Thermal stability of organic matter of typical chernozems under different land uses

Soil & Tillage Research 197 (2020) 104500

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Thermal stability of organic matter of typical chernozems under different land uses

T

Dmitry S. Volkova,b,*, Olga B. Rogovaa, Mikhail A. Proskurninb, Yulian R. Farkhodova, Larisa B. Markeevac a

Department of Chemistry and Physical Chemistry of Soils, V.V. Dokuchaev Soil Science Institute, Pyzhevsky per., 7/2, Moscow 119017, Russia Chemistry Department of M.V. Lomonosov, Moscow State University, Leninskie Gory, 1-3, GSP-1, Moscow, 119991, Russia c Skolkovo Institute of Science and Technology, Nobelya, 3, Moscow 121205, Russia b

ARTICLE INFO

ABSTRACT

Keywords: Chernozem Thermal stability Thermogravimetry

Soil organic matter of bulk samples and size fractions (by dry sieving) of chernozems with a rather with different history of use (from intact steppe to permanent bare fallow) was studied by TGA under pyrolysis conditions in an inert atmosphere to approach its transformation depending on the type of use. For the first time, thermal properties of samples of Kursk chernozem (Kursk Research Institute of Agricultural Production and V.V. Alekhin Tsentralno-Chernozemny Nature Reserve of Russia) were studied for a continuous 55-year field experiment with a well-documented history of use and showed that long-term agricultural use significantly changes the thermal parameters of soil organic matter, and this is manifested for aggregates of various sizes. The minimum representative weight for the thermal analysis of bulk soils is ca. 1 g (when grinding samples to 100 μm). The paper proposes a new parameter to assess the degree of organic matter degradation, the ratio of the mass loss in the 380–600 to 170–380 °C temperature ranges that is referred to as two-range thermal stability ratio. It is shown that this relative index discriminates soil samples depending on the land use type and does not require the knowledge on carbon content.

1. Introduction Soil is the world largest solid carbon reservoir, and it plays a key role in carbon stabilization (Lal, 2004). Chernozems are among the most fertile soils relevant for global food production and the main part of productive agricultural land in Russia is confined to chernozems. Being a huge carbon stock of more than 400 tons per hectare, chernozems form a significant carbon reservoir (Batjes, 2014; Mikhailova and Post, 2006). They are also characterized by a unique aggregate structure due to the peculiarities in organic matter composition. Nowadays, much attention is paid to the degradation processes of these soils under their active use, as well as to the restoration of their structure and unique hydrophysical properties (Chendev et al., 2018; Gusarov et al., 2018; Khaidapova et al., 2018b). Thus, the dynamics of changes in the structure, physical and chemical properties, and composition of chernozems is a relevant problem, and a detailed research of soil organic matter (SOM) properties of chernozems is topical. One of core mechanisms of stabilizing carbon in soil involves its accumulation in decomposition-resistant SOM (Lützow et al., 2006). Due to extreme complexity, SOM composition cannot be studied as ⁎

explicit changes in its structure. Therefore, indirect methods are being actively developed. They result in quantitative and reproducible parameters that can be associated with the changes in structure and therefore used for forecasting SOM changes. Many such approaches include physical fractionation of organic particles by size and density, fractionation by solubility using various chemical agents, or the assessment of the dynamics of microbiological destruction as well as thermal stability of SOM (Lützow et al., 2006). Among the latter is the classic thermal analysis that has been used in soil mineralogy for many decades (Plante et al., 2009). In fact, with state-of-the-art instrumentation, thermal analysis is an umbrella term for a family of methods. These methods provide a rather large set of parameters with high accuracy and reproducibility in an automated process thus significantly diminishing the human factor. Due to all this and the rapidity of data gathering without complex sample preparation, the relevance of thermal-analysis methods in SOM research is growing. In thermal analysis of SOM, the main hypothesis is that thermal stability can serve as an indicator of the relationship between biogeochemically labile and stable organic-matter fractions (Plante et al., 2009). Thus, thermal-analysis methods for SOM are based on the

Corresponding author at: Chemistry Department of M.V. Lomonosov, Moscow State University, Leninskie Gory, 1-3, GSP-1, Moscow, 119991 Russia. E-mail address: [email protected] (D.S. Volkov).

https://doi.org/10.1016/j.still.2019.104500 Received 26 June 2019; Received in revised form 7 October 2019; Accepted 1 November 2019 0167-1987/ © 2019 Elsevier B.V. All rights reserved.

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identification of the patterns in thermal curves (mainly TG, DTA, or DSC) and their chemistry-wise interpretation as well as the quantitative description of SOM stability by various indices. Up to 2009, this topic is covered in a review (Plante et al., 2009); most of the studies were carried out in an oxidizing atmosphere. Nowadays, SOM resistance to decomposition, apart from aggregate formation and sorption on mineral surfaces, is assumed to be related to its molecular structure (Han et al., 2016; Helfrich et al., 2006). Other factors are the accumulation of organic substances stable to microbiological digestion in soil, inaccessibility of SOM for microbial decomposition, and the formation of stable organomineral complexes (Krull et al., 2003; Kučerík et al., 2013; Oades, 1993; Six et al., 2002). However, using carbon isotopes, Dorodnikov et al. (2007) showed that thermal and biological stabilities are not explicitly related to one another. Siewert et al. (2012) compared the dynamics of biological mineralization of SOM by CO2 release during an 80-day laboratory incubation with their thermal-oxidative stability by thermogravimetry. These authors noted that the interactions between soil components such as SOM, inorganic matter, and microorganisms cannot simply be reduced to a single driving factor or dominant component. A recalculation should be made instead, with an appropriate theoretical concept considering possible factors that have not been clarified yet. Furthermore, the explanation of differences between averaged thermogravimetric curves for incubated and unincubated soil samples as exclusively microbial transformation and/or mineralization is too simplified. Thus, taking into account the available findings, it is possible only to speculate on the nature of soil transformation processes (Siewert et al., 2012). Evaluation of SOM by TGA and DSC usually is interpreted to provide some indicators. Among them, particularly noteworthy are the temperatures of thermal half-decomposition (T50) from a TG or DSC experiment as integral indicators of resistance to decomposition (in an inert atmosphere) or oxidation. The DSC-T50 index is defined as a temperature where 50 % of the total heat of reaction has evolved is measured under heating to 600 °C in a flow of synthetic air for various samples, such as charcoal, soot, graphite, black carbon-containing soils, charred plant tissues, lignite, and bituminous coal (Leifeld, 2007). The authors found that graphite had the highest 50 % burn-off temperatures, followed by soot, charcoals, bituminous coal, and charred pine wood. In addition, the authors conclude that DSC-T50 correlates significantly with the aromatic carbon content from NMR spectra (Leifeld, 2007). NMR spectra of charred wood and charred rice straw showed the same aromaticity (from 60.5–63.5 % of the total signal), but their thermal stabilities were completely different. Thus, we can expect this indicator to reveal differences in the organic matter of soils formed by various plant communities. Thus, aromatic compounds generally exhibit a higher thermal stability, which is accounted for by higher energies of conjugated double bonds compared to single aliphatic or CeO and CeH bonds. In addition, thermal stability increases with the molecular order. The latter finding is consistent with observations on the thermal stability and physical structure of soot and char from coal and wood. The reactivity of carbon black and coal with respect to thermal oxidation depends on the size, orientation, and organization of graphene layers (Vander Wal and Tomasek, 2003). In our opinion, such thermal-stability studies should not be limited to the assessment of the bulk soil. It is also necessary to investigate the properties of SOM that makes up soil aggregates of different size, as the change in the aggregate composition of soils under the influence of agrogenic factors directly reflects the changes in chemical composition (Chendev et al., 2017; Chernova and Bezuglova, 2018; Gao et al., 2015; Khaidapova et al., 2018a). The importance of researching not only the bulk but also soil fractions has been stated previously. Leinweber et al. (1992) analyzed SOM using DTA, TG, and pyrolysis-field ionization mass spectrometry and compared the data with SOM in particle-size fractions and bulk soil samples of two types of land use (untreated and

