Assessment of fallow grey forest soils fertility in sub-taiga zone of Omsk Priirtyshye

. There is a need to obtain information on the current state of their properties when reintroducing fallow soils into arable land. The aim of the research was to study fertility indicators of grey forest soils (Luvic Greyzemic Phaeozems) The research area was located in the city of Tara, Omsk region (south of Western Siberia). A field survey of soils and vegetation cover was carried out in plots with different age of fallow land with the establishment of the value of the above ground mass. The results of the research showed that the fallow area with age of less than 10 years is represented by a thick grey forest soil, with age of 10-20 years - by a medium thick grey forest soil. The value of air-dry aboveground mass of plants varied from 1.01 to 1.01 2.58 t/ha. High density, low porosity, and aeration porosity were found in soils throughout the profile. The humus content and reserves in soils are low and average; its distribution in profiles is markedly decreasing. Reaction of soil medium is acidic ( рН =4.4-4.8). The sum of exchangeable cations in humus horizons is elevated and high. In composition of cations calcium prevails (71.5-74.9 %) with magnesium share - 22.4-26.9 % and insignificant content of sodium (1.6-2.5 %). The surveyed soils can be reintroduced to arable land. However, it is necessary to provide measures to reduce acidity, improve organic matter balance, reduce density, and increase porosity of upper and subsoil horizons.


Introduction
According to scientists' estimates, the area of land in the world, for various reasons withdrawn from agricultural turnover and in fallow land, in the first decade of the twentieth century was more than 2 mln.km 2 [1].In some regions, fallow systems remain a way to effectively manage soil fertility [2,3].After the transfer of arable soils to the fallow regime, complex processes of their self-recovery begin.Numerous studies in different countries of the world testify to changes in soil properties in the postagrogenic period.The species composition of phytocenoses and the organic matter reserves in the soils are restored [4].Several works indicate restoration of the carbon content in soils [5][6][7], improvement of their physicochemical and physical properties [8][9][10][11][12] increase of microbiological and enzymatic activity [13,14].
Grey forest soils are zonal soils of the forest-steppe zone.They are also found in the transitional zone of sub-taiga.Their area in the Omsk region is 996 thousand hectares.
Semi-hydromorphic soils with groundwater table of 3-6 m (64%) prevail, automorphic soils with groundwater table deeper than 6 m are formed on more drained areas.In the sub-taiga zone grey forest soils have higher fertility compared to sod-podzolic soils and are a reserve for expansion of the arable land area, including through the introduction of fallow lands.At present, their reintroduction into arable land is taking place, which causes the need to obtain information on the current state of soil properties.At the same time, there is no data on the properties of grey forest soils of fallow land.The purpose of this research was to study the fertility indicators of grey forest soils of fallow lands of different ages in the conditions of the Omsk Priirtyshye region.

Materials and Methods
The object of the study was the subtaiga zone of grey forest soils (Luvic Greyzemic Phaeozems) sub-taiga zone of the Omsk region, located in the south of western Siberia.The study area was located near the town of Tara on the left bank of the Irtysh River.According to geomorphological zoning, it belongs to the Oshe-Irtysh watershed plain, which has an undulating surface with mane-shaped rises and numerous depressions.Soil-forming rocks are predominantly Quaternary alluvial deposits of the Irtysh terraces and floodplain, overlain by cover loams.
The climate is continental.The average annual air temperature is +0.6°C, annual precipitation is 437 mm.In summer period they fall 325-350, in winter -80-100 mm.The territory belongs to the zone with sufficient moisture.Winter is long, frost-free period is 100-110 days, vegetation period is 155-160 days.The height of snow cover is 35-45 cm.The depth of soil freezing in winter reaches 127-171 cm.According to the agrosoil zoning, the study area is included in the well-drained riverine subarea with a predominance of automorphic and semihydromorphic grey forest soils in the soil cover structure, swamp soils in depressions, light grey forest soils and alkalised forest soils.
The studied plots were located on a gentle slope of a mane type rise and had different ages in the deposit: up to 10 years and 10-20 years.A reconnaissance survey of the vegetation cover and plant biomass was carried out on them from plots with the following dimensions 1m 2 in 4-6 times repetition.Soil sections were laid down, morphological description of soil profiles, and sampling of genetic horizons were carried out.The soil density was determined using a known volume cutting ring.The carbon content of organic matter was determined in soil samples by the method of Tyurin I.V., pH of water and saline (1M KCl) of suspensions by potentiometric method, exchange-absorbed cations by displacement method 0.1M CH3COONH4 finitely defined Ca and Mg trilometric method, Na-method of flame photometry.

