the forests into sources of carbon dioxide.

The mineralization and humification of leaf litter collected in a mixed forest of the Prioksko-Terrasny Reserve depending on temperature (2, 12, and 22°C) and moisture (15, 30, 70, 100, and 150% of water holding capacity ( WHC)) has been studied in long-term incubation experiments. Mineralization is the most sensitive to temperature changes at the early stage of decomposition; the Q 10 value at the beginning of the experiment (1.5–2.7) is higher than at the later decomposition stages (0.3–1.3). Carbon losses usually exceed nitrogen losses during decomposition. Intensive nitrogen losses are observed only at the high temperature and moisture of litter (22°C and 100% WHC). Humification determined from the accumulation of humic substances in the end of incubation decreases from 34 to 9% with increasing moisture and temperature. The degree of humification CHA/CFA is maximum (1.14) at 12°C and 15% WHC; therefore, these temperature and moisture conditions are considered optimal for humification. Humification calculated from the limit value of litter mineralization is almost independent of temperature, but it significantly decreases from 70 to 3% with increasing moisture. A possible reason for the difference between the humification values measured by two methods is the conservation of a significant part of hemicelluloses, cellulose, and lignin during the transformation of litter and the formation of a complex of humic substances with plant residues, where HSs fulfill a protectoral role and decrease the decomposition rate of plant biopolymers.

Changes in carbon stocks and physical properties of gray forest soils during their postagrogenic evolution have been studied in the succession chronosequence comprising an arable, lands abandoned 6, 15, and 30 years ago; and a secondary deciduous forest (Experimental Field Station of the Institute of Physicochemical and Biological Problems of Soil Science, Russian Academy of Sciences, Pushchino, Moscow region). It is found that carbon stocks in the upper 60-cm soil layer gain with increasing period of abandonment, from 6.17 kg C/m2 on the arable land to 8.81 kg C/m2 in the forest soil, which represents the final stage of postagrogenic succession. The most intensive carbon accumulation occurs in the upper layer of the former plow (0- to 10-cm) horizon. It is shown that the self-restoration of gray forest soils is accompanied by a reliable decrease of bulk density in the upper 10-cm layer from 1.31 ± 0.01 g/cm3 on the arable to 0.97 ± 0.02 g/cm3 in the forest. In the former plow horizon of the arable–abandoned land–forest succession series, the portion of macroaggregates increases from 73.6 to 88.5%; the mean weighted diameter of aggregates, by 1.6 times; and the coefficient of aggregation, by 3.8 times. Thus, the removal of lands from agricultural use results in a gradual restoration of their natural structure, improvement of soil agronomical properties, and carbon sequestration in the upper part of the soil profile.

The study of carbon balance of terrestrial ecosystems is considerable important for the forecasts of climate and environmental changes. Using a geoinformation-analytical method and a climate-driven regression model, we have calculated the C-CO2 balance of natural steppe ecosystems of Khakassia and Tuva. Our research has shown that natural steppes of the region under study are a significant sink of carbon dioxide from the atmosphere. The intensity of this carbon flux in the region can be estimate as 184±41 gCm−2 yr −1. The annual absorption of carbon dioxide in the natural steppe ecosystems of Tuva and Khakassia is evaluated as 6.9±2.2 Mt C-CO2.

Fast growth of urban population during the last century led to massive abandonment of arable lands near big cities due to the outflow of rural population. After withdrawal, former arable lands undergo a process of self-restoration, and there are some periods of natural succession of vegetation and soils. This study was aimed to estimate the changes in plant biodiversity and organic carbon content in soils during postagrogenic evolution of former arable lands in various bioclimatic zones of the European part of Russia. The chronosequence studies were carried out in southern taiga (Albeluvisols, Kostroma region), zone of deciduous forest (Phaeozems, Moscow region), and steppe zone (Chernozems, Rostov region). Each chronosequence included arable, abandoned lands of different ages, and native (forest or steppe) cenosis. The content of organic carbon (Corg) and total nitrogen (Norg) was determined in mixed soil samples (0–20 cm layer). Total species richness (TSR) was used to quantify the biodiversity of plant communities. It was shown that the withdrawal of soils from agricultural use resulted in some alterations in species composition, increase of biodiversity, and a gradual recovery of native vegetation attributed to the bioclimatic zone. The highest values of TSR were observed in abandoned lands of 13, 15, and 30-yr old under grasslands. A notable increase in Corg content in the upper part of former arable layer was observed during the postagrogenic evolution for all bioclimatic regions. However, the tendency was more pronounced for postagrogenic soils in northern bioclimatic zones in comparison to southern ones: Albeluvisols > Phaeozems > Chernozems. Thus, Corg content in Albeluvisols under forest was 4.7 times higher than in arable soil. The same ratio (Corg in native cenosis: Corg in arable) for Phaeozems and Chernozems comprised 2.3 and 1.4, respectively.

