Saline fertility of fallow land massifs in the steppe zone of the Omsk Priirtyshye region

. Information on the properties of unused solonetz on deposits is necessary for studying the processes of their evolution, solving the issues of rational use of agricultural lands, and establishing the possibility of their reintroduction into agricultural turnover. The aim of the research was to study the fertility indicators of solonetz (Gleyic Solonetz) in the steppe zone of Omsk Priirtyshye, which are in the post-agrogenic stage of development. The research was carried out in Cherlak district of Omsk region (south of Western Siberia) on three fallow massifs with the age of less than 10 years; 10-20 years and more than 20 years. A field survey of soil cover and soil analyses were carried out. As a research result, it was established that fallow massifs are represented by the meadow-chernozem solonetz and the chernozem-meadow columnar solonetz of different types with low fertility. The unfavourable properties of solonetz were expressed physical solonetization, high density of illuvial horizons, low porosity, and unfavourable composition of exchange-absorbed cations, alkaline and strongly alkaline environment in the middle and lower parts of profiles, which are aggravated by arid conditions of steppe landscapes. Reintroduction of solonetz into agricultural turnover can be allowed only on arrays with predominance of medium and deep solonetz for cultivation of solonetz-resistant crops under moisture-saving technologies, no-tillage reclamation treatments, and chemical reclamation in the future.


Introduction
Solonetz are one of the widespread types of soils.Their area in the world is 120 million ha [1], in Russia -30.8 million ha [2].Solonetz belong to the group of low fertility soils, but they are characterised by great diversity and in different regions have different properties, which should be taken into account when using them.In the Russian Federation, large areas of solonetz are concentrated in the south of Western Siberia.In the Omsk region solonetz and solonetz complexes are spread in forest-steppe and steppe zones on 1.9 million ha, making up more than 10% of all soils.The wide spread of solonetz process is conditioned by natural conditions: poorly dissected relief, low drainability of the territory, heavy granulometric composition of rocks, frequent salinisation, semi-arid and arid climate.In the steppe zone solonetz, their complexes and solonetz soils occupy 26% of the total area of solonetz in Omsk region.
In spite of negative water-physical and physical-chemical properties, solonetz were widely used in arable land and under hay and pasture lands.To improve the properties of these soils in different regions of the world were applied: chemical amelioration with gypsum and phosphogypsum [3][4][5][6], systematic application of manure in combination with tillage [7][8][9], drip irrigation [10].Since the 90s of the twentieth century, a significant part of lands, primarily low fertility, including solonetz, was withdrawn from agricultural turnover [11].It has been shown that a long period of meliorated solonetz in fallow condition led to changes in carbonate and gypsum profiles [12][13][14], while in solonetz with solonetz horizons destroyed by treatments, the meliorative effect persisted for a long time [2].In the present period, the question of expediency of introducing fallow lands into agricultural turnover is topical.At the same time, there is not enough information about the properties of fallow solonetz in the south of Western Siberia, including the Omsk region, to solve this issue.The aim of the research was to study the properties and assess the fertility of solonetz of the steppe zone of the Omsk Priirtyshye at the postagrogenic stage of their development.

Materials and Methods
The object of the study was solonetz (Gleyic Solonetz) of fallow land massifs of different ages that are not currently used in farming.Soil survey was conducted in the summer of 2022 on the territory of land use located in the village of Preobrazhenka, Cherlak district, Omsk region (steppe zone of the south of Western Siberia).According to geomorphological zoning the territory belongs to the Kurumbelskaya depression, which is a part of the West Barabinsk Pliocene-Quaternary denudation-accumulative plain.The relief is flat.Soilforming rocks are Quaternary eluvial-deluvial cover carbonate loams and clays.
The climate is continental, with cold winters and hot summers.The average annual air temperature is +1.2°C, annual precipitation is less than 350 mm.They fall less than 275 mm in summer and less than 80 mm in winter.Humidification of the territory is insufficient in all years.The duration of frost-free period is more than 120 days, vegetation periodmore than 165 days.The snow cover height is less than 25 cm.The depth of soil freezing in winter reaches 124 cm and more.Landscapes of the study area represent drainless closed depressions, dissected by ravine network, with saline lakes, halophytic-different-grasshemlock-hemlock-typchak steppes on solonchaks and meadow-chernozem solonetz.According to agro-soil zoning of the Omsk region, it is included in the southern plainsteppe poorly drained area.
Soils of fallow lands with age up to 10 years, 10-20 years and more than 20 years were studied.On each of them were laid transects and excavations, the structure of soil profiles was studied.Soil samples were taken from genetic horizons for further analysis.Soil density was determined using a metal cutting ring of known volume.
The carbon content of organic matter was determined in soil samples by Tyurin I.V. method, pH of water suspension by potentiometric method, exchange-absorbed cations by displacement of 0.1M CH3COONH4 with final determination of Ca and Mg by trilonometric method, Na -by flame photometry.Granulometric composition was established by pyrophosphate method, soil moisture by thermostat-weight method.Porosity and aeration porosity were determined by calculation method.