Results and discussion
The botanical composition of the vegetation cover of fallow plots was similar to each other.It was represented by the dominance of Poa pratensis, Elytrigia repens, as well as Equisetum arvense, Stellaria holostea, Silenela tifolia, Potentilla anserina, Rumex acetosella, Vicia cracca, Hypericum perforatum, Ranunculus repens.Sonchus arvensis, Myosotis sylvatica, Phalaris arundinacea were rare.At the time of the survey (second decade of June 2022) bluegrass plants were in the phase of beginning to emerge and beginning to spike, the others -in the phases of budding and beginning of flowering.
Air-dry above-ground mass of plants was 1.01-2.58 in the fallow plot up to 10 years of age 1.01-2.58(average -2.08 t/ha).In the 10-20-year-old fallow area, the plant mass was The study of soil profiles showed that their structure is typical for grey forest type.However, there were differences in morphology of soils of different massifs.In the section of 1 plot of the deposit less than 10 years old, the profile was represented by a combination of horizons Ad (turf) -A1 (humus) -A1A2 (humus-eluvial) -A2B (eluvial-illuvial) -B1-B3 (illuvial) -Cca (soil-forming rock).A turf horizon with thickness of 5 cm was formed from the surface.The lower boundary of humus layer (horizons A1 and A1A2) was at the depth of 45 cm.The soil was classified as grey forest heavy loamy.Due to the considerable thickness of the humus layer in the upper part of the profile, the signs of past agrogenic impacts were not visible.
In contrast, the grey forest soil on the 10-20-year-old fallow land showed clear signs of use in arable tillage by the flat boundary of the A1 horizon and its abrupt transition to the B horizon.The turf layer was very thin, only 1 cm.Horizon A1 was a former arable horizon formed by the cultivation and mixing of horizons A1, A1A2 and possibly A2B.Its thickness was 22 cm.The soil was classified as grey forest medium loamy.
The structure of the middle and lower parts of soil profiles of different sites was similar.Horizons B were characterised by coarse lumpy structure, compaction, presence of clay deposits and ferruginous-manganese films on the surface of aggregates.The common features of morphology of soils of different sites were also close depth of carbonates occurrence (121-127 cm) and their form: veins, as well as rounded and oval nodules of 1.3-2.0cm in size.
Important physical properties of soils are density, porosity and aeration porosity, which depend on granulometric composition, structure, moisture content, soil formation features and type of utilisation.A number of researchers note the restoration and improvement of physical properties of soils that have been in fallow condition for a long time [9][10][11][12].The studied soils had high values of this indicator, which indicates their very strong density.In A1 horizons it was 1.41-1.52g/cm 3 , in the lower illuvial horizons increased to 1.52-1.65 g/cm 3 with slightly lower values in soil-forming rocks (Fig. 1, 2).Large values of soil density are associated with their loamy granulometric composition, dense packing of structural aggregates under the action of plant root systems and absence of mechanical influences.Soils of fallow massifs had low porosity.In humus horizons A1 it was 43.7-45.8%, in middle, more compacted illuvial horizons it decreased to values less than 40%.In soilforming rocks porosity again slightly increased.Very low aeration porosity was detected in all soil profiles.At moisture content of the upper horizons (A1, A1A2) of 21-27.9 per cent it was only 5.5-6.2 per cent in section 1 at the deposit less than 10 years old.In section 2 in the 10-20 years old deposit, the upper horizon A1 (former arable) was more aerated, with an aeration density of 12.2%.However, in the middle of the profile the aeration porosity values were very low.The highest porosity was observed in horizons of soil-forming rocks, which is associated with a decrease in their density as well as moisture content.