farmyard manure) from a 108-year-long agricultural experiment. Gregorich et al. (2015) used a Rock Eval method with nitrogen-atmosphere pyrolysis for studying samples of permanent pasture, cropland, and bare fallow, as well as sand, silt, and clay fractions. The correlation between TG-T50 index and mineralized carbon was revealed for samples of fractions isolated from the permanent pasture and arable cropping management practices. However, for the fractions isolated from the bare fallow, the bond was significantly less strong. In the study of the oxidative thermal stability of physical fractions selected by wet sieving from undisturbed and eroded soil (Nie et al., 2018), TG-T50 index showed a higher thermal stability of SOM fractions of eroded soil. It should be emphasized that long-term field experiments are very relevant in such studies as they provide unique data to estimate the trends in changes of key indicators of soil composition and properties under conditions of prolonged and controlled exposure (Johnston and Poulton, 2018; Richter et al., 2007). In general, among 620 long-term experiments with a duration of over 10 years, 66 were laid on chernozems, mainly in Eastern Europe and Spain (Debreczeni and Körschens, 2003). Still, of 62 long-term field experiments performed in Russia from 1964 to 1968, only 11 were laid out on chernozems of various land use systems, including three studies with preserved intact steppe areas as well. The above data indicate that the organic matter of soil aggregates, varying in size, has different thermal stability. One can also expect that under conditions of prolonged agrogenic exposure, not only the quantitative content of aggregates of different sizes changes, but also the qualitative composition of SOM characterized by thermal stability. Thus, in this study, the organic matter of bulk samples and size fractions (by dry sieving) of chernozems with different type of use was studied by TGA under pyrolysis conditions in an inert atmosphere to approach the SOM transformation depending on the type of agricultural land use. The data basis is a 55-year-long field experiment of the Kursk Research Institute of Agricultural Production and V.V. Alekhin TsentralnoChernozemny Nature Reserve. 2. Materials and methods 2.1. Long-term experiment location A detailed description of the territory and field experiments is given in the Guide for the “Soil Organic Matter dynamics in long-term field experiments and their modelling” International Scientific Symposium (Cherkassov et al., 2010). It is based on multiple studies of Russian researchers published in 1926–2009 years. The long-term field experiments of the V.V. Alekhin Central Chernozemic State Biospheric Reserve and the Kursk Research Institute of Agroindustrial Production are located in the forest-steppe zone of the Central Russia Heights. The territory of the experimental spots is typical enough for the Central Chernozemic Area. The climate is temperate-cold continental. Average annual temperature is 4.8–5.3 °C. The warm period of a year with the monthly average air temperature above zero is 230 days on average, and the vegetation period with temperatures above 5 °C is 186 days. By annual data, average winter duration is 135 days. Annual average precipitation is 540 mm, but every 3–5 years droughts of a different extent of intensity are common. In dry years, even in spring, moisture deficiency in topsoil can be observed. A major part of precipitation, 60–65 %, falls in the warm season. Annual evaporation in Streletskaya Steppe area is 636 mm. The maximum precipitation falls in July–August, in the same time the highest air temperature is observed; the moisture index stands less than 1, which is of great importance for the process of soil formation. Parent rocks in the area of the prevalence of deep chernozems of the forest-steppe belt of the Central Russia Heights are marl and chalk, and also tertiary sands of Poltavskiy layer (Afanasyeva, 1966; Lazarev et al., 2007; Mikhailova et al., 2000). Almost everywhere they are overlapped by loessial rocks of fluvio-glacial origin. The layer of 2

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loessial rocks is heterogeneous. It is distinctly divided into two parts: the upper layer of heavy loam 2–3.5 m deep and the underlying medium loam 5.5–8 m deep. Under the loams there lie clays frequently gleyed. Groundwater lies 12–14 m deep. The soil cover is mainly presented by combinations of typical and leached deep chernozems (black earth) which on virgin plots form patches related to the microrelief. Streletskaya Steppe of the Central Chernozemic State Biospheric Reserve (CCSBR) occupies 2046 hа, of which 1127 hа are under forest. For 200–300 years, Streketskaya Steppe has been used as hay fields and pastureland. Years ago, it was mowed. Scientific value of the Central Chernozemic Reserve is in the maintenance of several regimes of reservation on its territory, they are: steppe plots with an unmown regime and oak forest as an example of absolute reservation; steppe plots with moderate grazing and mown steppe variants as regimes of anthropogenic protection; a long-term fallow plot as an example of anthropogenic influence on soil. Unmown steppe is the oldest plot with an absolutely reserved regime (the last mowing was performed in 1940). Plant association is motley grasses-needle grass-awnless grasses. Soil is typical deep heavy loam chernozem. Mowed steppe are plots mowed annually. The stationary field experiment of Kursk Research Institute of Agroindustrial Production occupies 24.2 hа and is located on a plain area about 14 km east of the Central Chernozemic Reserve (see the map in Fig. 1). The soil of the plot is presented by a complex of deep typical chernozems with a small inclusion of leached chernozems. The structure of the soil cover is characterized by slight contrasts because the soil cover composition is remarkable for the supremacy of rather close soils. Typical chernozems are characterized by high total porosity. On arable plots, the total porosity is ca. 60 %. The high value of total porosity and water stable structure of soil aggregates result in favorable water-physical properties and primarily high water permeability of these soils. Due to a high value of moisture capacity of typical chernozems, the range of active moisture is satisfactory and amounts to 21–22 % in the layer of 0–40 сm. Humus content in the topsoil is 5.9–6.4 %, sometimes amounts to 7 %. The total nitrogen content in the top- and subsoil does not exceed 0.34 %. The 96–98 % nitrogen of deep chernozem is organic compounds. There are several blocks of experiments including “Efficiency of continuous crops” experiment, started in 1964 with winter wheat, potatoes, peas and maize, and fallow. The replication of the experiment is fourfold. The size of plots is 7.4 × 40 = 296 m2. The plot of continuous fallowing of typical chernozem (15 × 200 m), established 1964, which was modified 1998: ⅔ of the experiment area was retained under continuous soil fallowing, and ⅓ of it was allotted under idle land. At 2013 the long-term experiment with different types of tillage (and notillage) has started.

