И.Н. Курганова1, В.О. Лопес де Гереню1, А.В. Прищепов2,3

I.N. Kurganova1, V.O. Lopes de Gerenyu1, A.V. Prishchepov2,3 

1Институт физико-химических и биологических проблем почвоведения РАН

(Россия, 242290, Московская область, г. Пущино, ул. Институтская, 2/2)

2Университет Копенгагена, Факультет геонаук и управления природными ресурсами

(Дания, DK-1350 г. Копенгаген, 5 Остер Вольдгейт 10)

3Федеральное государственное бюджетное учреждение науки

Институт степи Уральского отделения Российской академии наук (ИС УрО РАН)

(Россия, 460000, г. Оренбург, ул. Пионерская, 11) 

1Institute of Physicochemical and Biological Problems in Soil Science of the Russian Academy of Sciences

(Russia, 142290, Moscow region, Pushchino, Institutskaya Str., 2/2)

2University of Copenhagen, Department of Geosciences and Natural Resource Management

(Denmark, DK-1350 København K, 5 Øster Voldgade 10)

3Institute of Steppe of the Ural Branch of the Russian Academy of Sciences (IS UB RAS)

(Russia, 460000, Orenburg, Pionerskaya Str., 11)

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Около 45,2 млн га степных экосистем были распаханы на юго-востоке России и в северном Казахстане в период между 1954 и 1963 гг. Это привело к огромным потерям органического углерода (С) из целинных почв, которые, согласно нашим оценкам составили 852 Mт C для верхнего 0-50 см слоя почвы в течение первых 20 лет их освоения и, вероятно, могли привести к росту концентрации CO2 в атмосфере.

About 45,2 million ha of steppe ecosystems were converted to croplands in the south-eastern Russia and northern Kazakhstan between 1954 and 1963. It resulted in huge losses of organic carbon (C) from virgin soils. According to our estimations, they accounted for 852 Mt C for upper 0-50 cm soil layer during the first 20 years of cultivation and likely could increase the CO2 concentration in the atmosphere. 

Introduction. Most land use changes affect significantly the amount of carbon (C) sequestered in vegetation and soil, thereby, shifting the C balance in ecosystems and affecting the climate through CO2 release into the atmosphere [5, 6]. The greatest C fluxes caused by Land Use Land Cover Changes (LULCC) are attributed to conversion of native vegetation to cropland and vice versa [7]. Globally, the changes in land use, such as cropland expansion yielded to release of 156 Pg C (1 Pg = 1015 g) to the atmosphere over the period 1850-1990, which can be compared to about half C release from of fossil fuels combustion over the same period [5]. Each cultivated hectare lost about 25% of its initial C stock to a depth of 1 m due to large conversion of forests or prairie to croplands [4]. Between 1954 and 1963, about 45,2 million ha of natural ecosystems (mainly steppes) in the south-east Russia and northern Kazakhstan were converted to croplands. It was the so-called Virgin Land Campaign (VLC). Despite the cropland expansions were also very massive in America, Africa, and Australia, their ecological consequences have been not estimated and understood up to now. There is no information on changes in C stocks caused by conversion of virgin steppes and prairies to croplands. Although, the overall negative impacts of Virgin Lands Campaign are widely acknowledged [2, 8, 9], the ecological cost of the VLC including C losses from soils is poorly quantified. Our study was aimed to assess the losses of soil organic carbon (SOC) due to the massive croplands expansion during Virgin Land Campaign in Russia and Kazakhstan.

Materials and methods. We assembled a database of initial SOC stocks and their changes in soil profiles collected from a VLC area. We calculated mean SOC stocks in the 0-50 cm layer for native grasslands and dominating soil types separately for Russian and Kazakhstan’s parts of the VLC area. The dominant soil types were Luvisols (LV), Haplic Chernozems (CH), Calcic Chernozems (calc. CH), Kastanozems (KS), and Calcisols (CL). All other soils (e.g. Umbrisols, Phaeozems, Fluvisols, Planosols, Arenosols, Solonetzes) were combined into other soils (Others, or os).

Two approaches were applied to estimate the SOC losses from the upper 50-cm due to conversion natural steppe soils to cropland. Firstly, the exponential Henin&Dupuis model [11] was fit to SOC stock changes (ExpFun approach) collected from literature. According to this model, change in SOC stock (∆SOC) due to conversion of management A (grassland) to management B (cropland) was determined as the SOC stock difference at equilibrium level A (SOCeqA) and B (SOCeqB): ∆SOC = SOCeqB-SOCeqA.

The mean annual rate of change in SOC stocks (MAR∆SOC, t ha-1 yr-1) was calculated for a duration T (20 years) according to simple exponential function:

MAR∆SOC =∆SOC [1 - exp(-kT)] / T                                                               (1),

where «k» is a time constant for C storage ‘rate’. Fitted rate (k) was 0,07±0,01 for 20-yrs period after conversion of grassland to cropland [11].