Most land use changes (LUC) significantly affect the amount of carbon (C) sequestered in vegetation and soil, thereby, shifting the C balance in ecosystems. Disintegration of the USSR and the followed collapse of collective farming system have led to abandonment of more than 58 million ha (Mha) of former croplands in Russia and Kazakhstan that comprise together about 90% of land area in the former USSR. This was the most widespread and abrupt LUC in the 20th century in the northern hemisphere. The spontaneous withdrawal of croplands in 1990s caused several benefits for environment including substantial C sequestration in post-agrogenic ecosystems. The new estimations of net ecosystem production (NEP) and changes in soil organic carbon stocks (∆SOC) in post-agrogenic ecosystems presented here are based on the uniform bio-climatic approach, and hereby, allow to update C balance of the former USSR. The total extra C sink in abandoned croplands in Russia (45.5 Mha) and Kazakhstan (12.9 Mha) is estimated to be 155 ± 27 Mt C yr− 1 and 31 ± 2 Mt C yr− 1, respectively. This additional C sink could cover about 18% of the global CO2 release due to deforestation and other land use changes or compensate annually about 36% and 49% of the current fossil fuel emissions in Russia and Kazakhstan, respectively. The extra C sink to the post-agrogenic ecosystems in Russia and Kazakhstan contributes possibly about 1/3 part to the total current C balance of the former USSR. Hence, the disintegration of the former USSR significantly affected national and global C budget over few decades after LUC.

Three consecutive freeze–thaw cycles (FTCs) have been performed in a laboratory experiment with lessivated burozems (Haplic Luvisols) under forest and agricultural crops in order to assess the contribution of physicochemical factors to the emission of CO2 from soils in consecutive FTCs. The use of native soils and their sterilized analogues (to exclude the effect of biotic factors) has allowed a quantitative description of physicochemical processes during the FTCs. It has been revealed that the ratio between the biotic and physicochemical factors during the FTCs largely depends on soil solution pH. The proportion of physicochemical factors is low (1.3–9.4%) in the frozen forest soil with low pH (3.9–4.1), while it is no less than 52% in the neutral arable soils, which indicates the prevalent contribution of physicochemical processes to the emission of CO2 from these soils under freezing. CO2 sorption has been observed in the thawed forest soils at an insignificant rate, which we estimate at only 2.3–3.6% of the total CO2 flux from the soil, which confirms the biotic nature of increase in CO2 emission bursts in the acid forest soils under thawing. In the arable soils, along with the enhanced CO2 emission caused by the activation of microbial community, a significant participation of physicochemical processes has been observed under thawing conditions, which makes up 17–20% of the total CO2 flux released during thawing.

than that from the background soil surface. In the dry season, it comprised 91 ± 7 mg C/m 2 per h from the background soil and 196 ± 16 mg C/m 2 per h from the termite mounds. In the wet season, the CO2 emission rate was considerably higher and reached 266 ± 40 and 520 ± 39 mg C/m 2 per h, respectively. The maximum rates of CO2 fluxes were determined in the wet season in some of the measurement chambers installed on ter� mite mounds; they reached 730–880 mg C/m 2 per h. Though termite mounds occupy about 4% of the area of tropical forest ecosystems, the overall effect of termites on the carbon budget was more significant; accord� ing to our estimates, it reached up to 10% of the total efflux of CO2 from the soil surface.

The decomposition of the prevalent peat-forming plants of oligotrophic bogs in the southern taiga subzone of Western Siberia is quantified in a model experiment. The decay rate (DecR) of Sphagnum fuscum, Chamaedaphne calyculata, Eriophorum vaginatum, and mixed sample (60% of Sph. fuscum and 40% of C. caltculata)is estimated as a C(CO2) emission from plant samples over 3 months at different temperatures (2, 12, 22° C) and at a moisture level corresponding to 60% of their water holding capacity. The dynamics of DecR of the prevalent peat-forming plants during the experiment is considerably influenced by both factors: the temperature and the type of plant litter. A remarkable increase in the C(CO2) emission rate is observed for all plant samples at the initial stages of decomposition. An increase in the DecR for all types of plants is observed first at 22° C and 1-2 weeks later at 2°C. During the 3 months of the experiment, the Sph. fuscum looses only 3-5% of the initial amount of C, whereas the C-losses from the C. calyculata and E. vaginatum vary from 6 to 18% depending on the temperature. The effect of plant species on the DecR is more pronounced and explains about 61% of the DecR variability. The temperature coefficient Q10 depends on the type of plant and varies from 0.97 to 1.3 in the low temperature range of 2-12°C and from 1.34 to 2.16 in the temperature range of 12-22°C.