Galium verum L., Salvia pratensis L., Iris pumila L., Adonis vernalis L., Vicia tenuifolia Roth., Stipa pennata, Stipa pulcherrima, and Stipa tirsa. Shelterbelt (Fig. 1, B, marker B) 51°37′17.1″N 36°15′42.0″E. since 1964 delimiting the experimental fields is the soil type in which the structure is being restored under the influence of forest vegetation and which, when compared with arable Chernozem, shows high structural indicators (Khaidapova et al., 2016). This is a shelterbelt forest without a living grass layer of ca. 60 years of age. Forest-forming species: Quercus robur, Fraxinus excelsio, Acer campestre. No tillage (Fig. 1, B, marker C, no-till) 51°37´46.2349″N 36°15´46.0598″E since 2013, a four-field grain crop rotation. The alternation of crops: winter wheat Tríticum aestívum, maize Zéa máys, barley Hordeum vulgare, oats Avéna satíva and peas Pisum sativum mixture. Sampling was carried out in 2017 after the completion of the first rotation. Overgrown fallow (Fig. 1, B, marker D; overgrown) since 1998 after permanent bare fallow from 1964 is a part of the site of the abovedescribed long-term experiment, which was no longer processed since 1998 and diverted to an overgrown fallow. Currently, the site is overgrown with mainly European feather grass (Stipa pennata). This type of soil use allows us to analyze the restoration of the soil for 17 years after a permanent bare fallow. Cropland under traditional cereal rotation (Fig. 1, B, marker E, cropland, ploughing) 51°37′17.1″N 36°15′42.0″E with mineral fertilizers since 1964; crop rotation: bare fallow, winter wheat, sugar beet Beta vulgaris, maize, and barley. Sampling was made after barley. Permanent bare fallow (Fig. 1, B, marker F; fallow) since 1964 is a site where the soil is annually treated: ploughing without sowing and fertilization; thus, since 1964, fresh organic matter has almost not entered this soil type. 2.3. Sampling General samples were taken in 2017 at sites (Fig. 1) with a radius of 5 m using an envelope method, according to GOST 17.4.4.02-2017 Russian State Standard, the depth of sampling is 0–15 cm in triplicate. The weight of each of general samples was 4–5 kg. General samples were dried for two weeks in air, then stored at room temperature. A mean sample of 0.5 kg was taken from the corresponding general sample, which was then crushed and sieved through a 1 mm sieve. Analytical samples of 50 g were taken from the mean sample. Then, they were manually sieved through a column of sieves with hole diameters of 0.25 mm, 0.5 mm, 1.0 mm, 2.0 mm, 3.0 mm, 4.0 mm, 5.0 mm, 7.0 mm, and 10.0 mm. The bulk soil and the fractions of aggregates < 0.25 mm, 0.5–1 mm, 1–2 mm, and 7–10 mm by sieving dried samples were used (the sieve kit and separation method were used according to the GOST 12536-2014 Russian state standard “Soils. Methods of laboratory granulometric (grain-size) and microaggregate distribution”).

2.2. Selection of soil samples Typical chernozems (Haplic Chernozems Loamic, Pachic) were used throughout. Sampling was carried out at sites of the Streletskaya steppe of the V.V. Alekhin Tsentralno-Chernozemny Nature Reserve and of long-term field experiments (Cherkassov et al., 2010) of the V.V. Dokuchaev Soil Institute on the territory of the Kursk Research Institute of Agricultural Production on the sites. Soil granulometric composition is a heavy silty clay loamy texture. The humus horizon of the studied soils (A + AB1) is 105–130 cm. Boiling up from 10 % HCl starts from a depth of 65–70 cm. The density of the arable layer (0–30 cm) varies from 1.20 to 1.25 g/cm3 (Kuznetsova, 2013). The soil sample types according to the field experiment are listed below. Native steppe vegetation (Fig. 1, A) 51°34′13.6″N 36°05′23.1″E; Steppe—here and below in parentheses are short names of the experiments of land site use types— is an example of intact typical chernozem. Samples were taken at the site of the annually tangible steppe of V.V. Alekhin Tsentralno-Chernozemny Nature Reserve. The vegetation cover is about 100 % with Bromus riparius L., Festuca sulcata Hack.,

2.4. Sample preparation prior thermal analysis Air-dry samples of native soils and separated aggregates were ground in an agate mortar. To ensure sample representativeness, about 100 g of a soil sample was ground in a Pulverisette 7 ball mill (Fritsch). After grinding, all the samples were processed through a 100-μm sieve (Retsch). Before measurement, the samples were stored in 5-mL polypropylene tubes (Axygen, USA) in the dark at room temperature. A sample of about 1 g taken with an accuracy of 0.1 mg was placed in a 5mL TGA crucible of Al2O3. The sample mass was measured using an Ohaus Adventure Pro analytical balance. 2.5. Determination of total organic carbon in bulk soils and size fractions The content of organic carbon in the bulk soil and in size fractions was determined by catalytic combustion using a Vario Macro Cube 3

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Fig. 1. Location of sampling points. A, the positions of natural steppe (red marker, V.V. Alekhin Tsentralno-Chernozemny Nature Reserve) and agrogenic soils (yellow marker, Kursk Research Institute of Agricultural Production); B, points of selection of agrogenically transformed soils: markers: B, shelterbelt; C, no tillage; D, overgrown fallow; E, cropland; and F, bare fallow.

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instrument (Elementar, Germany) at a temperature of 960 °C; a weighed portion, 20 mg.

the sample residue, and high temperature were avoided in order to not to melt the residue to the crucible bottom. Next, the crucible was cooled down in a helium flow, after which the sample residue was completely removed, and the next analysis was performed in the same crucible. The total analysis time per sample, involving all stages, was 22 h.

2.6. Thermogravimetric instrumentation Thermogravimetric (TGA) measurements were carried out on a STA 449 F5 Jupiter synchronous thermal analyzer (NETZSCH-Gerätebau GmbH) with a SiC furnace for the operation in the temperature range from room temperature to 1650 °C in a TGA mode (a TG Pin S thermogravimetric sensor for temperatures from room temperature to 1650 °C and for large samples). The analyzer was controlled using NETZSCH Measurement software (version 6.1.0). The equipment was pre-calibrated using a standard set of metals with a purity of 99.99 % (NETZSCH-Gerätebau GmbH) covering a temperature range of 80–1150 °C (indium, aluminum, tin, zinc, and gold) in an atmosphere of argon of a grade of 5.0 at heating rates 1, 5, and 10 K/min. For operation in an inert atmosphere (as both the purge and protective gases prevent pyrolysis products from entering the balance). Helium of a grade of 6.0 (99.9999% He) from Helium Plant of Gazprom Dobycha Orenburg (Russia) was used. For operation in an oxidizing atmosphere, oxygen of 99.9 % O2 was used (Tsentrogaz, Moscow, Russia). The oxidizing atmosphere was used to purify the thermal analyzer from condensed pyrolysis products between runs.