The second approach (LnFun) based on dataset of Poeplau et al. [10] reports on the relative SOC changes as % of initial SOC stocks after grassland conversion to cropland in temperate zone of Europe depending on time (more than 170 pair sites). The dynamics of SOC decrease (∆SOC, %) depended logarithmically on period after LUC (age, yrs):

∆SOC (%) = -6,2·ln(Time) – 8.1                                                                    (2).

Using this equation, the reduction of initial SOC stocks for the time interval of 20 yrs (period after conversion of grassland to cropland assumed for this study) was 27%. Then, we applied this percentage to estimate the changes in initial SOC stock (SOCgrass) for each soil type:

∆SOC (Mg ha-1) = -0,27·SOCgrass                                                                   (3).

Mean annual change in SOC stocks (MAR∆SOC, t C ha-1 yr-1) for the first 20 yrs after grassland to cropland conversion was calculated by simple division:

MAR∆SOC = ∆SOC/20                                                                                     (4).

To present the dynamics of cropland expansion and contraction on annual basis from 1954 to 2010, we used the sowing area statistics (official statistics' reports) at province (oblast) level for Russia and Kazakhstan explicitly for all study years. Sowing area statistics were allocated within the cropland mask using spatial allocation approach, when areas with higher potential yields received higher probability to be cultivated first. To prototype the potential for cropland productivity, the spatially-explicit dataset with the estimated yield potential for wheat production was used from Global Agro-Environmental Zones database – GAEZ (2008). The soil types [1] were summarized within the cropland maps for each study year and we calculated soil shares, which we used later to calculate SOC stocks.

Bookkeeping models were applied to estimate the total changes in SOC stocks due to the land conversion. These models are used to calculate annual emissions and C accumulations at any scale - from the regional level to national or global [3, 4].

Results. The initial SOC stocks in the upper 0-50 cm in virgin steppe soils of south-eastern Russia varied between 237±8 t C ha-1 in Haplic Chernozems and 105±5 t C ha-1 in Kastanozems. Because of lower net primary production caused by the limited precipitation, the SOC stocks in virgin soils of Kazakhstan steppe area were significantly lower: from 154±15 t C ha-1 in Chernozems to 52±2 t C ha-1 in Calcisols and 54±20 t C ha-1 in “other soils” category (mainly in Solonets). The total stocks of organic C in virgin soils before their conversion in 1954-1963 accounted for ~6,13±0,73 Gt C and about 2/3 parts of this amount (~3,94±0,68 Gt C) were sequestrated in the soils of Russian Federation. On average, the virgin soils of steppe zone in south-eastern Russia contained 36,5% more organic C compared to virgin soils in Kazakhstan.

After the conversion of natural ecosystems to cropland, the SOC stocks strongly decreased in all soil types. The rate of SOC losses in the individual soil types varied widely, but the results depended strongly on the calculation approach. Henin&Dupuis model (ExpFun approach) showed the rate of C losses during the first 20 yrs between -0,45 and -1,52 t C ha-1 yr-1 in Russian soils and was about 2 times less in the soils of Kazakhstan. Logarithmic function (LnFun approach) showed much higher rates of SOC losses during the first 20 yrs of VLC for all soil types – from -0,69±0,03 t ha-1 yr-1 in Calcisols of Kazakhstan to -3,16±0,11 t ha-1 yr-1 in Chernozems of south-eastern Russia. The weighed mean rate of SOC losses in the upper 50-cm layer over 20-year period after LUC amounted to -0,96÷-2,15 t C ha-1yr- 1 for Russian soils and was substantially lower -0,44÷-1,36 t C ha-1yr- 1 for soils in Kazakhstan.

According to estimations based on experimental data, the relative decrease of SOC-stocks in the upper 0-50 cm layer after conversion of steppe ecosystems to croplands (compared to their initial level) changed from 9,6% (Calcic Chernozems) to 19,4% (Luvisols) in Russian soils and varied negligible (10,5-12,0%) in the soils of Kazakhstan. According to ExpFun approach, the total loss of SOC stocks for the first 20 yrs after VLC amounted to 852±36 Mt C (or 0,95 t C ha-1 yr-1). More than 70% of total SOC decrease were in Russian soils. Chernozems contribution to the total SOC losses in both countries comprised about 42% and was much higher than that of other soils. Kastanozems (in Kazakhstan) and category of «other soils» (in Russia) contributed about 30% to the total SOC losses during VLC.

Conclusions. Therefore, the first estimations of SOC losses induced by VLC in Russian Federation and Kazakhstan based on experimental data, simulation results and bookkeeping modeling showed clearly that this unprecedented natural ecosystem transformation in the middle of 1950s resulted in essential decrease of SOC stocks in steppe soils. About 852 Mt C were lost only during the first 20 yrs after grassland to cropland conversion. About 55% of this amount was lost as CO2, and likely could affect the planetary greenhouse effect. 

The study was supported by the RFBR (project no. 18-04-00773a) and the ERA.Net RUS Plus Science & Technology CLIMASTEPPE project (ID № 559). 


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