The processes of the organic matter (OM) mineralization in forest soils developed under temperate continental (Moscow oblast, Russia), Mediterranean (the central and western parts of Spain), and tropical monsoon (southern Vietnam) climates were studied under laboratory conditions. The potential and specific rates of the OM mineralization (PR min and PR min/Corg, respectively), the ecophysiological parameters of the microbial communities status (Cmic, qCO2, and Cmic/Corg), and the sensitivity of the rate of the OM mineralization to the rise in temperature were evaluated by the temperature coefficients (Q 10) determined in the humus horizons (0–10 cm, without forest litter). The average values of PR min for the climatic zones decreased in the following order: Mediterranean (57.1 ± 10.6 mg C/kg per day) > temperate continental (23.8 ± 7.1 mg C/kg per day) > tropical monsoon (10.4 ± 1.6 mg C/kg per day). The lowest resistance of the soil OM to mineralization as evaluated by the PR min/Corg values was found in the Albeluvisol and Phaeozem of the temperate continental climate and in the Acrisol of the Mediterranean climate. The highest Q 10 coefficients were attributed to the OM mineralization in the forest soils of the temperate continental climate. This allowed us to conclude that the observed and expected climate changes with an increase in the mean annual air temperature should lead to the maximum intensification of the OM mineralization processes in the forest soils of northern regions.

The analysis of daily, seasonal, and annual dynamics of CO2 emission from soils under different stands of monsoon tropical tall-tree forest was performed on the basis of field observations conducted at the Russian-Vietnamese Tropical Research and Technology Center of the Russian Academy of Sciences. Under a tropical climate, the main factors responsible for the rate of carbon dioxide emission from the soils are shown to be the soil type and the topographic position of the area studied along with the type of vegetation. Depending on these factors, the rate of CO2 emission from the soils was 65–178 mg C/(m2 h) during the dry season and 123–259 mg C/(m2 h) during the wet season. The daily dynamics of CO2 emissions from the soils of the tropical zone was weakly pronounced in both the wet and the dry season owing to the insignificant diurnal fluctuations of soil temperature. The investigations carried out allowed making an expert evaluation of the annual CO2 fluxes from the soils under different stands of monsoon tropical tall-tree forest in southern Vietnam. They amounted to 900–2000 g C/(m2 yr) depending on the forest type.

The contributions of root and microbial respiration to the total emission of CO 2 from the surface of gray forest and soddypodzolic soils were compared under laboratory and field conditions for the purpose of optimizing the field version of the substrateinduc ed respiration method. The magnification coefficients of respiration upon the addition of saccharose (kmic) were first determined under conditions maximally similar to the natural conditions. For this purpose, soil cleared from roots was put into nylon nets with a mesh size of 40 µm to prevent the penetration of roots into the nets. The nets with soil were left in the field for 7-10 days for the compaction of soil and the stabilization of microbial activity under natural conditions. Then, the val� ues of kmic were determined in the rootfree soil under field conditions or in the laboratory at the same tem� perature and water content. The contribution of root respiration as determined by the laboratory version of the substrateinduced respiration method (7-36%) was lower compared to two field versions of the method (27-60%). Root respiration varied in the range of 24-60% of the total CO2 emission from the soil surface in meadow ecosystems and in the range of 7-56% in forest ecosystems depending on the method and soil type.

The effect of the temperature and moisture on the emission of N2O from arable soils was studied in model experiments with arable soils at three contrasting levels of wetting and in a wide temperature range (from −5 to +25°C), including freeze-thaw cycles. It was shown that the losses of fertilizer nitrogen from the soils with water contents corresponding to 60 and 75% of the total water capacity (TWC) did not exceed 0.01–0.09% in the entire temperature range. In the soils with an elevated water content (90% of the TWC) at 25°C, the loss of fertilizer nitrogen in the form of N2O reached 2.35% because of the active denitrification. The extra N2O flux initiated by the freeze-thaw processes made up 88–98% of the total nitrous oxide flux during the entire experiment.