2.8. Evolved gas analysis by mass spectrometry Mass spectrometry measurements of the formed gases were performed using a QMS Aeolos 403 quadrupole mass spectrometer (NETZSCH-Gerätebau GmbH), which was connected to the STA 449 F5 Jupiter unit through a gas-transfer line, consisting of an interface and a capillary (quartz glass, 75 μm i.d, ca. 2 m long) thermostatically controlled at 200 °C. The measurements were carried out in an atmosphere of helium of a grade of 6.0; ionization was carried out in an electronimpact mode with an ionization energy of 70 keV. Detection was performed in a bar-graph mode (m/z range, 1–200; accumulation per an m/z channel, 0.5 s; resolution, 50; a CH-TRON detector; SEM voltage, 2000 V, amplifier mode, AUTO-D; range, L E-09; offset ON); the total measurement time per cycle was ca. 100 s. 2.9. Data processing The obtained TG curves were processed in NETZSCH Proteus–Thermal analysis software (version 6.1.0). The mass loss was calculated in the range of 30–170, 170–380, 380–600, and 600–1000 °C, the final mass and TG–T50 index (hereinafter, T50) in the range 170–600 °C were also determined. Further data processing was performed using MS Excel 2010 software. When constructing DTG plots, the original TG curves were smoothed using 12 points in the NETZSCH Proteus software (the “D” software option of the smoothing operation). Statistical analysis was performed using MS Excel 2010 and Statistica 12 (StatSoft) using the methods of non-parametric statistics by Wilcoxon, Mann-Whitney, and Box & Whisker plot statistics.

2.7. Thermogravimetric measurements All soil samples were measured in an Al2O3 crucible (99.7 %, 5 mL, cat. No. NGB809163). The samples in the crucible were not compacted, the sample layer was only flattened by gently tapping the crucible. The main parameters of TG measurements are given in Table 1. All the experiments were carried out in the same crucible without additional purification. Each measurement consisted of two stages: pre-purging the system at higher gas flowrates to remove residual atmospheric gases in helium and the main stage. After the main stage, the formed pyrolysis product deposits were annealed in an oxygen flow without removing

3. Results and discussion

Table 1 Parameters of thermogravimetric measurements of soil and fraction samples. Parameter Preliminary purging Temperature, °C Time, min Purge gas flowrate through the furnace Protection gas flowrate (through the balance and then through the furnace) Main measurement Starting temperature, °C End temperature, °C Heating rate, K/min Frequency of data acquisition, points/K Purge gas flowrate through the furnace Protection gas flowrate (through the balance and then through the furnace) Total gas flowrate System purging Starting temperature, °C Time, min Purge gas flowrate through the furnace Protection gas flowrate (through the balance and then through the furnace) Total gas flowrate System cooling Starting temperature, °C End temperature, °C Cooling rate, K/min Purge gas flowrate through the furnace Protection gas flowrate (through the balance and then through the furnace) Total gas flowrate

The nature of soil use is an important factor that controls the SOM content, the structure (Helfrich et al., 2006) affects the quantity, quality of leaf cover, its decomposition rate, and the stabilization processes of the SOM in general (Kogut et al., 2012; Six et al., 2002, 1999). Thus, we selected soils with significantly different history and intensity of use within the whole long-term field experiment. The selection of fraction sizes is based on previous findings on the change in the aggregate composition of chernozem depending on land use (Kholodov et al., 2019) for the same samples. As was previously shown, among main components, with the obvious nonuniform distribution of aggregates of different sizes, are aggregates of 1–2 mm (prevailing in all soils and containing the main carbon stock) (Kogut et al., 2012) and smaller, with a pronounced accumulation particles with a size of < 0.25 mm. Aggregates of 7–10 and 0.5–1 mm belong to the in-between variants, which are the most variable and therefore make a significant contribution to the differences in chernozem soils.

Value 30 10 100 mL/min He 50 m L/min He 30 1000 1.0 100 50 m L/min He 20 m L/min He 70 mL/min 1000 2 200 mL/min O2 20 m L/min He

3.1. Representative sample mass selection Heat losses for all studied soil samples had the form shown in Fig. 2. This is a fairly typical thermogravimetric curve for soil (Kučerík et al., 2018; Siewert, 2004; Siewert and Kučerík, 2015) with no temperature intervals with mass losses close to zero. This is not surprising as soils are extremely multiphase and multicomponent systems. This should be considered from the viewpoint of the representativeness of thermal analysis as for any kind of physicochemical analysis. In fact, SOM content in soils is at a level of several percent. In the studied samples, the carbon content ranges from 5.4 to 6.0% in bulk soils to 2.6–4.5 % in fractions (Table 2).

220 m L/min 1000 ∼ 25 50 20 m L/min He 20 m L/min He 40 mL/min

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Fig. 2. TG curves and their corresponding DTG curves for 6 samples (1.00 g each) of a shelterbelt sample.

In the literature on TGA of soils, unfairly little attention is paid to representativeness of soil sample. In the case of bulk soils, the representativeness of the sample is provided (in addition to the correct sampling of a general sample and averaging and sampling of an analytical sample) by a sufficient mass and a controlled degree of grinding of the directly analyzed sample, as specified in ISO 11464:2006 Soil quality – Pretreatment of samples for physicochemical analysis (Standardization, 2008) and ISO 23909:2008 Soil quality – Preparation of laboratory samples from large samples (Standardization, 2006). Thus, Leinweber et al. (1992) used a 300 mg sample; recently, soil masses of 15–50 mg (Duguy and Rovira, 2010; Nie et al., 2018) or as high as 0.5–1 g (Siewert et al., 2012) have been used. In the case of preliminarily isolated fractions of humic substances, masses are from ca. 15 mg (Boguta et al., 2017) to 50–100 mg of isolated humic substances (Grisi et al., 1998; Kucerík et al., 2004) This is partly due to the fact that DTG and, especially, DSC are often used along with TG, and it is technically impossible to use masses of more than few tens of mg in these methods. In the case of pure substances or two- or three-component mixtures, as well as isolated mineral fractions, this is not a problem. Humic substances that are extracted from soil samples in tens of grams are also representative. However, in the case of whole soils, the evaluation of the representative mass is a separate task. Here, we investigated the reproducibility of heat loss for different masses of soil samples. For this, a 100 g portion of a shelterbelt sample was ground in an agate ball mill and processed through a 100-μm sieve. Each sample, weighing from 50 mg to 2.50 g, was measured 6 times in an inert He atmosphere in one large crucible with a volume of 5 ml. From the data, we obtained (i) the standard deviation of the mass loss over the entire temperature range 32–1000 °C and (ii) standard deviations of the mass loss and T50 for the most informative section for SOM — as will be discussed below—170–600 °C. For total losses (32–1000 °C), Fig. 3 (A), the difference is significantly less noticeable, because total losses, including water loss as a result of dehydration at low temperatures as well as dehydration and