Results and discussion
The soil survey of fallow lands indicated the predominance of solonetz.On a plot of young fallow land less than 10 years old, located near a small birch forest, the soil cover was represented by a complex of solonetz of different types with solonetz.Studying the soil profile structure of section 1 allowed us to diagnose the soil as solonetz meadowchernozem silty medium columnar loamy.It was characterised by a combination of horizons Ad (turf) -A1 (suprasolontic, humus-eluvial) -B1 (illuvial-solontic), B2-B3ca-B4ca (illuvial) -Cca (soil-forming rock).The thickness of the suprasolontic horizon was 17 cm.It had whitish silicon oxide powder in the lower part, which indicates the development of the silicification process.There were no signs of former ploughing in the structure of the upper horizons, which indicates the restoration of physical solonetisation in the postagrogenic period.The profiles of crustal columnar loam solonetz with A1 horizon thickness -3 cm, as well as meadow fine-drained low-humus loam solonetz with the lower boundary of humus layer at the depth of 12 cm were uncovered with the help of excavations.Carbonates were found at a depth of 54-60 cm.
On the middle deposit with the age of 10-20 years, solonetz of chernozem-meadow silts of different types were found: shallow, medium and deep.Profile structure was typical for solonetz.In contrast to the young deposit, the soils showed signs of ochre staining from 40 to 68 cm.The thickness of A1 horizons in shallow solonetz was 5-6 cm, in medium solonetz -12 cm, in deep solonetz -18 cm.The depth of carbonates varied in a wide range: from 44 to 106 cm.
The array of the old deposit more than 20 years old was characterised by a combination of shallow and medium columnar loamy chernozem-meadow solonetz.The thickness of humus-eluvial horizons varied from 6 cm in shallow to 16-18 cm in medium solonetz.Carbonates were detected at the depth of 40-57 cm, signs of gleying -in horizons B, at the depth of 33-75 cm.Iron neoplasms in horizons B had bright red-ochre colour.
Solonetz horizons B1 in all solonetz had columnar or prismatic structure, below located illuvial horizons -nutty structure.Common for all solonetz was the presence in the lower part of their profiles of layered rocks with heterogeneous granulometric composition, alternation of sandy loam and loamy layers with thickness from 1-2 to 5-7 mm.Probably, these rocks are of lacustrine origin, as the territory is characterised by the presence of saline lakes, in particular, the fallow areas are located near the saline lake Ulzhai.
Thus, it was established that the surveyed fallow lands have a soil cover with a wide distribution of solonetz.The surveyed soils for the illuvial part of the profile showed clear signs of physical solonetisation in the form of their hardness and dense composition.To characterise soil fertility, density, porosity and aeration porosity were determined by genetic horizons (Table 1).
Profiles of solonetz were sharply differentiated by density values.The lowest values of the index were characteristic of A1 horizons (1.02-1.29 g/cm 3 ); they allow us to assess them as loose and weakly compacted.In solonetz horizons the density sharply increased up to 1.48-1.52g/cm 3 .The exception was a solonetz on an old deposit with a low-density value, which could be due to the presence of cracks between columnar aggregates.Also, the effects of former mechanical tillage of the soil probably persisted.Throughout the illuvial soil strata, as well as in the soil-forming rocks, densities were high to very high, which will have unfavourable effects on the root systems of cultivated plants.
Total porosity also differed from horizon to horizon.It had the highest values in humuseluvial horizons A1, the lowest values in illuvial horizons and soil-forming rocks.In upper horizons it was evaluated as satisfactory and excellent, in solonetz horizons B1 and in layers located below -as unsatisfactory.At the same time, soils had high values of aeration porosity even in the upper part of illuvial horizons.However, this was due to low soil moisture, absence of precipitation during the study period, which led to air filling of intercellular pores and channels.Unsatisfactory air content was characteristic only for the lower part of illuvial horizons and soil-forming rocks.