The contributions of root and microbial respiration to the CO2 emission from the surface of gray forest and soddy-podzolic soils under meadow and forest vegetation were determined in field and laboratory experiments. In the field, a new modification of the substrate-induced respiration (SIR) method was applied. According to this method, the contribution of root respiration was estimated at 41–50% for meadow cenoses and 33% for forest cenoses; similar values were obtained in the course of separate incubation of roots and soil in laboratory (42–57% and 29–32%, respectively) and with the use of the laboratory version of the SIR method (35–40% and 21–31%, respectively). The analysis of difference between the values of root respiration and microbial respiration obtained by the field and laboratory methods for the same experimental plots and the comparison of advantages and disadvantages of these methods made it possible to outline the ways for the further improvement of the field version of the SIR method.

The assessment of the changes in the organic carbon pool in the soils of the Russian Federation that occurred in 1990–2004 was carried out using approximation, soil-geoinformation, and simulation approaches. As a result of the changes in the system of land use, after 1990, the organic carbon storages in the 0- to 20-cm-thick soil layer could be 196–319 Mt depending on the methodology of the calculation applied and taking into account the abandoned area of 14.8 million ha. As compared to the beginning of the 1990s, the organic matter stock in the former plow layer increased by 1.6–5.8%. The great scatter of the data is mainly related to the incertainty of the estimates of the area of arable soils not used any more in agriculture.

The effect of droughts and drying-wetting cycles on the respiration activity of agrogray soils was studied in field and laboratory experiments. The alternation of drought periods and rains during the vegetation season did not increase the annual emission of CO2 from the soils under a sown meadow and an agrocenosis. In laboratory experiments, the wetting of dried soil released 1–1.5% of Corg with a high decomposition constant n × 10−1 day−1 and a very short renewal time (2.1–2.4 days); therefore, an abrupt change in the wetting conditions did not intensify the loss of soil carbon under field conditions.

The effect of different land-use practices on the carbon budget in old arable gray forest soils of Russia was studied in field experiments. A short-term (for 6–7 years) cessation of mineral fertilization had no negative effects on the carbon budget in the agrocenoses studied. Only the combination of zero fertilization with the return to monoculture and the introduction of black fallows created a negative budget of humus in the soil. The regrassing of the eroded arable soil for 24 years increased the humus reserve in the 0- to 60-cm layer by a factor of 1.6–1.7. The average annual accumulation of carbon and nitrogen after the restoration of the perennial vegetation was 106–128 g C/m2 and 11–16 g N/m2, respectively.

Experimental studies and the analysis of published data have shown that carbon reserves in soils generally increase upon soil exclusion from agricultural use. The rate of carbon accumulation in the abandoned soils depends on the soil type, the time elapsed since the soil abandoning (the restoration period), and the thickness of the layer for which the rate of carbon accumulation is determined. For the upper 20-cm-thick layer, it varies from 66 to 175 g C/m2 per year in dependence on the type of soil and averages 111 g C/m2 per year. The highest rate is typical of the first 10–15 years of soil restoration. According to our calculations, the carbon sequestration in the upper 20-cm-thick layer of Russian soils due to changes in land use was 184–673 Mt C in 1990–2005 and may reach 282–1030 Mt C by 2020.

Knowledge is scarce on mineralization of soil organic carbon (SOC) in and N2O emissions from tundra soils in periods of alternate freezing and thawing. Our objectives were to study the CO2 and N2O emissions from two silty gleyic soils formed in different climate zones (a gleyic Cryosol located in the Russian tundra, and a stagnic Gleysol located in an oak stand in central Germany) during freeze–thaw events. Soils were adjusted to a matric potential of −0.2 kPa and emissions were measured in 3-h intervals during an incubation period of 50 days including three freeze–thaw cycles. CO2 emissions from the German oak forest soil were twofold higher than those of the tundra soil. The ratios of the mean CO 2 production rate before the freezing to the mean CO2 production rate after thawing ranged from 0.63 to 0.73 for the forest soil and from 0.85 to 0.89 for the tundra soil. The specific CO2-C production rate (CO2-C/SOC) was 0.16 for the tundra soil and 0.57 for the forest soil. The results indicate that bioavailability of SOC was markedly smaller in the tundra soil than in the forest soil. Large N2O emissions were found for the German forest soil, but no N2O emissions were observed for the tundra soil. The main reason for the absence of N2O emissions was most likely the negligible availability of nitrate for denitrification. There was some indication that the initial increase in mineralization of SOC induced by freezing and thawing differs between soils from various climatic regions, probably mainly due to a differing bioavailability of the SOC and differing releases of nutrients after thawing.