decomposition of minerals at high temperatures, are higher, while the spread of values due to organic-matter heterogeneity is levelled against such a high background. However, samples of 2.5 g show significantly lower losses than samples of 0.25–1 g. Fig. 3 (B) shows that for the temperature range 170–600 °C, the scatter of mass loss is largest for 50-mg samples, decreases with an increase in weight, and stabilizes at 1–2.5 g. For T50 in the range of 170–600 °C, 2.5-g and 0.05-g samples are different, Fig. 3 (C). This behavior can be due to fact that the nature of processes inside the sample layer depends on the sample mass (Wendlandt, 1986). Besides, an increase in the sample depth in the crucible leads to a temperature gradient within it, and to reduce it, the heating rate should be slowed down. Thus, based on all these factors, we selected a sample mass of 1.00 g as representatively reflecting SOM when grinding samples to a particle size of less than 100 μm. Fig. 2 shows that for a sample of 1.00 g at a heating rate of 1 K/min, TG and DTG curves are quite similar; hence, these conditions are used in all subsequent experiments. Considering the metrological parameters of the mass loss and T50 values, we limited the experiments to a single measurement for each sample, as each run—heating, annealing residues in oxygen, and cooling the furnace—took about a day. Nevertheless, we believe that this demonstrates the need for accurate metrology of thermal-analysis data. In general, we recommend similar methodological measurements for any TGA systems, samples, and sample-preparation protocols. The above conditions could be used as indicative. Still, the data obtained even for slightly different conditions should be compared with great care. 3.2. DTG curve interpretation Fig. 2 shows a typical view of the first derivative of TG (DTG) for a shelterbelt sampel as an example. When considering TG and DTG curves together, the following ranges can be distinguished: (I) from the heating start to 170 °C; (II) 170–600 °C; and (III) 600–1000 °C. To

Table 2 Organic carbon contents in bulk soils and fractions. Sample

Bulk soil 10–7 mm 2–1 mm 1–0.5 mm < 0.25 mm

Organic carbon contents, % wt. Steppe

Overgrown fallow

Fallow

5.79 ± 0.28 5.4 ± 0.4 5.79 ± 0.28 6.01 ± 0.31 5.56 ± 0.11

3.95 3.68 3.95 3.83 3.94

2.79 2.85 2.79 2.84 2.65

± ± ± ± ±

0.35 0.26 0.35 0.17 0.49

6

± ± ± ± ±

0.09 0.01 0.09 0.06 0.03

Cropland

Shelterbelt

No tillage

3.55 3.47 3.44 3.51 3.45

4.45 3.23 3.39 3.36 3.18

3.46 3.51 3.55 3.51 3.42

± ± ± ± ±

0.06 0.03 0.13 0.05 0.07

± ± ± ± ±

0.34 0.08 0.03 0.06 0.05

± ± ± ± ±

0.15 0.05 0.06 0.07 0.07

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Fig. 3. Comparison of (A) the total mass loss in the range 32–1000 °C, (B) mass loss in the characteristic range of organic-matter pyrolysis range 170–600 °C, and (C) T50 in the range 170–600 °C. All data obtained for the bulk soil masses from 0.05 to 2.5 g in a He atmosphere at a heating rate of 1 K/min. Confidence intervals calculated based on the hypothesis of a normal distribution, P = 0.95, n = 6.

interpret the processes occurring in these temperature ranges, we used the analysis of evolved gases by quadrupole mass spectrometry by detecting ions with m/z in the range from 2 to 300. Here, we mainly focus on ions with m/z of 18 and 44, which correspond to water and carbon dioxide as main contributors to the total ion current and, therefore, are the main products of the pyrolysis of organic matter (Boguta et al., 2017) as well as the dehydration processes of clay minerals (Fordham and Smalley, 1983). At high temperatures (over 600 °C), a surplus release of relatively large amounts of CO was observed, apart from CO2. All other gas-phase components are at trace levels compared to these components and, although of considerable interest, such a discussion is beyond the scope of this paper. Fig. 4 shows TG and DTG curves along with the corresponding water emission curve (by detection of m/z 18 attributed to H2O); numbers 1 through 5 indicate water-release maxima. A comparison of these maxima with the DTG curve shows that the mass loss in 30–170 °C is due to water release (emission maxima 1 and 2), and the maximum of the DTG curve coincides with the maximum 1 of the water release curve. The maximum 2 corresponds to the DTG shoulder curve. This is water with a different binding energy, physisorbed and released from crystalline hydrates (Kučerík et al., 2013, 2018; Wang et al., 2011).

Losses in 110–120 °C are closely related to the clay content in soils (Siewert, 2004). To separate these types of water, we conducted an experiment when a sample was first heated to 100 °C, held for 1 h, and cooled to 30 °C in a helium flow, and then the main heating program was launched. Upon re-heating, a peak with a maximum of 130 °C appeared on the DTG curve (Fig. 5). Peaks 3–5 in Fig. 4 are in the range II, 170–600 °C. They correlate well with two maxima on the DTG curve. Fig. 6 shows the CO2 the emission curve. One can clearly see that the main fraction of the emitted CO2 gas falls within this range, with the maxima at the second derivative coinciding with the maxima of the DTG curve. 3.3. Temperature range justification Based on the mass-loss ranges and emission ranges of the major product gases, we selected temperature ranges for calculating the thermal stability indices of SOM. In the literature, the choice of specific boundaries varies. e.g., Oudghiri et al. (2016) identified four ranges: 100–200, 200–400, 400–600, and 600–900 °C. Another study selects three ranges, 40–220, 220–430, and 430–650 °C (Boguta et al., 2017). Guo et al. (2016) divided SOM into three categories based on thermal

Fig. 4. TG curve (black), DTG curve (red), and a water-emission curve (blue) at the MS detector, m/z 18 and its second derivative (purple). Numbers are peaks of water emission. 7

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Fig. 5. TG curve of an air-dry sample (blue dashed), its DTG curve (blue), TG curve of an absolutely dry sample obtained by heating to 100 °C, keeping for 1 h and cooling to 30 °C directly in the thermal analyzer (red dashed), and its DTG curve (red) and at the MS detector, m/z 18 (bright blue).