The most important factor of soil fertility is the amount of organic matter and its qualitative composition.For arable soils of the forest-steppe zone of the region a decrease in humus content was noted [15].The carbon content of organic matter in the upper layer of fallow grey forest soils was small: 1.79-2.71%(Table 1).Its distribution in soil profiles was sharply decreasing, which is characteristic of soils with the development of eluvial processes associated with leaching of soil formation products, including humus compounds.Thus, in the A1A2 horizon of thick grey forest soil the amount of carbon was lower by 3.3 times.The humus content in the A1 layer of the fallow land soil of 10-20 years according to the gradation developed by Orlov D.S. and Grishina L.A., was low (3.08%), soils on fallow land less than 10 years old -average (4.68%).Accordingly, the calculated humus reserves in the 0-20 cm layer of the fallow land soil less than 10 years old are estimated as average (133 t/ha), in the soil of a fallow land plot aged 10-20 years as low (94 t/ha).Soils in the 0-100 cm layer had average humus reserves (204-222 t/ha).
One of the main functions of soil humus is accumulation of chemically bound energy in its composition.We have calculated energy reserves in soils based on the equivalent: 1 t/ha of humus corresponds to 21.654 gJ/ha.The results showed that the energy reserves in the metre layer of soils were 4411-4803 gJ/ha, with 46-60% of energy concentrated in the upper 0-20 cm layer of soils.
No less important in the formation of soil fertility are their physical and chemical properties.The results of their determination are given in Table 2.The reaction of the environment affects the conditions of plant nutrition and soil properties.The results of the conducted research showed that grey forest soils have a pronounced acidity.The upper humus horizon of soils on two fallow plots had the same values of pH of salt suspension (рНKCl), indicating their average degree of acidity.The underlying horizons of grey forest soil up to the depth of 102 cm on the fallow land up to 10 years old were characterised by strongly acidic reaction of the environment.In grey forest soil of older fallow land (10-20 years) horizons B1-B3 had medium and weak acidity.The sum of exchangeable bases in humus horizons of soils was evaluated as elevated and high (14.7-16.7 mmol/100g).There was a regular decrease in its values in horizons B (14.7-16.7 mmol/100g) and soil formation rocks (14.9-18.8mmol/100g).In the composition of exchange-absorbed cations, elevated and high content of calcium and very high content of magnesium were found.They accounted for 71.5-74.9 and 22.4-26.9% of the sum of cations in the humus horizons, respectively.The amount of exchangeable sodium was very small (1.6-2.5 %).The distribution of cations in soil profiles showed a decrease in calcium content with depth.

Conclusion
Thus, the results obtained allow us to assess the fertility of the soils studied and give recommendations on their introduction into arable land.Grey forest soils from fallow lands up to 10 years old and 10-20 years old are characterised by high density, low total porosity, and aeration porosity, which is associated with the peculiarities of soil formation and lack of tillage.Factors limiting soil fertility are small humus reserves, acidic reaction of the medium and high density of the middle horizons.Soils can be used as arable land, but it is necessary to take measures to regulate the reaction of the environment and the balance of organic matter: application of organic fertilisers, crop rotation with perennial leguminous grasses, deep loosening, and, if necessary, liming.

Fig. 1 .
Fig. 1.Density, porosity and aeration porosity in genetic horizons of grey forest soil (section 1) of fallow land up to 10 years old

Table 1 .
Organic matter carbon content and stocks (Corg) grey forest soils fallow lands

Table 2 .
Physico-chemical properties of fallow grey forest soils