Soil temperature and moisture are the main ecological factors regulating the processes of production and emission of CO 2 from soil surface. The CO 2 evolution rate from cultivated clay Phaeozem (Russia, Moscow region; 54°50'N, 37°35'E) were studied under field conditions from November 1997 to October 2002. The daily mean CO 2 evolution rate varied widely - from 0.9 to 246 mg C/m 2 /h. The total annual CO 2 flux from cultivated Phaeozem averaged 352 ± 148 g C/m 2 /year, the interannual variability amounted to 42%. We found significant linear trends ( R = 0.46- 0.55, P < 0.001) reflecting the relationship between CO 2 emission and soil temperature through the whole observation period and during spring and autumn seasons as well. The exponential equations described these relationships for the same periods more adequately than the simple linear equations (R = 0.62-0.68, P < 0.01). The temperature coeffici - ent Q 10 comprised 2.3 (for the whole data set) and was essentially higher 3.2-3.6 during the spring and autumn. The correlation between CO 2 evolution rate and soil moisture was insignificant for the whole period, winter, spring and autumn seasons as well. During the summer, correlation between CO 2 evolution rate and soil moisture was positive and very close (R = 0.74, P < 0.001), indicating that the soil moisture content was a main factor limitative the rate of CO 2 emission from soil for this period.

Fluxes of carbon dioxide from arable gray forest soils (clay Humic Luvisol) were studied under field conditions during 36 months. The mean daily CO2 evolution rates varied — from 0.9 to 246 mg C·m −2·hour −1. The annual flux of CO2 from arable gray forest soils averaged 4.2 t C·ha−1. Contribution of cold period (November — April) ranged from 12 to 16% of average annual flux. The exponential Q10 function over whole the period of investigations and over the different classes of soil temperature (Ts) and moisture (Ms) were calculated. Q10 value for all the data obtained was equal to 2.64 (n=104) and increased from approx. 0.8 at Ms 30%. Significant differences of Q10 function were observed between different classes of Ts. The highest Q10 values were 13.3 at Ts = 5–10°C (Ms = 20–30%) and 8.8 at Ts = 0–5°C (Ms > 30%).

The land use system in Russia changed considerably after 1990: 30.2 million ha of croplands were abandoned. Based on the own field investigations that were carried out in abandoned lands of different age (Luvic Phaeozems, deciduous forest zone; Moscow region, 54°50′N, 37°37′E), it has been shown that after 4–5 yr of abandonment, the former croplands acted as a stable sink of CO 2 . The net ecosystem production (NEP) in the post-agrogenic ecosystems averaged 245 ± 73 g C m -2 yr -1 for the first 15 yr after land use change that corresponds to an estimated 74 ± 22 Tg C yr -1 for the total area of abandoned lands in Russian Federation. Currently, the Russian territory acts as an absolute sink of atmospheric CO 2 at a rate about 0.90 Pg C yr -1 . Using three different approaches, it was demonstrated that after 1990, the carbon sequestration in Russian soils (0–20 cm layer) has averaged 34 ± 2.2 Tg C yr -1 . This soil C forms net biome production (NBP) where carbon lifetime is much longer than in ‘Kyoto forests’. Thus, the post-agrogenic ecosystems in Russia provide with the additional CO 2 sink in NEP and NBP that could annually compensate about 25% of the current fossil fuel emissions in the Russian Federation. DOI: 10.1111/j.1600-0889.2010.00467.x

A quantitative assessment of the carbon balance was performed in gray forest soils of the former agricultural lands abandoned in different time periods in the southern part of Moscow oblast. It was based on the field measurements of the total and heterotrophic soil respiration and the productivity of biocenoses. Geobotanical investigations demonstrated that the transformation of the species composition of herbs from weeds to predominantly meadow plants occurred in five–ten years after the soil was no more used for farming. The amount of carbon assimilated in the NPP changed from 97 g C/m2 year in the recently abandoned field to 1103 g C/m2 year in the 10-year-old fallow, and the total annual loss of carbon from the soil in the form of CO2 varied from 347 to 845 g C/m2 year. In five years, the former arable lands were transformed into meadow ecosystems that functioned as a stable sink of carbon in the phytomass and the soil organic matter.