oxidation: labile (200–380 °C), recalcitrant (380–475 °C), and refractory organic matter including black carbon (475–650 °C). Much attention has been paid to this by other authors (Kučerík et al., 2018; Siewert and Kučerík, 2015), who proposed dividing a TG curve into 10 °C intervals and building an autocorrelation function. We believe that it is more logical to divide a TG curve under specific experimental conditions into ranges based on the character of the mass loss revealed by the DTG curve. In fact, even identical samples would produce different TG curves if experimental conditions are not the same. The heating rate, gas flow and composition, crucible shape and material, and many other factors would influence the curve shape, and this is a common feature of thermal-analysis methods. When we compare the thermal behavior of soils in either an inert or an oxidizing atmosphere, we have anyway divided TG curves into four ranges, as shown by Fig. 7: (1) 32–170 (150) °C; (2) (150) 170–380 °C; (3) 380–600 °C; and (4) 600–1000 °C. The selected boundaries of the ranges are based on the minimum rate of change of mass (i.e. local minima on the DTG curve). As we focus on SOM, the ranges (2) and (3) are most interesting from this viewpoint. We underline that the range boundaries should not be considered constant as they are conditional on the one hand and derived from the actual data on the other. Comparing the curves in the oxidizing and inert atmospheres shows that the oxidation

starts at 150 °C and pyrolysis, at 170 °C. However, we compare the mass-loss ratios in these ranges and, thus, use the ranges of 170–380 and 380–600 °C in all these cases. In the range of 150–170 °C, the mass loss for different samples in an oxidizing atmosphere is almost identical, i.e., such a simplification does not affect the resulting trends. Thus, in the range of 170–600 °C, predominant is the pyrolysis of organic matter. Outside this range, major processes are water emission and decomposition of mineral components (Fernández et al., 2011; Kučerík et al., 2018). As two maxima rate of mass loss have the same shape and position for all the selected samples (Fig. 7), it is logical to assume that the pyrolysis of significantly different organic substances occurs in these ranges. However, each of these alleged groups of substances is a complex mixture, and it is impossible to draw a chemically justified boundary between these ranges. Therefore, we selected temperature ranges based on the DTG curves of bulk soils, which are also given in Fig. 7. The range of 170–600 °C is divided into two subranges at 380 °C, this temperature is set as this temperature is at the end of DTG plateau for steppe and shelterbelt samples, and it shows the minimum rate of mass change for the rest of samples. Nevertheless, it should be emphasized that this 380 °C boundary is quite conditional and, most likely, will be different for other conditions. However, we consider such a temperature-range separation principle versatile.

Fig. 6. TG curve (black), DTG curve (red), a CO2 emission curve (green) at the MS detector, m/z 44, and its second derivative (purple). 8

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Fig. 7. TG curves (solid lines) and the corresponding DTG curves (dashed lines) of the studied solid soils: steppe (blue), shelterbelt (black), no-till (purple), overgrown fallow (brown), cropland (purple), and bare fallow (red).

Fig. 9. Cluster analysis of COM weight loss for all variants of samples for two ranges of temperature up to 380C and 380–600 C. Cluster 1- soils of no-till, overgrown fallow, cropland, and bare fallow. Cluster 2- soils of steppe and shelterbelt.

is soil not cultivated, steppe and forest belt. They are characterized by high losses in the low-temperature range both for bulk soils and for aggregates of all sizes. At the same time, the mass loss of organic matter in both groups of soils turns out to be close in the high temperature range. In this case, the thermal stability of the whole SOM differs sharply from the matter of individual aggregate fractions. Among aggregates, SOM is distinguished by the largest and smallest fraction (Fig. 8). Fig. 9 demonstrates that the largest differences in SOM mass loss between the SOM of fractions of two different groups of soil, cultivated and not-cultivated for a long time, are related to the low-temperature range. Besides, the entire set of the obtained mass losses was analyzed by principal components analysis (PCA) for two components (Fig. 10). Two principal components were the type of agrotechnical impact and the size of the aggregates, including non-scattered material. By the first component (Fig. 10, A), depending on the agrotechnical impact, the data are combined as follows: the points above the abscissa axis correspond to the bulk soils and the fractions less than 0.25 mm (outlined with blue circles), and other fractions are below the axis. At the same time, selected temperature ranges of 170–380 and 380–600 °C are clearly separated by the ordinate axis (quadrants I/IV and II/III, respectively). The groups are also divided by the abscissa. Quadrants I and II contain samples of bulk soils and aggregates of less than

Fig. 8. Tree diagram (multi: for 6 variables−COM of the variants of different landuse; for 5 variables−COM of the different size aggregates.

3.4. Thermogravimetry data interpretation Multivariate cluster analysis made it possible to separate soil samples and their fractions by the mass loss into two groups. The first is characterized by relatively small losses in the temperature range up to 380 °C and combines the cultivated soil. It is noteworthy that within this group, the soil of bare fallow stands out sharply. The second group 9

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Fig. 10. Separation of the pool of mass loss data of the analyzed soil samples by principal component analysis (two components). Red dots correspond to data at 600 °C; blue dots, to 380 °C. А, depending on type of the land use; В, depending on the size of aggregates.

0.25 mm, while III and IV, larger aggregates. By the second PCA component (Fig. 10, B), a group of the largest aggregates of 7–10 mm in size and the smallest below 0.25 mm (orange circles) are clearly distinguished. Thus, thermal stability of SOM of these aggregates is similar throughout the temperature range under question. The relationship between these components seems possible: the smallest aggregates can be decomposition products of the largest ones, which is quite inherent in actively tilled chernozems (Shein and Milanovskiy, 2014). SOM for this PCA component for bulk soil is attributed to aggregates of 1–2 mm (blue circles). Aggregates of 0.5–1 mm make an intermediate position. Probably, they are a mixture of incompletely disaggregated large particles or newly formed particles aggregating to the size of 1–2 mm, which does not contradict the above conclusion.

The low-temperature range (LT, 170–380 °C) is associated with the pyrolysis or thermal oxidation of carbohydrates and other aliphatic compounds as simple lipids and amino acids, that is, with the most easily decomposable fraction of organic matter, as well as the beginning of the decomposition of lignin fragments, primarily aliphatic side chains and depolymerization of lignin (Brebu and Vasile, 2010; Kawamoto, 2017). The high-temperature range (HT, 380–600 °C) is interpreted as the pyrolysis of aromatic compounds or polyphenols corresponding to a stable fraction of organic matter (Dell’Abate et al., 2000; Dell’Abate et al., 2002; Plante et al., 2009) and the secondary reactions: pyrolysis of lignin, the release of aromatic compounds, and the formation of polyaromatic compounds (Kawamoto, 2017). According to Dembicki (1992), at a sufficiently high content of organic matter in the composition of soil organomineral material and the 10

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Fig. 11. Change in total mass loss normalized to carbon content in the range of (A) 170–380 °C; (B) 380–600 °C; and (C) 170–600 °C with an increase in the degree of agroecological impact from the minimum (intact steppe) to the maximum (long-term bare fallow).