Fertility of solonetz is also determined by their chemical and physicochemical properties (Table 2).The maximum carbon content of organic matter was in humus-eluvial horizons A1.In solonetz horizons and lower along the profiles it significantly decreased.Carbon stocks in the 0-20 cm layer of soils varied within 57.3-66.0t/ha, in the 0-100 cm layer -98.7-113.6 t/ha.Note.CEC -cation exchange capacity.Horizon thickness is given in Table 1.Dash means the indicator was not determined.
The conversion of carbon to soil humus was carried out.The humus content in A1 horizons was 4.10-6.86%,according to the gradation of Orlov D.S. and Grishina L.A. it was estimated as medium and high, lower down the profile it was low.Humus reserves in 0-20 cm layer of solonetz (99-114 t/ha) were low and medium, in 0-100 cm layer (201-312 t/ha) -medium.It should be noted that the humus content in fallow solonetz approached its quantity in zonal chernozems and meadow-chernozem soils.Taking into account the weakening of humus accumulation process in arid steppe conditions, the humus state of the studied solonetz can be considered satisfactory.
Humus content in soils determines their energy potential.Each ton of humus on the area of 1 ha corresponds to the equivalent of 21.654 gJ.Solonetz of fallow massifs were characterised by energy values in the humus composition of 2137-2460 gJ/ha for the 0-20 cm layer and 4359-6760 gJ/ha -for the 0-100 cm layer.The share of energy in the metre column, contained in the upper 0-20 cm layers, ranged from 34 to 49% of its total amount.
The reaction of the medium was distributed unevenly over the horizons of solonetz depending on the influence of soil formation processes.The A1 horizon is characterised by a medium close to neutral.In humus-eluvial horizons of solonetz it is slightly acidic.Even solonetz horizons B1 had pH values less than 7.0.This is due to the destruction of the mineral part of the soil as a result of the salinisation, changes in the composition of exchange-absorbed cations and the appearance of hydrogen ions [15].Soils had relatively small values of cation exchange capacity in A1 horizons (16.0-18.0mmol/100g), which is associated with a small content of particles smaller than 0.01 mm (16.9-28.7%).In solonetz horizons of heavier granulometric composition, but with lower content of organic matter, the value of this indicator increased by 11-31%.
Calcium prevailed in the composition of exchange-absorbed cations in the young deposit of solonetz with the share of 68.8% in horizon B1 and 50-52.9% in horizons B1, B2.The high content of exchangeable magnesium is characteristic: from 24.4% in the upper horizon to 45% in the illuvial solonetz and podsolonetz horizons.The amount of sodium in illuvial horizons was 12.4-13.0% of cation exchange capacity (CEC).This indicates the role of not only sodium, but also magnesium in the formation of solonetzification features of soils.On the 10-20 years old fallow land, the content of exchangeable sodium in the solonetz horizon was only 7.0% of CEC.The largest share of sodium (15.5% CEC) was characterised by shallow solonetz on the fallow land more than 20 years old.That is, all solonetz studied were low-sodium solonetz.

Conclusion
The results of studies conducted on deposits of different ages indicate a low fertility of meadow-chernozem and low-sodium loamy chernozem-meadow solonts of the steppe zone of Omsk Priirtyshye, which are at the postagrogenic stage.They were characterised by profile morphology and properties typical for these soils, expressed physical solonetzification.The studied soils had low and average humus reserves in 0-20 cm layer and average reserves -in 0-100 cm layer, relatively low share of exchangeable sodium (7-15.5% CEC) and increased share of magnesium (24.4-45%).The factors limiting the fertility of solonetz were high density and low porosity of the illuvial horizons, unfavourable composition of exchange-absorbed cations, alkaline and strongly alkaline reactions in the middle and lower parts of the profiles.Medium and deep solonetz can be acceptable for reintroduction into agricultural turnover for cultivation of solonetz-resistant crops using moisture-saving technologies under arid climate conditions, no-tillage ameliorative treatments, and chemical amelioration if economically feasible.

Table 1 .
Physical properties of fallow land solonetz

Table 2 .
Organic matter carbon content (Corg) and physico-chemical properties of fallow land solonetz