selected size fractions of the aggregates, one can neglect the effect of possible differences in the composition of clay minerals—mainly smectites (Alekseev, 2017)—of agrogenically transformed and native soils on the pyrolysis process in this high-temperature range. Under the conditions above, the mass losses of the aggregates and bulk soil of all land use types and their fractions by dry sieving were determined. For a correct comparison, the results were normalized to the total carbon content (Table 2) and histograms were constructed by grouping the mass loss depending on the type of land use, Fig. 11, or on the particle size in fractions, Fig. 12. Statistical analysis was performed using nonparametric statistics (Friedman ANOVA and Kendall’s W) and Box & Whisker plots were built (Fig. 13) and compared in pairs by the Wilcoxon test. On these histograms, the main contribution to the confidence interval is the error in the determination of total organic carbon, this explains the difference in the confidence intervals for even similar values of mass loss. For example, for the overgrown fallow (Fig. 11), confidence intervals for both bulk soil and fractions are significantly larger than for other soil types, as the errors in determining carbon in this land-use, type are relatively large. This is also due to the representativeness of the sample, as carbon analysis by high-temperature catalytic combustion often requires sample masses of tens of

milligrams (on average, up to 200–250 mg) resulting from the technical requirements of carbon analyzers. The presence of post-mortem residues in the soil—small plant roots, microscopic leaf residues, etc., which are not SOM in the strict sense—may introduce a significant error in the analysis results. In absolute values, aggregates of all sizes, as well as the bulk soil, lose most of their mass in the range of 170–380 °C (Fig. 11, A), namely, 65–73 % of the total weight loss in the range of 170–600 °C (Fig. 11, C). In the range of 170–380 °C, largest SOM losses are found for a set of shelterbelt soil aggregates, which differs considerably from all other land use types, which do not significantly differ from each other according to the Wilcoxon test (Table 3, LH). At the same time, the graph shows a trend in the thermal stability of organic matter in the row: overgrown fallow > cropland > bare fallow > > shelterbelt; with equal stability of steppe, no tillage, and cropland. In the range of 380–600 °C (Fig. 11, B), this trend manifests itself much stronger as there remains much less nonspecific SOM. The smallest SOM losses are for steppe, the largest, for the shelterbelt and bare fallow; these mass loss values do not differ statistically (Table 3, HT). The remaining types (overgrown fallow, cropland, and no tillage) occupy a middle position and do not statistically differ from each other. 11

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Fig. 12. Change in total mass loss, normalized to carbon content in the range of (A) 170–380 °C; (B) 380–600 °C; and (C) 170–600 °C depending on the fraction after dry sieving.

For thermal stability, it forms a row: steppe > no-till = overgrown fallow > cropland > bare fallow = shelterbelt; the differences are statistically significant. Shelterbelt differs from other soil types by the type of plant residues that serve as a source of organic matter: the leaf cover of woody plants prevails. In other land use types, the vegetation is predominantly cereal or grass-grass. It is mentioned that coniferous, deciduous woody, and herbaceous plants easily differ in the composition and ratio of basic structural phenolic groups (Hedges and Mann, 1979; Kovalev and Kovaleva, 2011; Kovaleva and Kovalev, 2009; Thevenot et al., 2010). According to Brebu and Vasile (2010), these indicators determine the rate of decomposition of different plant lignins, which means their different thermal stability. Predominantly vanillin and sinergil lignins, characteristic of deciduous oak, beech, and maple forests, begin to decompose at relatively high temperatures. The underground, and not above-ground parts of plants contribute to the remnants of the plant roots. And in this case, the steppe type stands out as most sustainably formed over the centuries of lack of ploughing, the shelterbelt as a system with a predominance of deciduous tree leaf cover and a minimum fraction of

dying underground part (the latter to some extent combines this land use type with bare fallow. And the three other types can be united as intermediate, with the beginning processes of restoring the mortal grassy biomass of the root system (overgrown fallow and no-till) and annual ploughing (cropland). A higher resistance to thermal decomposition of lignins in grass plant straw compared to woody plants was also shown (Watkins et al., 2015). They show that lignins isolated from wheat straw and flax fiber lose almost 10 % less mass than pine straw lignin in the temperature range from 30 to 800 °C, and the main mass loss occurred at temperatures of 320–340 °C. The last type in the group (cropland) is least stable, Fig. 11, although the Wilcoxon test does not confirm the significance of the differences (Table 3, HT part). Let us consider the absolute mass loss of aggregates within each land use type in two temperature ranges. In 170–380 °C, the greatest loss (by 20–30 % more than other types) both in bulk and aggregates is shelterbelt, which is associated with the readily decomposable leaf cover material as a SOM source, unlike all others, with the predominance of cereal stubbles (no-till and cropland), especially against motley grass (steppe and overgrown fallow). In land use types with the minimal agrotechnical impact (steppe, shelterbelt, and overgrown fallow), the 12

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till < cropland < overgrown fallow = bare fallow < shelterbelt; for other sizes: steppe < overgrown fallow < no-till < cropland < shelterbelt < bare fallow (Fig. 12, C). Based on the mass loss, we calculated temperatures at which half of the substance pyrolyzed in the range of 170–600 °C (T50). The results are shown by increasing the agroecological impact from the minimum (intact steppe) to the maximum (long-term bare fallow) (Fig. 14, A) and, depending on the fraction, after dry sieving (Fig. 14, B). In Fig. 14, confidence intervals are calculated from the errors in T50 at the stage of metrological evaluation of the results (Fig. 3, C) and are ± 0.8–0.9 °C. Fig. 14, A shows that T50 gradually and significantly, almost by 40 °C, increases from steppe to bare fallow. From a chemical point of view, this can be interpreted as an increase in the number of high-energy molecular bonds, breaking of which requires high energies. These results are in good agreement with those by Nie et al. (2018), where fractions of an eroded soil also had a higher T50 (Fig. 15). As an indicator of thermal stability for aggregates of different sizes, T50 mainly showed significant differences between the value for the bulk soil (which does not differ significantly from the aggregates of 1–2 mm) and the other three sizes (Fig. 14, B). At the same time, the aggregates visually form the row: 1–2 mm > 0.25 mm > 0.5–1 mm > 7–10 mm. The correspondence of the thermal stability of T50 of soils and aggregates of 1–2 mm (Fig. 14) confirms the conclusions by Kogut et al. (2012) on the role of this aggregate fraction in the formation of the water-resistant structure of chernozem and, probably, the predominant content of lignin components in it. In this work, we also calculated a parameter called the two-range thermal stability ratio (the term does not claim to be versatile). This index is the ratio of mass losses in the high-temperature (380–600 °C) to the low-temperature range (170–380 °C). The convenience of this relative index is that, like T50, it does not require the knowledge on the total carbon content to correctly compare different samples, as the carbon content is corrected by the ratio form. However, T50 index is the temperature at which half of the substance decomposes in the considered range and, calculation-wise, it does depend on the whole SOM decomposition range boundaries only. The proposed two-range thermal stability ratio is a somewhat subtler tool: it depends on subranges that we divide the whole decomposition range. Strictly speaking, the number of these subranges can be more than two, but these two subranges is readily justified by chemistry. The chemical meaning of this index is the ratio of more thermostable components (aromatic and strongly cross-linked aliphatic) to less stable (aliphatic). A direct evidence of this requires fairly laborious research. However, if we browse the data on the thermal stability of pure organic compounds (Chow et al., 2007; Johns et al., 1962), it is clear that most aromatic compounds decompose at temperatures above 400 °C, while carboxylic acids, their esters and some carbohydrates, in the range of 200–400 °C. At the same time, fully aromatic and heterocyclic

Fig. 13. Results of statistical analysis (nonparametric Friedman ANOVA and Kendall’s W) for total mass loss normalized to carbon content, with different use types in the range of (A) 170–380 °C and (B) 380–600 °C.

greatest mass loss occurs in aggregates below 0.25 mm, whereas in cases of active land use (cropland and bare fallow), in larger fractions (Fig. 12, A). In 380–600 °C, there is a minimum loss in the mass of the bulk soil and aggregates for steppe soil, and the maximum is in aggregates of the shelterbelt and bare fallow (Fig. 12, B). It is noteworthy that in all soil types, the smallest mass loss is for a fraction of 1–2 mm. Regardless of mass loss distribution between the aggregates, the mass loss of aggregates of 0.25 mm increases in the row: steppe < no-

Table 3 Pair-to-pair differences in sample mass loss in two temperature ranges normalized to SOM contents; red show no difference and green are significant difference according to the Wilcoxon test.

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Fig. 14. Temperature change of T50 half-decomposition in 170–600 °C with (A) increasing agroeconomic impact from zero (intact steppe) to the maximum (long-term bare fallow) and (B) depending on the fraction after dry sieving.

compounds with condensed aromatic systems have the highest thermal stability. A loss of aromaticity leads to a decrease in thermal stability (Korshak et al., 1976). As far as we are aware, this index was not previously used in the field of thermal analysis of soils; we propose it as a criterion for the degradation of SOM along with T50. Certainly, like many similar indicators, a correct comparison using two-range thermal stability ratio is possible only if the conditions for recording TG curves and the sample preparation are identical. Comparison of the results based on two-range thermal stability ratio (Fig. 16) shows that steppe has the highest index value. Thus, the lower is this index, the more depleted are thermally unstable components in SOM. Fig. 16, A clearly demonstrates the decrease in losses of thermally unstable components, apparently, readily decomposed aliphatic species, when shifting from samples of intact steppe and shelterbelt in which such components prevail, to bare fallow, where unstable components are mostly depleted. Fig. 16, B shows that this pattern is true not only for the bulk soil, but also within each analyzed size fractions of aggregates. Statistical analysis of SOM distribution of soils and fractions according to the proposed two-range thermal stability ratio (Fig. 17) shows that this index value is maximum and almost the same for SOM

Fig. 15. Statistical analysis (nonparametric Friedman ANOVA and Kendall’s W) of T50 half-decomposition temperature in 170–600 °C for different size fractions.

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Fig. 16. Change in two-range thermal stability ratio in the range of 170–600 °C (A) with increasing degree of agroeconomic impact from zero (intact steppe) to maximum (long-term bare fallow) and (B) depending on the fraction after dry sieving.

for bulk soils and aggregates of 1–2 mm in size. This means that during the formation and existence of soil aggregates, particles of this size are most active because they concentrate the maximum amount of organic matter available to microorganisms and/or metabolic products of soil microbiota, which is confirmed by several studies (Kogut et al., 2012; Milanovskiy and Vasilyeva, 2014). For other size fractions, the tworange thermal stability ratio decreases with aggregate size. 4. Conclusions We investigated the thermal properties of chernozem samples of a continuous 55-year-long field experiment with a reliably documented history of land use and showed that long-term agricultural use significantly changes thermal stability parameters of SOM, which manifests itself in aggregates of various sizes. 1 It is shown that the SOM of the formed deciduous tree shelterbelt soil and steppe soil formed over the centuries of lack of ploughing under predominant grassy vegetation are the least thermally stable over the whole decomposition temperature range and contain the

Fig. 17. Statistical analysis (nonparametric Friedman ANOVA and Kendall’s W) of two-range thermal stability ratio in 170–600 °C for different size fractions.

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maximum amount of thermally unstable compounds. Among the aggregates of these soils, the aggregates with a size of 1–2 mm are the most thermally stable, the least stable are 7–10 mm in size and less than 0.25 mm, which is confirmed by the whole set of applied statistical criteria. 2 Moreover, all its aggregates contain predominant amounts of thermally unstable SOM, the most thermally stable are aggregates of 1–2 mm. 3 SOM recovery processes that occur in soil of the overgrown fallow and no-tillage farming if compared with the native soils (bare fallow and cropland, respectively) are not the same. Predominant SOM accumulation occurs in aggregates of 0.5–1 and 1–2 mm.

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Our results once again demonstrate that thermogravimetry serves a reliable tool for monitoring soil degradation processes. However, methodological aspects of such studies are still not developed in full. This study shows the importance of using sufficiently large soil weighed portions (at least 1 g) to provide representative SOM samples. In addition, the choice of temperature ranges in which SOM is assumed to be thermally stable and unstable is quite conditional and depends on experimental conditions. Nevertheless, we believe that the principle of choice of subrange boundaries should be based on the data of a thermal experiment itself, the features of TG or DTG curves. Thus, the subjective part in the decision is reduced. With systematic observation of agricultural land, a statistically significant increase in T50 in the range of 170–600 °C (or a similar range determined according to principles similar to those described in this work), as well as a decrease in two-range thermal stability ratio introduced in this work, the same range may indicate the manifestation of SOM degradation processes and the need for measures to compensate for a decrease in thermally unstable SOM components. At the same time, state-of-the-art thermogravimetry is a highly automated method of analysis, which almost does not require consumables and operator time, and thus can be used in small laboratories. Finally, we would like to highlight that thermal stability has gained considerable experimental and theoretical experience in many branches of chemistry, and it is most important in the field of polymer chemistry. Thermal stability of polymers is thought to be influenced by many factors, such as functional groups, molecular weight and size, molecular weight distribution, degree of branching, cross-linking, crystallinity, and amorphousness. However, the main cause of polymer thermal degradation is bond scission related to their energies (Ray and Cooney, 2018). Keeping soils in mind, the pioneering work by Tyan et al. (1999) should be mentioned, who showed that the addition of clays dramatically increases the thermal stability of polymers. It is expedient to assume that soil clay minerals may play a similar role with respect to SOM and this is subject of quite relevant further research. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was done as a part of the collaboration project with Skolkovo Institute of Science and Technology (Digital Agriculture Lab, CDISE). The authors acknowledge the financial support of The Ministry of Science and Higher Education of the Russian Federation, budget project 0591-2019-0024.

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